T h e Y i e l d M a n a g e m e n t C o m p a n y Electrostatic Compatibility in Photolithography An OEM perspective

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1 T h e Y i e l d M a n a g e m e n t C o m p a n y Electrostatic Compatibility in Photolithography An OEM perspective Jeff Bruner Compliance Engineering Project Manager KLA-Tencor RAPID Division

2 Topics -Case studies of ESD damaged reticles -ESD challenges at smaller device design rules -Weapons to control ESD in photolithography -Avoiding ESD-induced reticle defects 2

3 Electrostatic Discharge (ESD) Is Seeing Increasing Emphasis in Lithography SEMATECH ESD Forum gaining momentum Second annual meeting Web-based grapevine for information sharing SEMATECH funded research Parameters for management of ESD proposed for inclusion next ITRS roadmap ESD management becoming recognized as a requirement for yield with new smaller design rules 3

4 ESD Damage in Lithography Can Be Catastrophic A single reticle defect can cause a yield crash Step-and-repeat process duplicates errors Subtle defects can impact speed and binning Long time-lag to discovery at functional test 4

5 Reticle Defects Cause Yield Excursions Bridging on wafer caused by reticle CD errors 5

6 Reticle Construction and the Litho Environment Promote ESD Events Quartz reticle substrate is a good insulator Thin layer of conductive chrome is easily damaged 4:1 typical reduction at stepper Gaps are below well 1 micron Insulative pellicle can be charged to 20 kv Non-ESD-compliant tools and handling equipment contribute Reticle boxes and SMIF pods generate charge Charged to 20 kv New versions with ESD dissipative materials More study needed 6

7 Conducted Model Reticle Damage Is Well Understood Reticle is charged Charged reticle is grounded Charge jumps gaps in chrome features Chrome features are damaged Damage most likely to occur Small gaps Sharp corners 7

8 Higher Energy Reticle Damage On Old Design Geometry 8

9 Higher Energy Reticle Damage On New Design Geometry Transmissive View Reflective View 9

10 Inductive Model Reticle Damage Recently Characterized Recent innovative study by Intel, Ion Systems, DPI, SEMATECH Field Induced damage Mathematical model developed Breakdown voltage for a 0.13-micron feature are only a few hundred volts Electric field polarizes a neutrally charged chrome feature Charges are segregated within the chrome Charge segregation can be self-amplifying The induced charges jump gaps in chrome features Induced charges arc again when field is removed Long traces on interconnect layers are most susceptible 10

11 Reticle Image - Transmitted Light AIMS Review Case Study #1 -- Critical ESD Defect Causes Yield Crash Leading Asian Foundry experienced yield crash Cause traced to reticle Defect barely visible in transmitted light Reticle returned to mask shop Defect found on inspector using Simultaneous Transmitted and Reflected Light AIMS review tool confirmed defect prints Foundry improved ESD program Foundry added SEMI E78 to equipment specs Implements reticle requalification program

12 Transmissive View Case Study #2 Transmissive View ESD Damage at Progressive Levels Contact landing pad on M1 layer crosses about 1/3 of the chip Three views Transmissive Reflective Reflective View Summed No damage shows in the transmissive view Reflective image shows variations in reflectivity Summed View Darkness in summed image indicates early stage damage Antireflective coating on edge has been slightly damaged Damage is due to ESD Plate will continue to degrade Inspection needed to identify bad plate early

13 Transmissive View ESD Damage Evolves Similar location on same plate Transmissive view shows blue (not black) patterns, indicating transmission through damaged chrome Reflective View Partial bridge in the quartz is partially transmissive and partially reflective. Damaged chrome clear in the summed view Caused by multiple ESD events Summed View Defect will print soon

14 Transmissive View Things Get Worse Reflective View Summed View Similar location on same plate Transmissive view shows shows further chrome break-down (blue areas) Complete bridge shown in red in the transmissive image Enlarged area of damaged chrome shows up in black on the summed light image

15 Transmissive View Reflective View Summed View The Damage is Done Roughened chrome and bridge is visible in 4X demag In a higher-energy discharges, the roughened chrome would be completely melted and/or vaporized and the bridge not likely to form Damage must be characteristic of repeated lowerenergy discharges Chrome foam

16 What is the Bridge? Very high NA 100X objective, shows that the bridges are transparent at the green wavelength used for the photograph Nomarski 100X shows the yellow is the original unperturbed chrome Ringed green area is damaged chrome Texture of the bridge is smooth unlike the damaged chrome Is the bridge a chrome buildup? Or is it a dip in the quartz?

17 Final Answer Is Atomic Force Microscope (AFM) study shows bridge is convex, 0.2 microns high. Bridge is chrome oxide which has migrated from the adjacent damaged chrome during ESD discharge events. The chrome oxide bridge forms an electrically conductive path. Bridge is stable. Successive ESD events seem to damage the chrome like rings on a tree. The defect will print the same way a phase defect on a phase shift mask would print -- the light coming through the defect is out of phase with the light immediately adjacent to it.

18 Transmissive View Corner Damage E-Fields concentrated at corners Reflective View

19 Mouse Bites

20 ESD-induced Reticle Damage Increases at Newer, Smaller, Design Rules Older, Larger geometry Infrequent ESD events Higher voltage Higher energy events Catastrophic damage High temperature Vaporized chrome Newer, Smaller Geometry Frequent ESD events Lower voltage Lower energy events Cumulative damage Lower temperature Melted chrome 20

21 Weapons to Tame the ESD Gremlin in Photolithography 1. Establish ESD control program per spec 2. Avoid exposing reticles to static fields 3. Use static dissipative carriers 4. Utilize proper handling process 5. Increase humidity to 50% 6. Select ionizers for sensitive operations 7. Catch ESD damage early with periodic reticle inspections 8. Specify SEMI E78 compatible tools and handling devices 21

22 Avoid Exposing Reticles to Static Fields Plastic and glass paneled mini-environments Consider ESD dissipative clear plastic windows Picks and handling equipment Large plastic and other insulative surfaces Tool skins Furniture & Clothing Insulated wheeled carts Carriers, reticle boxes, and SMIF pods Much more study needed A huge opportunity 22

23 Catch ESD Damage Early With Periodic Reticle Requalification XXXX 23

24 Specify ESD Compliant Lithography Equipment On SEMATECH member purchase specs Know your tools do not cause ESD damaged reticles Testing is easy and painless KLA-Tencor reticle inspection tools are now Third Party certified Passed at Level 1: < 1 nc charge (0.13 & 0.10 um design rules) ESD Control (nc) Contamination (Volts/cm) Equipment Malfunction (nc) Level Level Level Level

25 Conclusions ESD damage to reticles can be catastrophic ESD events change character and become more likely at smaller geometries An effective program to control static and monitor process control is required Well designed equipment and facilities are essential Continual vigilance is mandatory 25