NEW STANDARD: GUIDE FOR THE HANDLING OF RETICLES AND OTHER EXTREMELY ELECTROSTATIC SENSITIVE (EES) ITEMS WITHIN SPECIALLY DESIGNATED AREAS NOTICE

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1 Background Statement for SEMI Draft Document 4783A NEW STANDARD: GUIDE FOR THE HANDLING OF RETICLES AND OTHER EXTREMELY ELECTROSTATIC SENSITIVE (EES) ITEMS WITHIN SPECIALLY DESIGNATED AREAS NOTICE: This background statement is not part of the balloted item. It is provided solely to assist the recipient in reaching an informed decision based on the rationale of the activity that preceded the creation of this Document. NOTICE: Recipients of this Document are invited to submit, with their comments, notification of any relevant patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this context, patented technology is defined as technology for which a patent has issued or has been applied for. In the latter case, only publicly available information on the contents of the patent application is to be provided. Background Statement This is the second version of this ballot Document. Draft Document 4783 was balloted in the summer of It was failed by the NA Metrics Technical Committee at the NA SEMICON West 2011 Meetings and returned to the ESD/ESC Task Force. A large number of changes were made based on the responses received to Document Most of these changes were to use language consistent with it being a Guide to provide recommendations rather than requirements (e.g., must to should ) and editorial changes to better comply with the SEMI Standards Style Manual. Some additional wording changes were made to improve accuracy and clarity and to include feedback received from members of the ESD Association. Among users and manufacturers of semiconductors, MEMS devices and flat panel displays, the effects of electrostatic surface charge are well known. Charged surfaces attract particles (i.e., electrostatic attraction [ESA]) and increase the defect rate. Charged products are sometimes difficult to handle and cause equipment jamming or breakage. Finally, electrostatic discharge (ESD) damages products and reticles, as well as causing numerous equipment malfunctions. Static control methods have been employed by semiconductor and equipment manufacturers to reduce the effects of static charge while handling product or reticles. But static charge problems continue to occur due to methods and materials used to construct the cleanroom as well as activities within the cleanroom itself. SEMI has issued E78: Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) in Production Equipment and E : Guide to Assess and Control Electrostatic Charge in a Semiconductor Manufacturing Facility to address electrostatic issues that occur within the equipment and manufacturing facility. This Document is a Guide for establishing optimum electrostatic compatibility of the handling environment for reticles and other items that are extremely electrostatic sensitive (EES) to electrostatic charge, voltage and electric field. This Guide is complementary to SEMI E78 and E129 and is intended to improve the protection of the most electrostatic damage-susceptible items. For the purposes of this Document, extremely electrostatic sensitive (EES) items are those that are affected by any combination of electrostatic charge, electrostatic voltage or electric field. This Document can be used as a Guide for equipment manufacturers during the design and testing of their equipment and by those who either use or produce reticles and other EES items. Process technology used in the manufacture of semiconductors and electronic devices continues to achieve increases in active feature density and device complexity. With increased levels of integration, longer interconnects and smaller conductor separations, sensitivity to field-related problems increases. This Document provides recommendations for addressing the problem of damage through closer examination of electric field as a supplement to existing static charge mitigation techniques. This Document defines principles for handling reticles and other EES items within a specially designated controlled environment and recommends appropriate levels of electric field to maintain within that environment. This Document presents recommendations about grounding and material selection that may conflict with established methods of electrostatic charge control and should be applied only within a specially designated and clearly identified area. Wherever this Document makes reference to reticles, this should be regarded as an example of an EES item that has been studied in depth and which is being used for illustration purposes. The principles being discussed may also be i

2 relevant to other items that exhibit extreme electrostatic sensitivity so the guidance should not be regarded as exclusive to reticles and reticle handling. Such items include small geometry device structures on wafers, charged device model (CDM) sensitivity of packaged devices in back end processing, and device structures on glass in flat panel display manufacturing. Review and Adjudication Information Task Force Review Committee Adjudication Group: ESD/ESC Task Force NA Metrics Technical Committee Date: 2011/10/ /10/26 Time & Time Zone: PST PST Location: SEMI Headquarters SEMI Headquarters City, State/Country: San Jose, CA San Jose, CA Leader(s): Arnie Steinman (Electronics Workshop) David Bouldin (Fab Consulting) Mark Frankfurth (Cymer) Standards Staff: Paul Trio (SEMI NA) Paul Trio (SEMI NA) This meeting s details are subject to change, and additional review sessions may be scheduled if necessary. Contact the task force leaders or Standards staff for confirmation. Telephone and web information will be distributed to interested parties as the meeting date approaches. If you will not be able to attend these meetings in person but would like to participate by telephone/web, please contact Standards staff. ii

3 SEMI Draft Document 4783A NEW STANDARD: GUIDE FOR THE HANDLING OF RETICLES AND OTHER EXTREMELY ELECTROSTATIC SENSITIVE (EES) ITEMS WITHIN SPECIALLY DESIGNATED AREAS 1 Purpose 1.1 The purpose of this Document is to minimize the negative impact on productivity caused by static charge and electric fields in semiconductor manufacturing equipment and facilities. It is a Guide for establishing optimum electrostatic compatibility of the handling environment for reticles and other items that are extremely electrostatic sensitive (EES) to electrostatic charge, voltage, and electric field. This Guide is complementary to SEMI E78 and E129 and is intended to improve the protection of the most electrostatic damage-susceptible items. NOTE 1: For the purposes of this Document, EES items are those that are affected by any combination of electrostatic charge, electrostatic voltage, or electric field. 1.2 This Document can be used as a Guide for equipment manufacturers during the design and testing of their equipment and by those who either use or produce reticles and other EES items. 1.3 Process technology used in the manufacture of semiconductors and electronic devices continues to achieve increases in active feature density and device complexity. With increased levels of integration, longer interconnects, and smaller conductor separations, sensitivity to field-related problems increases. This Document provides recommendations for addressing the problem of damage through closer examination of electric field as a supplement to existing static charge mitigation techniques. 2 Scope 2.1 The scope of this Document is limited to the definition of principles for handling reticles and other EES items within a specially designated controlled environment and recommendation of an appropriate level of electric field to maintain within that environment This Document presents recommendations about grounding and material selection that conflict with established methods of electrostatic charge control (e.g., those defined in ANSI/ESD S20.20 for devices of 100 volt Human Body Model [HBM] sensitivity) so the guidance given here should be applied only within a specially designated and clearly identified area. 2.2 This Document references SEMI E78, SEMI E129, SEMI E43, and other methods of measuring electrostatic parameters. The set of Documents is complementary, providing guidance on managing different aspects of electrostatic risk under a wide range of conditions. 2.3 While this Document makes frequent reference to electrostatic fields and effects, for the purposes of this Guide this should also be considered to include alternating, variable, and transient electric and electromagnetic fields. All such fields can potentially generate electrical stress within an EES item. 2.4 Wherever this Document makes reference to reticles, this should be regarded as an example of an EES item that has been studied in depth and which is being used for illustrative purposes. The principles being discussed may also be relevant to other items that exhibit extreme electrostatic sensitivity so the guidance should not be regarded as exclusive to reticles and reticle handling. Such EES items may include small geometry device structures on wafers, packaged devices in back end processing where charged device model (CDM) sensitivity may be a problem, and device structures on glass in flat panel display (FPD) manufacturing. NOTICE: SEMI Standards and Safety Guidelines do not purport to address safety issues associated with their use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices and determine the applicability of regulatory or other limitations prior to use. 3 Limitations 3.1 General This Guide contains general recommendations Specific field-related problems or certain devices may require or allow different levels of electric field than are recommended in this Document The calculation of electric field induction involves some simplification, but the effects of such simplification will be within the typical measurement accuracy of accepted field measurement techniques. Page 1 Doc SEMI

4 4 Referenced Standards and Documents 4.1 SEMI Standards SEMI E43 Recommended Practice for Electrostatic Measurements on Objects and Surfaces SEMI E78 Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment SEMI E129 Guide to Assess and Control Electrostatic Charge in a Semiconductor Manufacturing Facility 4.2 ESD Association Standards and Advisories 1 ANSI/ESD STM3.1 Ionization ANSI/ESD SP3.3 Periodic Verification of Air Ionizers ESD TR Selection and Acceptance of Air Ionizers ANSI/ESD S20.20 Development of an Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices) 4.3 Other Documents International Technology Roadmap for Semiconductors ITRS 2 NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions. 5 Terminology 5.1 Definitions carrier a device for holding wafers, dies, packaged integrated circuits (ICs), or reticles for various processing steps in semiconductor manufacturing. [SEMI E78] extremely electrostatic sensitive (EES) item any item that is very highly susceptible to degradation or malfunction caused by electrostatic charge, voltage or field, even when handled under conditions that would normally be classified as electrostatic discharge (ESD) controlled electromagnetic interference (EMI) any electrical signal in the nonionizing portion of the electromagnetic spectrum with the potential to cause an undesired response in electronic equipment. [SEMI E33] electrostatic compatibility charge control adequate to allow the manufacturing of products and the interequipment transfer of products, reticles, and carriers without electrostatic problems. [SEMI E129] electric field-induced migration (EFM) the movement of normally stationary atoms or molecules on a surface as a consequence of the presence of an electric field. NOTE 2: Related Information 1 describes the characteristics of EFM in chrome-on-glass reticles electrostatic discharge (ESD) the rapid spontaneous transfer of static charge induced by a high electrostatic field. [SEMI E78] minienvironment a localized environment created to isolate product from contamination and people. [SEMI E78] product any item intended to become a functional semiconductor device. [SEMI E78] EES minienvironment carrier a transport method for extremely electrostatic sensitive (EES) items that excludes electric fields by surrounding the EES item with a Faraday Cage (i.e., a conductive enclosure). 5.2 Acronyms AFM atomic force microscope ANSI American National Standards Institute CDM charged device model 1 Electrostatic Discharge Association, 7900 Turin Road, Building 3, Suite 2, Rome, NY , USA. Telephone: ; Fax: , 2 ITRS Global Communication Center, SEMATECH, 2706 Montopolis Drive, Austin, TX 78741, USA; Page 2 Doc SEMI

5 5.2.4 CD critical dimension EES extremely electrostatic sensitive EFM electric field-induced migration EMI electromagnetic interference ESA electrostatic attraction ESD electrostatic discharge ESDS electrostatic discharge sensitive FPD flat panel display IC integrated circuit ITRS International Technology Roadmap for Semiconductors MR magneto-resistive SMIF standard mechanical interface 5.3 Symbols C capacitance Q charge V voltage 6 Considerations About Assessing the Risk from Electric Field NOTE 3: Appendix 1 of this Document contains a detailed treatment of the interaction between an electric field and a fieldsensitive object (in this example, a reticle). NOTE 4: Related Information 1 describes the methods that were used for determining the recommended electric field level for reticles. A similar approach may be adopted to determine the field sensitivity of other EES items. 6.1 Electric fields cause a number of undesirable effects in electronic device manufacturing environments Field induction can affect EES items such as reticles or FPDs without any physical contact between the source of the electric field and the sensitive item Field induction can cause damage within such EES items without any transfer of static charge to or from them Electric fields that do not cause ESD may still be a hazard to small device structures In some circumstances, the damage caused by exposure to electric fields can develop continuously over an extended period of time, resulting in gradual deterioration and eventual failure of the affected item. 6.2 Measurements of parameters such as electric field are difficult to make The nature of an object (i.e., insulator, conductor, or in most cases a mixture of both), its geometry, its surroundings, and the measuring equipment itself are only a few of the factors affecting the accuracy of an electric field measurement The levels of electric field that can cause progressive damage in EES items such as reticles may be lower than it is practical to measure with hand-held apparatus It is not possible to measure the internal electric field within an item such as a reticle or a packaged device. This internal field may be the result of static charge generated on its surface or fields through which it may pass when it is moving through equipment. However, sensor devices that can be handled in the same manner as the item of interest and that can record the electric field exposure that the item might experience are becoming available A field measurement made at a particular location and time may not be representative of the field that may be present in the same location at another time, or in another apparently identical location Electric fields may be transient in nature and may leave no permanent evidence that they have been present Not all field measurement equipment is sensitive to rapidly changing or transient fields. 6.3 It is difficult to relate the measurement of an electrostatic quantity like electric field to the effect it may have on a sensitive item. Page 3 Doc SEMI

6 6.3.1 Tests conducted with one sensitive item may not be representative of the effects that may be produced in another apparently similar item Almost every reticle in use has a unique conductor pattern and variations in the pattern of conductors and insulating spaces alter the field induction that will take place upon exposure to an electric field. 6.4 Due to the variable nature of reticle designs and the inability to quantify the field induction that takes place at different places in a reticle, it is not possible to establish definitive damage thresholds for production reticles. Therefore, guidance values for electric field exposure have been determined following experimentation with specially designed test reticles. NOTE 5: Refer to Related Information 1 for details of the methods used and technical references. 6.5 It may be impossible to define electric field levels that guarantee field-related problems are totally eliminated Due to the progressive deterioration of a reticle that can be caused by electric fields and the possibility of similar continuous damage effects in other field-sensitive items, no particular value of electric field can be considered as safe. A weak electric field that is present for a long period may possibly cause more significant damage than a strong electric field that is present for a shorter time. 6.6 Damage may be caused by internal or external electric fields Internal electric fields can be caused by an electrically isolated part of an object becoming charged electrostatically while another part remains uncharged. The epoxy encapsulation of a packaged device may be tribocharged during handling and this can create an electric field that emanates from the device If the leads of such a charged device are now connected to ground, a balancing charge attracted by the charge on the epoxy encapsulation will flow onto the leads. Externally, the object may appear to have been neutralized by connection to ground, but internal charge separation and hence an internal electric field will still exist The strength of the internal electric field and hence the risk of field-induced damage will have been increased by grounding, even though the device may now appear to be electrically neutral and hence safe External electric fields that can penetrate a field-sensitive object may be generated in many ways, but an electric field interacts with a field-sensitive object in exactly the same way regardless of its source An electric field coming from static charge on an insulating surface is equivalent to the same strength of electric field originating from a conductor connected to a power supply, in terms of the damage it can induce However, all sources of electric field are not equivalent in terms of how they can be treated Electric fields from static charges present on insulating surfaces or isolated conductive objects can be eliminated by using air ionization, but fields emanating from powered sources cannot be eliminated in this way Fields from powered sources can only be eliminated by shielding them with a grounded conductive enclosure (i.e., Faraday Cage). The ability of a Faraday Cage to shield alternating or rapidly varying fields depends on both the conductivity and continuity of the material used for its construction and the rate of change of the field. A metal Faraday Cage protects against field penetration from any source Field perturbation will occur whenever conductors are introduced into an electric field. Such field perturbation can result in a greater electric field strength being present within a field-sensitive object, without there being any change in the charge state or voltage of the field source A field sensitive object is itself likely to perturb an externally produced electric field and may cause the local field strength to increase to much higher levels than might be measured in the absence of the object Similarly, the field strength within a field-sensitive object can be increased by changing its position relative to other conductive surfaces such as equipment panels, robot arms, and workbenches Such field perturbations may not be intuitively obvious. For example, the placement of a conductive ring around a planar array of isolated conductors (as in the case of a chrome border around the pattern area of a reticle) can cause the direction of the electric field between the isolated structures to reverse. The movement of any similar conductive structure (such as a ring-shaped carrier frame) close to a field-sensitive item could cause significant field perturbation and consequential damage. 6.7 Field induction is virtually instantaneous. It can take only picoseconds for an electric field to induce damage in a field-sensitive object such as a reticle, so even transient electric fields and perturbations caused by the movement of tools and equipment during material handling can be sufficient to cause damage. Such transient fields may not be detectable unless specialized high-bandwidth monitoring equipment is used. Page 4 Doc SEMI

7 7 Considerations for Assessing the Risk from Electrostatic Discharge (ESD) 7.1 When an ESD event happens during the handling of a charged, packaged device, there may be a very rapid transfer of a balancing charge onto the device leads. There may be no neutralization of the original charge on the device package material. When a charged object is connected to ground, even through a dissipative connection, the charge transfer that takes place delivers a balancing charge onto the device, not a neutralizing charge. This is illustrated in Figure 1. Figure 1 Charging During Packaged Device Handling 7.2 Since it is virtually impossible to eliminate the in-process charging of devices that takes place inside tools or equipment, measures should be taken to protect the devices even in the charged condition. 7.3 It is also important to recognize that measures that have been put in place to reduce the generation of ESD events in general often to quite justifiably avoid equipment lock-ups and particle attraction can actually increase the risk of damage to a sensitive device. 7.4 This Document explains that ESD avoidance and sensitive device protection do not always require the same methods and that grounding is not always the preferred method If a charged EES item is connected to ground through a dissipative contact, there may not be a damaging discharge at the moment of grounding as a controlled, balancing charge flows slowly onto the device pins. In normal ESD-prevention terms, there is no ESD event on contact. However, the gradual transfer of a balancing charge to the circuitry can raise the potential difference across an internal circuit junction to the critical point, causing internal damage. This happens as a direct consequence of connecting the pins to ground and allowing static charge elsewhere on the device to attract a balancing charge into the circuitry. The device can now appear to be macroscopically neutralized. There will have been no ESD event at the moment of grounding and no event registered on any ESD sensors used to monitor the production area, but the device may have been damaged internally. 8 Apparatus 8.1 Electrostatic Charge Measurement For measuring the charge generated on product, reticles, or carriers as defined in SEMI E78, the Faraday Cup test method is shown in Figure 2 and is described in more detail in SEMI E43. However, when using this method to measure charge on EES items, inaccurate results may be obtained due to the presence of balanced charges that produce internal electric fields as described previously (refer to 6.6.1) Insulators and complex EES items are capable of simultaneously being charged with both polarities of static charge. Measurement with a Faraday Cup will indicate the net excess charge on an object, not the amount of charge separation within it If a tribocharged item being tested has been grounded at any time during its movement into the Faraday Cup, for example by the use of an equipotential bonding scheme for ESD management, a balancing charge may have been placed on it that will nullify the charge measurement An electrostatic fieldmeter or voltmeter may be capable of indicating whether this condition exists, but this may not always be possible. Page 5 Doc SEMI

8 Shielding Outer Cup Isolated Inner Cup Faraday Cup In Electrometer Ground Figure 2 Faraday Cup Charge Measurement 8.2 Electric Field Measurement The instrument used for making electrostatic field measurements is known as an electrostatic fieldmeter. Instructions concerning its use should be obtained from the instrument manufacturer and SEMI E43. Typically, an electrostatic fieldmeter measures at a distance of 2.54 cm (1 inch) The measurement configuration shown in Figure 3 illustrates the effect of the instrument on the measurement. In most cases the presence of the fieldmeter will increase both the flux from the charged surface and the convergence of the electric field lines. The fieldmeter will generally indicate a higher value of electric field than would be present without the fieldmeter An electrostatic voltmeter can be used as an alternative to the fieldmeter measurement. For small objects or surface areas, an electrostatic voltmeter is appropriate. Refer to E43 for further information Under appropriate conditions, electrostatic voltmeters exhibit a high degree of accuracy and stability that is independent of the distance from the charged object. The electrostatic voltmeter probe can be located very close to a charged surface without arc-over, and it is able to resolve the field from a small charged object For measuring transient electric fields a high bandwidth electrostatic voltmeter with an output to a computer or a storage oscilloscope may be needed. Electric Field Lines 1999 Electrostatic Fieldmeter (volts/cm) Charged Surface Charged Surface Figure 3 Electrostatic Field Measurement 2.54 cm (1 inch) For recording the electric field within the restricted handling environment of equipment, carriers, and process chambers where there is insufficient access for hand-held devices or where the presence of the probe itself would significantly alter the measurement, customized sensor devices may be required that can take the place of the EES item under consideration The electric field configuration in the presence of such a sensor device should be as close as possible to the field configuration with the EES item present. Such a device can sample virtually all the environments through which the EES item may pass and can record process- or handling-induced charging. It can also record the field exposure duration as well as the field strength. Therefore, this may be the most suitable way of assessing electric field risk for EES items. Page 6 Doc SEMI

9 9 Identification of EES Classified Zones 9.1 Owing to the different principles employed for EES item handling and the conflict with conventional ESD precautionary handling methods that use equipotential bonding (i.e., grounding) schemes, it is recommended that zones where EES items are handled be segregated and clearly marked The recommended symbol for identification of an EES item, handling zone, or equipment that is compatible with EES handling techniques is shown in Figure 4. Figure 4 Recommended Symbol for Identifying an EES Item, Compatible Handling Zone, or Equipment 10 Principles for EES Classified Zones 10.1 The primary objective is to maintain an EES item in the absence of any static charge or electric field. There are two different aspects of protection for EES items charge management and field management Charge Management The safety of charge management by grounding, even through a resistive contact, will depend on the nature of the circuitry within the sensitive item, how rapidly excess charge flows to ground, and the route it takes. Grounding may also cause internal electric fields to increase as described previously. For these reasons, grounding of the EES item may need to be avoided, when possible, within an EES classified handling zone Within the EES zone, other methods of charge management, such as air ionization, are employed as an alternative to grounding. When charge cannot be safely neutralized, changes to handling methods may be needed to assure that charge generation does not occur. Materials that contact EES items may need to be selected to minimize triboelectric charge generation Air ionization is inherently capable of neutralizing charge at a rate that is unlikely to cause damage, so it can be used in place of grounding In any application of air ionization, it is necessary to assure that the ionized air reaches the charged surface. Air ionization operates relatively slowly, and sufficient time should be allowed for the ionization system to work Air ion streams respond to the presence of static charge by reacting to the electric field that is produced by the charge, so it is important to recognize that ionizers do not actually prevent the generation of static charge or electric field Ionizers supply positive and negative ions to neutralize either polarity of static charge. Ionizers should be carefully balanced to avoid generating a charge on isolated conductive objects. Ionizers that are incorrectly balanced or badly maintained may create a risk rather than reducing the risk. Refer to ESD Association documents ANSI/ESD STM3.1, ANSI/ESD SP3.3, and ESD TR for more information regarding the use and testing of air ionization Regular monitoring of ionizer efficiency and balance is recommended for the effective operation of an EES handling zone. The frequency of checks depends on many factors including the inherent stability of the ionizers being used, so it cannot be recommended in this Document. Operators of EES zones should satisfy themselves that auditing intervals and monitoring methods are adequate to maintain the effective operation of ionizers Field Mitigation Field Reduction Avoiding the accumulation of static charge should minimize electric fields. This may be best done through the careful selection of materials and by employing air ionization as described previously. Page 7 Doc SEMI

10 Corona-type air ionizers work by generating strong electric fields at emitter tips. Care should be taken to ensure that any electric fields produced by such air ionizers cannot reach an EES item. Alpha or x-ray ionizers produce no strong fields so may be more suitable for use close to EES items Shielding Ionizers cannot neutralize the electric fields that are generated by powered systems. So wherever power is used, steps should be taken to minimize stray electric or electromagnetic fields by using shielding Shielding with metal is recommended to achieve the best electric and electromagnetic field reduction around powered systems and great care should be taken when making joints in the shield to ensure that high frequency fields cannot escape. Methods and materials that are suitable for electromagnetic interference (EMI) shielding purposes are preferred for shielding in an EES zone The only fail-safe means of protecting an EES item from externally generated electric and electromagnetic fields may be to enclose it within a fully conductive Faraday Cage. Such a field-protective, EES minienvironment carrier should be used to house EES items whenever possible Shielding efficiency is dependent on the conductivity and density of the material that is used to make the shield. Inherently conductive or metallized plastics and even metal wire meshes may not be sufficient to achieve complete field shielding at all frequencies. The most suitable shielding material to use may be sheet or machined metal, with metal-to-metal connections being made between any separate parts. It should be noted that additional shielding may increase the risk of field perturbation as described below Field Perturbation Changing the proximity of an EES item to any conductive surface or other objects will change the field configuration and field strength that the EES item may experience. Refer to Appendix 1 A1-4.1 for an example of placing a reticle in a grounded enclosure. To keep field perturbation effects to a minimum, EES items should be kept as far as possible from conductive surfaces and other objects It is recommended that EES items be handled with inherently insulating (i.e., field-transparent) end effectors or tools so that any electric fields that may be present in the handling environment or created by the items themselves being tribocharged are not perturbed. For example, adding an insulating surface layer or contact pad to an otherwise conductive end effector is not equivalent to a fully insulating end effector because the body of the tool is not field-transparent and hence will cause field perturbation If it is not possible to implement fully insulating end effectors in equipment that handles EES items, it may be necessary to adjust the timing of the handling sequences. If air ionization is used, it should have sufficient time to neutralize any static charge that may have been generated on the item or its surroundings before a grounded end effector approaches the EES item to move it An EES item may itself be tribocharged during handling and this may generate an internal electric field. Placing such a charged item inside a Faraday Cage will result in field perturbation that could increase the risk of damage. Therefore all previously described precautions should be followed to neutralize static charge and ensure that the use of a Faraday Cage does not itself increase the risk of the EES item being damaged. Refer to Appendix 1 A Interfacing EES Zones and Carriers to Other Zones 11.1 When transferring an EES item from an EES zone to an electrostatic discharge sensitive (ESDS) handling zone where equipotential bonding is used and where the item may be either purposely grounded or closely approached by other grounded objects, excess static charge should be neutralized before transfer takes place. Hence, ionization is recommended at all transfer points, especially where EES items are being placed into and removed from EES minienvironment carriers When EES items are within an EES minienvironment carrier, the handling of that carrier should comply with external handling standards as follows If the outside environment is designated as an ESDS handling zone, ESDS norms should apply. For example, the minienvironment carrier should be connected to an equipotential bonding point when it is placed on a load port for opening If an EES minienvironment carrier is being handled outside a controlled zone, no special handling precautions for electrostatic protection are recommended. Page 8 Doc SEMI

11 11.3 Some equipment in a facility may be designated EES compatible while other equipment may not. Whenever an EES compatible minienvironment carrier is used with non-ees compatible equipment, ESDS handling practice should be adopted Such interoperability allows facilities to upgrade their handling systems only where it is considered necessary or appropriate to do so. Thus the adoption of EES handling or the use of EES compatible minienvironment carriers does not necessitate a complete change of the established handling methods, ESD precautions, certification programs, or operator training in the wider facility Likewise the use of EES compatible minienvironment carriers within an ESD protected area does not degrade the electrostatic protection rating of that area towards the sensitive objects being handled. Since grounding of ESDS items has been shown to increase the risk of them being damaged rather than to reduce it, the adoption of EES compatible carriers is an improvement to ESD protection zones Hence existing certification schemes and auditing practices are not altered by the adoption of EES handling EES is merely appended to them However, the use of EES compatible minienvironment carriers may change the recommendation for certain ESD precautions in parts of the facility or may alter the specification that is needed. For example, if reticles were previously transported in non-ees compatible minienvironment carriers or were handled bare, air ionizers throughout the area might have been recommended. When using EES compatible minienvironment carriers, some of this ionization might not be necessary. Depending on site conditions and other requirements, ceiling ionizers may still be needed to control EMI. They may not be recommended to protect the EES items now protected by the EES compatible minienvironment carriers. Hence the specifications for ionizer location and performance may be reduced. 12 Guide Recommendations for EES Zones Table 1 Suggested Parameters for Use Within an EES Classification Zone Parameter Value Notes Ambient electric field #1 < 500 V/m Value is based on EFM risk in chrome-on-glass reticles Field recovery time < 60 s Field levels should be recoverable to the allowable ambient level (<500 V/m) within this time of a normal transient stress event, such as the ending of a manufacturing process step. Maximum transient field 5000 V/m Transient stress should be present for no longer than 1 second before field recovery process starts. #1 Ambient electric field see Related Information 1 R The levels in Table 1 should be appropriate for the handling of chrome on glass reticles. Electrostatic fields may also damage other specialized components such as compound semiconductors, FPDs, or magneto-resistive (MR) disk-drive heads. Safe handling of these devices may need to use different levels The levels in Table 1 should be measured with an electrostatic fieldmeter or other field measuring devices. The measurement instrument should have sufficient response time to allow measurement of transients. Refer to In all cases it is desirable to achieve the minimum possible field strength, static charge levels, and recovery times from transient events. The lower the stress that is present and the shorter its duration, the longer an EES item may remain within the environment without suffering potential damage. Page 9 Doc SEMI

12 APPENDIX 1 COMPUTER SIMULATION OF ELECTRIC FIELD INTERACTIONS WITH RETICLES NOTICE: The material in this appendix is an official part of SEMI [XXXXXX] and was approved by full letter ballot procedures on [A&R approval date]. A1-1 Introduction A1-1.1 Electrostatic damage is most commonly caused by rapid charge transfer, through a spark (i.e., ESD event). A1-1.2 Charge moves in reaction to the electrostatic force of attraction or repulsion exerted on it by another charge. This force is represented by the concept of an electric field the field strength indicates the magnitude of the force that a unit charge would experience at that point and the direction of the field line is the direction of the force. A1-1.3 Hence, for charge to move there must be an electric field present to exert the force that moves it. Without an electric field present, charge will not move. A1-1.4 Computer simulation can produce a map of the electric field configuration around any object, so it can indicate when there may be a risk of ESD or other field-related damage. A1-2 How an Electric Field Causes ESD Damage A1-2.1 Figure A1-1 shows schematically the stages of field induction when an electric field (which produces a voltage gradient) passes through a reticle. A Figure A1-1a is the state of a uniform electric field created by two vertical electrodes at different potentials before a reticle is introduced. The voltage changes linearly from one electrode to the other. A When the reticle is introduced as shown in Figure A1-1b, each isolated conductor comes to an intermediate potential that is induced by the external electric field. A Electrons are forced to move within the conductors under the influence of the external electric field. This charge displacement continues until it produces a balancing electric field that cancels out the external electric field. Thus, at equilibrium there is no net electric field present within each conductor. A The electric field is seen to have been amplified in the gaps between the isolated reticle structures. Any array of isolated conductors will function as an electric field amplifier in this way. The degree of field amplification depends on the length and relative separation of the conductors and the orientation of the array with respect to the external electric field. Typically, a reticle can amplify the ambient field strength by up to 1000x. A Such an amplified electric field may be sufficient to initiate a discharge between the structures. After discharge has taken place, the charge redistribution within the reticle will be as shown in Figure A1-1c. Charge has been displaced internally and there is now no electric field remaining between the reticle structures. A When the external electric field is removed as shown in Figure A1-1d, each of the isolated reticle structures has a charge imbalance the structures have been charged by field induction. Note that in this condition there has been no charge transfer to or from the reticle by any external source, only the charge within the reticle has been forced to move. A When the external field is removed, the displaced charge within the reticle creates an internal electric field that is exactly opposite to the previously applied field. This condition may cause further discharges as the displaced charge returns to its original location. A After the field induction cycle, the reticle may have been damaged twice as the internal charge has been forced to move in two opposite directions. The reticle has remained electrically neutral throughout the process and no charge has been added to or taken from it. Page 10 Doc SEMI

13 + + a) b) c) d) Figure A1-1 Schematic Representation of the Field-Induced Damage Process in a Reticle (the Graph Represents Voltage as a Function of Position in the Plane of the Conductors) a) Uniform electric field between two charged plates before the reticle is present b) Reticle introduced. Charge (i.e., electrons) within the conductors moves in response to the field. At equilibrium, the field is zero within the conductors, thus the field is amplified in the gaps between the conductors. c) Discharges have occurred between the conductors and charge is now redistributed, which removes the internal field. The conductors have now been charged by field induction even though no charge has been transferred to or from the reticle. d) When the external field is removed, the redistributed charge creates its own field. More discharges may occur as the charge returns to its original location. A1-3 Details of the Simulations A1-3.1 Computer simulation of electric field interactions with isolated conductors on an insulating support has been carried out using commercially available software (e.g., Opera-2d). Voltage contours (i.e., lines of equal potential) are plotted around the simulated objects. A Two-dimensional rather than three-dimensional analysis has been used to simplify the modeling. Such two-dimensional models consider that the structures extend to infinity in front of and behind the plane being modeled. Since electric field strength is greatest at edges and points that are reduced by this simplification, this means that the simulations slightly understate the true field strength that would be present around a threedimensional object. A While these limitations of two-dimensional simulation do affect the results, the differences between the calculated values in the simulations and the true values in a three-dimensional structure may be small enough to not significantly affect the conclusions that can be drawn from the simulations. A1-3.2 Structures Simulated A Reticle in Cross-section Figure A1-2 Structure Used to Represent a Reticle in Cross-Section Page 11 Doc SEMI

14 A A reticle in cross-section is simulated as a block of insulating material representing the glass substrate, supporting 100 isolated conductive lines representing the image area of a reticle. This is shown in Figure A1-2. A On either side of the 100 isolated conductors are 2 wider strips of conductor representing the chrome border. When these are used to represent a continuous chrome field around the image area, these two strips are constrained to be at the average potential of the reticle. When a reticle without a continuous chrome border is being simulated, these two strips are allowed to float independently. A Reticle Environment A To represent free space conditions the model is given floating boundary conditions. Such simulations represent the situation with no other field-perturbing objects in close proximity to the simulated object. A When a uniform electric field is to be applied to an object in free space, the two side boundaries are given specific voltages that create a uniform electric field between them. The simulated structure is allowed to float within this field to determine the induced potentials and field configurations that would result. A When a nonuniform electric field is to be simulated to represent static charge on a nearby object, that object is given a fixed potential and a grounded plane is introduced that represents a work surface, equipment wall, or other such surface. This defines an electric field with a known strength and initial configuration prior to the insertion of the object to be simulated. A Fixing the potential of an insulating or isolated charged object is physically incorrect, since its potential will vary with its separation from ground according to the relationship Q = CV, where Q is the charge, C is the capacitance, and V is the voltage. However, the inaccuracy introduced by this simplification of the model should be small and should not invalidate the conclusions drawn, which are comparative. A1-4 Simulated Scenarios A1-4.1 Static Charge on a Reticle A Since a reticle is normally handled by contacting the chrome border that surrounds the image area and is outside the pellicle enclosure, this is where static charge is most likely to be deposited. A reticle that has 5 kv on the chrome border is simulated in Figure A1-3. a) b) Figure A1-3 5 kv on the Chrome Border of a Reticle (the Lines are Equipotentials or Voltage Contours) a) Reticle in Free Space Potential Difference Within Reticle 80 mv b) Reticle in a Large Grounded Box Potential Difference Within Reticle 3 kv A Figure A1-3a shows the voltage contours surrounding the reticle in free space conditions, meaning there are no nearby objects that affect the field emanating from the charge on the reticle. The entire reticle is at almost the same potential, with there being only 80 mv of potential difference between the edge and the central part of the image area. A Figure A1-3b shows the same reticle enclosed in a large grounded box. The presence of the grounded enclosure perturbs the field conditions around and within the reticle, resulting in 3 kv of potential difference induced between the central region of the reticle and the chrome border. Page 12 Doc SEMI

15 A It is seen that the proximity of a grounded surface is a factor that strongly affects the risk of damage to a field-sensitive object like a reticle if it is charged. Voltage alone is not hazardous, since there will be no charge movement within the reticle as a direct result of it (Figure A1-3a). When a nearby object disturbs the field conditions, this can perturb the situation and create a high risk of field-induced damage (Figure A1-3b). A The high field strength in the reticle is produced long before the grounded object (in this case the box) makes contact and provides a path to ground through which the charge on the reticle might be removed. This shows that attempting to discharge an EES item by grounding is highly likely to damage it, regardless of whether or not the path to ground is resistive. A1-4.2 The Effect of a Guard Ring A The chrome border around the edge of a reticle combined with an insulating channel between the chrome border and the image area is sometimes referred to as the guard ring. This is often considered to act as a protective structure, reducing the risk of ESD damage to the sensitive image area of the reticle. A The presence of a continuous chrome border around the image area of a reticle indeed has a significant effect on the penetration of an electric field, as shown in Figure A1-4, but the effect is not simply to attenuate the field. a) b) Figure A1-4 The Effect of the Chrome Border (Guard Ring) on Field Induction in a Reticle a) Voltage Contours as They Would Be Without a Chrome Border Present b) Voltage Contours with a Chrome Border Surrounding the Image Area A Figure A1-4a shows the voltage contours as they would be if a chrome border was not present on the reticle and the reticle was placed in a uniform horizontal electric field. The voltage contours are seen to pass uniformly through the structure, inducing a constant potential gradient from one side of the reticle to the other, as shown in Figure A1-1. A Figure A1-4b shows the situation with a continuous chrome border around the image area. The chrome border comes to the average potential of the reticle and the presence of this conductive ring at this average potential strongly disturbs the electric field. The field bends around and its direction is actually reversed at the points where it passes through the pattern area on the way to the chrome border (as indicated by the overlaid arrows). A The effect of the chrome border can be seen clearly in Figure A1-5, where the induced voltages from the simulations of Figure A1-4 are plotted. It can be seen that for this particular orientation there is indeed an attenuating effect on the average field strength within the reticle due to the presence of the guard ring. A However, the reticle features at the edge of the image area closest to the guard ring experience a field strength that is just as high as it would have been without the guard ring present the key difference is that the direction of the potential gradient is reversed. A The field induction characteristic illustrated in Figure A1-5 explains the ring of fire distribution pattern for much of the ESD damage that is seen in reticles. Reticles with significant amounts of ESD damage frequently have a high concentration of damage sites in close proximity to the edge of the pattern. This has often been Page 13 Doc SEMI

16 explained as being due to static charge that is deposited on the chrome border from a nongrounded handling tool, which then jumps into the image area by means of a spark. A The considerations of Figures A1-3 and A1-4 show the true reason for the concentration of ESD events in this area. Field induction causes the local field strength to be highest here. Damage would occur here due to field induction whether charge was placed on the chrome border or not. As Figure A1-3 shows, attempting to remove any charge from the reticle with a grounded handling tool would perturb the internal field and might induce damage in the reticle even without electrical contact being made. Figure A1-5 Induced Potential and Potential Gradient in a Reticle With and Without a Guard Ring (Sloping Bars Indicate Potential Gradient at the Edges Near the Guard Ring ) A1-4.3 Insulating Versus Conductive Supports A It is common practice to handle ESD sensitive items with grounded tools and end effectors, which is referred to as equipotential bonding. The objective behind this is to maintain all objects at the same electrical potential so that when they contact each other during handling procedures static discharges should not occur. A However, it is quite easy for items to be tribocharged during handling or processing, so they may unavoidably develop a static charge. A A high resistance to ground (i.e., Ω) at the point of contact is commonly prescribed so that if any static charge does flow to or from the ESD sensitive item during handling, the current and hence the power dissipated will be low. A While this practice does indeed reduce the likelihood of a damaging static discharge between a tool and the object that it is handling, it has an unfortunate effect on the field induction that will take place within a fieldsensitive item as has been shown in previous examples. Page 14 Doc SEMI

17 a) b) Figure A1-6 Field Induction With a) Conductive / Static Dissipative or b) Insulating Supports (the Lines Are Equipotentials or Voltage Contours) A Figure A1-6 represents the field induction pattern in a reticle that is resting on supports, for example in a reticle pod. The base of the pod defines the ground plane in the simulation, since equipment load ports are always grounded and pod bases are always conductive or static dissipative. A high voltage is simulated immediately above the reticle, representing static charge that may be present on the pod handle or on an operator s gloved hand. A Figure A1-6a shows the field configuration if the reticle is resting on conductive or static dissipative supports that are connected to ground. Figure A1-6b shows the same situation except that the reticle is supported by fully insulating (i.e., field-transparent) support structures. The potential differences present between the structures in the reticle image area are several times higher when the chrome border is grounded through the supports than when it is allowed to float to an intermediate potential along with the other isolated reticle structures. A It has been confirmed experimentally that field-induced damage inside a reticle pod is worse if the reticle is grounded on the supports as shown in Figure A1-6. NOTE 6: See reference by Rudack, Pendley, Gagnon and Levit in Related Information 1. A Figure A1-7 shows a similar situation to Figure A1-6 but the field source is now displaced to one side, representing static charge on an operator s gloved hand while loading a reticle onto a support structure using a reticle pick. A Figure A1-7a has conductive reticle supports, while Figure A1-7b has fully insulating supports. The scenario is almost identical to that of Figure A1-6 except that the consequences of field distortion by the chrome border / guard ring in the asymmetric field coming from the side of the reticle are shown. a) b) Figure A1-7 Asymmetric Field Induction During Reticle Loading Onto a) Conductive / Static Dissipative Supports and b) Insulating Supports. The Arrows Represent the Direction and Magnitude of the Field at Each Edge of the Reticle. (the Lines Are Equipotentials or Voltage Contours) Page 15 Doc SEMI

18 A In Figure A1-7a the highest potential gradient induced in the reticle is at the point nearest to the source of the field, which can be seen by the density of the voltage contours at that point. By contrast, the highest potential gradient in Figure A1-7b where the reticle is allowed to float on insulating supports is on the side of the reticle furthest from the source of the field. A The direction of the field as it passes through the features in the image area close to the chrome border is also seen to be different in the two cases, as represented by the arrows above the figures. This has a very significant implication for the loading of a reticle onto conductive supports; as the reticle approaches, but does not yet make contact with the supports the field configuration will be similar to that in Figure A1-7b. As soon as the reticle makes electrical contact with the supports the chrome border immediately comes to ground potential and the electric field configuration switches to that of Figure A1-7a. A As soon as the reticle is grounded by the supports, the potential differences in the reticle reverse and increase. The damage caused may be significantly worse in case a) than in case b) and may involve sequential discharges taking place in two different directions. A1-5 Conclusions A1-5.1 In all situations studied, grounding a reticle should increase the risk of field-induced damage. A1-5.2 Grounding does not reduce the damage sustained by a field-sensitive item like a reticle. A1-5.3 Hence, it has been demonstrated that while equipotential bonding does indeed help to prevent external ESD events between an object and its handling means, if the object being handled is field-sensitive, grounding creates a much greater risk of damaging the object itself. NOTE 7: The use of field-transparent (i.e., fully insulating) handling arms and tools to avoid field perturbation effects around EES items necessitates the use of air ionization. Page 16 Doc SEMI

19 RELATED INFORMATION 1 DEVELOPING NEW RECOMMENDATIONS FOR THE ELECTRIC FIELD EXPOSURE OF RETICLES NOTICE: This Related Information is not an official part of SEMI [XXXXXX] and was derived from the work of the global Metrics Technical Committee. This Related Information was approved for publication by full letter ballot procedures on [A&R approval date]. Contributed by Gavin Rider, PhD. gavinrider1@aol.com R1-1 Previous Guidance R1-1.1 All guidance values published in SEMI Standards and the ITRS prior to publication of this Document have been based on managing ESD risk or contamination rates. ESD risks for reticles were evaluated by exposing production reticles to calibrated electric fields and also by stressing specially designed test reticles in the same way. R1-1.2 Since those guidance values were developed, further research has shown that a physically different damage mechanism (i.e., EFM) was responsible for some of the reticle damage that had previously been attributed to ESD. EFM causes the continuous critical dimension (CD) degradation of certain features in chrome-on-glass reticles at induced potential differences well below the onset threshold for an ESD event. R1-1.3 Since EFM involves a completely different physical process to ESD, it was apparent that guidance designed to protect against ESD would not necessarily be effective at protecting against EFM. R1-1.4 The following experimental quantification of EFM was conducted to establish accurate thresholds and rates for reticle damage below the ESD threshold, to find out more about the physics of EFM, and to confirm whether or not ESD was involved. A more complete description and treatment of the data can be found in the references. R1-2 Experimental Quantification of EFM Risk in Reticles R1-2.1 Stress Testing R Previous field induction experiments had been conducted with Canary test reticles, but those experiments did not produce quantifiable data about the local electric field level at the point of damage. It was decided to apply voltages directly to reticle structures and thereby to create the local electric field directly. R This approach should allow fully calibrated and reproducible stress testing of the structures and accurate timing of the stress duration. R To enable direct comparison with the earlier field-induced damage data, a new test reticle was designed that had similar electrode structures having an almost identical local electric field configuration at the places where damage would be produced. There are 8 columns of test cells in groups of 5 x 5, with line widths from 1 µm to 10 µm. The width of each spur line is the same as its spacing from the chrome border. To monitor surface conditions across the reticle, a blank cell with no spur line is positioned at the intersection of the borders around the 5 x 5 groups of cells. The structure of the test reticle is shown in Figure R1-1. Figure R1-1 Design of the EFM Test Reticle R Voltage was applied to individual test structures in a systematic way to apply calibrated electrical stress over the range 1 V to 100 V for different periods of time. Page 17 Doc SEMI

20 R Voltage was also scanned continuously from 0 V to 100 V and the current flowing was recorded so that current-voltage characteristics of the test cells could be produced. R The tests were conducted with positive and negative polarity. R1-2.2 Results R After the stress testing was completed, the test cells were imaged using optical microscopy and atomic force microscopy. R Line scans were conducted using a CD Atomic Force Microscope (CD-AFM) to accurately determine CD variation produced. R The current-voltage measurements indicated that current flowing on the reticle surface had a highly nonohmic characteristic, with the current increasing nonlinearly with increasing field strength at the positively biased electrode. This indicated that field-generated positive charge carriers play a role in the surface conduction. R The CD-AFM line scans as shown in Figure R1-2 revealed progressive alteration of the left chrome line edge topography starting with the formation of a meniscus at the point where the chrome meets the quartz. As the stress duration and stress voltage increase, the extent and character of the CD variation change. a 7a b 7b 7c c 100 nm 7d d 1 µm Figure R1-2 Line Profiles of 1 µm Test Cells: a) Spur Stressed at 100 V for 300 s b) Spur Stressed at 100 V for 15 s c) Spur Stressed at 50 V for 15 s d) Unstressed R The rate of line edge modification at 50 V was measured at 3 nm/s. At 100 V, the rate of edge modification of the chrome line increased to over 6 nm/s. Page 18 Doc SEMI