XV International PhD Workshop OWD 2013, October High Aspect Ratio Micro-interconnections in Multilayer Printed Circuit Boards

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1 XV International PhD Workshop OWD 2013, October 2013 High Aspect Ratio Micro-interconnections in Multilayer Printed Circuit Boards Janusz Borecki, Tele and Radio Research Institute, Warsaw, Poland ( , prof. Jan Felba, Wroclaw University of Technology, Wroclaw, Poland) Abstract The paper presents the issues related to the problem of forming of high aspect ratio (AR) 1 micro-interconnections in multilayer Printed Circuit Boards (PCBs). In described work the microinterconnections were made by metallization of laser drilled blind microvias with use magnetron sputtering deposition of copper. The researches were realized among others with use of Taguchi Method of planning experiments, and use of statistical analysis elements. The applied method of experiments planning allowed to significantly reduce (hundreds times) the number of necessary for implementation the single experiments. In addition, the applying of statistical analysis elements allowed chose the appropriate levels of parameters of the laser ablation process with which use the best results of experiment were obtain. The examination of microvia quality made with use of multicriteria analysis (in relation to: shape, symmetry of microvia, clearance of microvia bottom, and repeatability of process) allowed to elaborate the process parameters of blind microvias drilling which an aspect ratio reaches a value of 6.5, with a diameter of 25 µm. In the next sequence the process conditions of microvia wall metallization by magnetron sputtering of copper have been developed. Produced in this process a thin conductive layer covering the microvia wall can be thickened by using electrochemical plating technique. As a result, it was developed a method of forming of the low-ohmic micro-interconnections in multilayer PCBs. 1) AR - aspect ratio - the relation of via depth to via diameter. 1. Introduction At the present time there is more and more demand for functional and portable electronic equipment, such as: portable personal computers, tablets, modems, multimedia mobile phones, etc. The construction of these type devices requires, inter alia, the application of semiconductor structures with high number and high density of terminals. As a consequence, it is needed to fabricate the PCBs with high density of interconnections. The technological capabilities of standard techniques of PCBs production are insufficient to meet the demands of fine-line technology. It is necessary, in this regard, the use of the advanced forming techniques for network connections on individual layers of PCB s and interconnections between them [1]. The one of the best ways to increase the density of connections in PCBs is the use of metalized blind microvias technique [2]. The applying of that technique allows: - significant reduction (even 20 times) of the parasitic elements such as capacitance and inductance, which strongly influence on the limit of signals propagation; - reduction of the area occupied by interconnection and thereby increase of connections density on the individual conductive layer of PCB; - reduction of the number of PCB s conductive layers even up to 20%; - reduction of PCB s dimensions, even up to 40 %. Additionally, the applying of blind microvias directly in solder pads allows to significantly reduction of nets length and increasing of PCB area on the outer layers for assembly of higher number of electronic components. In this work the micro-interconnections were formed as a laser drilled blind microvias which were subsequently metalized in process of magnetron sputtering of copper. According to the definition contained in the standard IPC/JPCA-2315 (Design Guide for High Density Interconnects and Microvias), the microvia is the via which diameter is equal 150 µm or less. Considering that a standard through holes typically have diameter about 300 µm or higher, can be seen the scale of mentioned above advantages. 2. Laser drilling of microvias In this work the microvias were formed with use the UV Nd:YAG (λ=355 nm) laser drill machine ESI 5200 type from Electro Scientific Industries Inc. The microvias were formed with different diameter, mainly from 163 to 25 µm, and at the different 363

2 depth, 90 and 163 µm. Consequently, the aspect ratio of the microvias was in the range from 0.55 to 6.5. The process of blind microvias laser drilling is a multistep and very complicated process in which many parameters influences on each other [3]. In the first step, the window in copper layer (microvia TOP) with use higher energy is made, and after that in the second step the appropriate microvia to the target land (microvia BOTTOM) in dielectric layer is made. Finally in the second step the microvia shape and clearance of microvia bottom is defined. Schematically it is presented in figure 1. a) b) UV UV and the others (from B to H) have three. A First Pulse Enabled B Repetitions Rate C Velocity of laser beam movement D Effective Spot Size (ESS) E Inner Diameter of spiral (ID) F Number of spiral turns G Laser Power H Z offset (location of laser beam focus above the surface of copper layer) Distance between laser pulses (BS) Effective Spot Size (ESS) Fig.1. The multistep process of blind microvia drilling. a) 1 step ; b) 2 step Laser microvia drilling can be made by two methods, depending on the microvia diameter. These methods are: spiral mode (diameter about 150 µm and higher) and trepan mode (diameter less than 150 µm) which are shown in figure 2. a) b) Laser ON Laser OFF Fig.2. The methods of microvia laser drilling. a) spiral mode ; b) trepan mode In order to identify the key parameters of laser drilling process the technique of cause-effect diagram was used. In this technique, by method of successive iterations, the most important parameters that may influence on the results of experiment were chosen. The final form of the diagram is shown in figure 3, and graphic interpretation of some of them is presented in figure 4. First Pulse Enabled Inner Diameter of spiral Repetitions Rate Number of spiral turns Velocity of laser beam movement Laser Power Bite Size Zoffset Fig.3. The cause-effect diagram of process of laser drilled microvias. Each of the parameters (see figure 3) was indicated by the letter symbol from A to H, as it is shown below, wherein the one of them (A) has two levels Microvia shape Inner Diameter of spiral (ID) Spiral Diameter Fig.4. The graphical interpretation of parameters of microvia laser drilling in spiral mode. For example, the specified levels of individual process parameters, for 150 µm blind microvia drilled in spiral mode in 18 µm copper layer and 72 µm dielectric layer (90 µm microvia depth), are presented in table 1. The levels of process parameters for 150 µm blind microvias drilling Distance between spiral turns (RP) Tab.1. Parameter Unit I step II step L1 L2 L3 A - ON ON OFF - B [khz] C [mm/s] D [µm] E [µm] F G [W] H [mm] The examination of the experiment requires the numerous of single experiments in which each time the level of only one of them is changed. On the base of the number of selected parameters and their levels it should be made 4374 of single tests. In this situation it is helpful to use the Taguchi Method of planning the experiment [4]. This method allows tens or even hundreds times reduce the number of necessary to implementation the single tests. In this case it is possible to realize whole experiment by means of only 18 single tests. The way of experiment describes a Taguchi orthogonal array L18 type, which is shown below in table 2. Each single test was made three times (called as repetitions) in random order to eliminate of permanent errors which can be generated by the operator. 364

3 Tab.2. Taguchi orthogonal array L18 type Test No. A B C D E F G H The evaluation of quality of formed microvias was carried out on the base of microscopic observations of cross-sections, and for several criteria such as: - shape of microvia (diameter of microvia bottom and tilt angle of microvia wall); - symmetry of microvia; - clearance of microvia bottom; - repeatability. On the base of that the diameter of microvia on the TOP in each simple test was equal 150 µm, the tilt of microvia wall and the shape of microvia was closely related to the diameter of microvia bottom. In the analysis of experiment results made in Taguchi Method the quality of microvias, in respect to each criterion, has to be interpreted in a numerical way. From this reason, for example in case of clearance of microvia bottom the three-level scale of quality evaluation was used, where: 1 means totally clean microvia bottom; 2 means the slight residue of dielectric on microvia bottom, mainly located in the centre; 3 means the residue of dielectric covering the whole microvia bottom. The examples of microvias cross-sections evaluated in this respect are presented in figure 5. a) b) c) Fig.5. The examples of microvia bottom clearance. a) level 1 ; b) level 2 ; c) level 3 For all other criteria were adopted the same rule: the lower level of evaluation (1, 2 or 3) means the better achieved result of experiment. In order to obtain answer about the repeatability of process and which level of each process parameter should be used to receive best results of experiment, the main effects analysis was made. On the beginning the arithmetic average of received results for given level of a specified parameter (e.g. A1, A2, B1, B2, B3, C1, C2, C3,, etc.) was calculated. Besides of that, this analysis was also carried out for signal to noise (S/N) parameter. That parameter refers to repeatability of individual tests (repetitions) in the single test (one of the eighteen defined in table 1). The higher value of S/N parameter means higher repeatability, but that parameter is defined in different way, in dependence on that the result of the experiment is better when is higher (formula 1) or when is lower (formula 2). where: r number of repetitions (observations) in simple test y i result of i observation (1) (2) In the research work the S/N parameter was calculated according with formula 1 in reference to first criterion (d D - diameter of microvia bottom), and for the other criteria (symmetry of microvia, clearance of microvia bottom) it was calculated according with formula 2. For example, the results of experiment in reference to first criterion (d D - diameter of microvia bottom) are presented in table 3. Test No. Tab.3. The results of experiment measurements of diameter of microvia bottom Series of test dd δ (S/N)HB [µm] [µm] The presented results are vary widely but repeatable. 365

Besides of that, none of individual result do not exceed the value of microvia diameter on the TOP (150 µm) what may initially provide the "right" of microvia (cylinder or slightly scratched inverted

Parameter Tab.4. The results of main effects analysis for diameter of microvia bottom in reference to dd in reference to (S/N)HB 1 2 3 1 2 3 A 94.70 90.19-39.38 38.90 - B 94.22 90.11 93.00 39.28 38.

with green fill of cells in table 4). With the use of these parameters was performed checking experiment, and the expected shape of microvia was achieved.

Finally, it was chosen the B1 level because the difference in value of S/N parameter between 39.28 (for B1) and 39.30 (for B3) is very small and can be considered as the same versus 38.84 (for B2).

The use of more advanced statistical analysis allows better understand the influence of each process parameters on the final result of the experiments.

In other words, it allows identify the process parameters and their percentage strength, which have the greatest influence on the results of the experiment.

4 Besides of that, none of individual result do not exceed the value of microvia diameter on the TOP (150 µm) what may initially provide the "right" of microvia (cylinder or slightly scratched inverted sheared cone; non barrel). The final results of main effects analysis in reference to first criterion (d D - diameter of microvia bottom) are presented in table 4. Parameter Tab.4. The results of main effects analysis for diameter of microvia bottom in reference to dd in reference to (S/N)HB A B C D E F G H On the base of presented results it can be seen that the best shape of drilled microvias were achieved with use of process parameters on the levels: A1, B1, C1, D1, E3, F3, G3, H1 (results marked with green fill of cells in table 4). With the use of these parameters was performed checking experiment, and the expected shape of microvia was achieved. It should be noted that in case of B parameter the best results were obtain for different levels: B1 in reference to average value of result, and B3 in reference to S/N parameter. Finally, it was chosen the B1 level because the difference in value of S/N parameter between (for B1) and (for B3) is very small and can be considered as the same versus (for B2). In this case, the most important is difference between (for B1) and (for B3) in reference to average value of result. The use of more advanced statistical analysis allows better understand the influence of each process parameters on the final result of the experiments. One such analysis is ANOVA analysis of variances. It enables determining the impact of main effects and it helps to find the correlation between the process parameters and the shape of microvia. In other words, it allows identify the process parameters and their percentage strength, which have the greatest influence on the results of the experiment. The ANOVA analysis of variances was calculated for the same criteria as in case main effects analysis, and the final results of analysis are presented in table 5. The composed mathematical calculations used in the ANOVA analysis of variance makes it impossible to present it in this article, but if is very useful tool to study the multi-factor experiments. The presented in table 5 results shows that the quality of formed blind microvias, in reference to diameter of microvia bottom, mainly depends on laser power (parameter G with the strength about %) and number of spiral turns (parameter F with the strength about %). The percentage part of errors e p on the level % means influence of unspecified factors not included in the experiment. Tab.5. The results of ANOVA analysis of variances for diameter of microvia bottom Parameter SS x v V F SS P [%] A B C D E F *** G *** H e T e p On the base of undertaken analysis the parameters of blind microvias drilling process have been developed. These parameters are different for each type of the microvia in dependence of microvia depth and aspect ratio. The examples of metallographic cross-sections of blind microvias with different aspect-ratio are presented in figure 6. Ø 350 µm ; AR = 0.46 Ø 25 µm ; AR = 6.5 Ø 250 µm ; AR = 0.65 Ø 163 µm ; AR = 1.0 Ø 82 µm ; AR = 2.0 Ø 54 µm ; AR = 3.0 Ø 33 µm ; AR = 5.0 Fig.6. The examples of microvias with different aspect ratio (AR). 366

2. Metallization of microvias The forming of deep blind microvias with low diameter (with high aspect ratio) is a partly successful in making of micro-interconnections in multilayer Printed Circuit

It should be noted that the standard techniques of metallization such as copper electroplating process enable the generation of conductive layer on the walls of blind microvias, whose aspect ratio

For this purpose, a magnetron WMK50 type, with unique construction, designed and manufactured in Faculty of Microsystem Electronics and Photonics of Wrocław University of Technology was used.

5 2. Metallization of microvias The forming of deep blind microvias with low diameter (with high aspect ratio) is a partly successful in making of micro-interconnections in multilayer Printed Circuit Boards. Large difficulty in this issue is the metallization of walls of deep microvias which aspect ratio is much higher than 1. It should be noted that the standard techniques of metallization such as copper electroplating process enable the generation of conductive layer on the walls of blind microvias, whose aspect ratio does not exceed the value of 0.8 [5]. Accordingly, the metallization of deep blind microvias in this work has been made in process of magnetron sputtering of copper. For this purpose, a magnetron WMK50 type, with unique construction, designed and manufactured in Faculty of Microsystem Electronics and Photonics of Wrocław University of Technology was used. The unique construction of magnetron is characterized by working with very high density of power dissipated on the target, which is several times higher than those achieved in conventional systems [6]. The possibility of changing of power dissipated on the target in magnetrons of WMK series allows for smooth transition from standard mode (argon mode) to self-sputtering mode (without the participation of working gas). The one of the advantages of selfsputtering mode is the significant elongation of mean free path of particles minted from the target, which is very useful in case of its deposition in very deep microvias [7]. The aim of the experiment was to investigate the possibilities of conductive layer deposition on microvia walls with use standard mode and self-sputtering mode. The basic parameters of the magnetron sputtering of copper process in standard and self-sputtering mode are presented in table 6. more uniform deposited layer, the dynamic mode was used as a second way of metallization. In this way the test samples (4 test samples in 1 row) were moved many times in front of the target. Schematically it is presented in figure 7. a) b) MAGNETRON TEST SAMPLES (2 pcs x 2 rows) MAGNETRON TEST SAMPLES (4 pcs x 1 row) MOVEMENT Fig.7. The scheme of metallization microvias process made by magnetron sputtering of copper. a) static mode ; b) dynamic mode The thickness of the deposited layer indirectly depends on the time of process, and it seems to be obvious that the thicker layer allow to receive low electrical resistance of formed connections. However, the magnetron sputtering process is a high-energy process which causes heating up of the substrate (test samples). From this reason the time of process in standard mode was limited to 180 seconds. Because the exposure time of test samples in front of target in dynamic mode is several times shorter, the time of process in this mode was set at 900 seconds. The quality of microvia metallization was estimated on the base of microscopic observations of metallographic cross-sections from microvias as well as on the base of measurements of electrical resistance of conductive layer deposited on microvia wall. The examples of cross-sections made from metalized microvias are presented in figure 8. Parameter The basic process parameters of magnetron sputtering of copper Unit Standard mode Tab.6. Self-sputtering mode Pressure at the end of process [Pa] 2.67 * * 10-3 Pressure of Ar [Pa] 4.00 * 10-1 ~ 0 Target current [A] Voltage [V] Target power [W/m 2 ] 4.32 * * 10 9 During the investigations in one way (static mode) the test samples with drilled microvias (2 test samples in 2 rows) were located in magnetron chamber in distance about 190 mm centrally in front of the target. In view of directional movement of particles of sputtered material the most of them are deposited on the top conductive layer of test PCB, but not on the microvia wall. From this reason, in order to increase the efficiency of conductive layer deposition on microvia wall, and also to obtain a Ø 150 µm ; AR = 0.6 Ø 75 µm ; AR = 1.2 Ø 25 µm ; AR = 3.6 Fig.8. The examples of blind microvias metalized by magnetron sputtering of copper. For measuring the electrical resistance of formed interconnections the four-point method, and the laboratory multimeter KEITHLEY 2001 type (7 1 / 2 digits) were used. This enabled the measurement of very low resistance with an accuracy of ± 1µΩ. The results of electrical resistance measurements have been calculated for different modes (standard, self-sputtering, static and dynamic) in relation to the size of metalized microvias. The final results are presented in figure 9 and

6 Resistance [Ω] Fig.9. The average electrical resistance of interconnections in reference to standard and self-sputtering mode. It can be seen that the lower resistance of metalized microvias was achieved in case of standard mode. This is due to the fact that in case of self-sputtering mode the free path of particles minted from the target is relatively long, and on the microvia wall which is almost parallel to the direction of the moved particles, the thinner conductive layer is deposited (in comparison to standard mode). Nevertheless, to metalize of deep and high aspect ratio microvias it is recommended to use the self-sputtering mode. The priority in this case is to make the continuous conductive layer on the entire length of microvia, which can be thickened in specialized electroplating processes. Resistance [Ω] 35,0 30,0 25,0 20,0 15,0 10,0 5,0 0,0 Aspect Ratio (AR) standard mode self-sputtering mode 0,6 0,7 0,9 1 1,5 1,8 2,4 3,6 współczynnik Aspect Ratio kształtu (AR) otworu metoda static mode statyczna metoda dynamic dynamiczna mode Fig.10. The average electrical resistance of interconnections in reference to static and dynamic mode. As in the previous case, are observed different values of connections electrical resistance in reference to static mode and dynamic mode. Consequently noticeable is the effect of direction of target particles movement in relation to microvia wall surface. Changing the position of microvia wall during the process in dynamic mode helps to achieve more uniform conductive layer, but only if the aspect ratio of microvia not exceed the value of After that the possibility of making the continuous conductive layer on the entire length of microvia significantly decreases. It is caused by the low number of sputtered particles which can reach the microvia bottom area in relatively short time, at the moment when the microvias are moved in front of the target. Summary The applied in the researches of blind microvias laser drilling process the Taguchi Method of experiments planning allowed to significant reduce of necessary to implementation single tests (from 4096 to 18). Additionally, the carried out analysis of achieved results (main effects analysis and ANOVA analysis of variances) allowed define the levels of laser ablation process parameters which guarantee the drilling of deep (up to 163 µm) blind microvias with low diameter even about 25 µm (aspect ratio reaches the value about 6.5). Additionally, the application of magnetron sputtering of copper technique, especially in self-sputtering mode, allowed to make high aspect ratio interconnections which is very promising in the development of modern techniques of high-tech PCBs production. Bibliography [1] Buetow M.: On The Road To, Printed Circuit Fabrication, January 2003, pp [2] Borecki J.: Interconnection for High Density PCB Ways of Design Optimization, Proceedings of XXVI International Conference of IMAPS Poland Chapter, September 25-27, 2002, Warsaw, Poland, pp [3] Hackiewicz H., Koziol G., Borecki J.: Contribution to UV Laser Ablation Process of Sound PCB Microvias, Proceedings of 27 th International Conference and Exhibition IMAPS-Poland 2003, September, 2003, Ostaniec Hotel, Podlesice Gliwice, Poland, pp [4] Peace G.S.: Taguchi Methods, Addison-Wasley Publishing Company, 1993 [5] Borecki J.: High Density Interconnects An Overview of Plating techniques, Proceedings of XVI IEEE-SPIE Symposium on Photonics, Electronics and Web Engineering, May 31 June 5, 2005, Wilga, Poland, Vol. 6159, pp U1-9 [6] Posadowski W.M.: Niekonwencjonalne układy magnetronowe do próżniowego nanoszenia cienkich warstw, Oficyna Wydawnicza P. W., Wrocław 2001, ISDN [7] Posadowski W.M., Brudnik A.: Optical Emission Spectroscopy of Self-Sustained Magnetron Sputtering, Vacuum 53, 1999, pp Author: Ph. D. (E. Eng.) Janusz Borecki Tele and Radio Research Institute Ratuszowa 11 Str Warsaw, Poland tel fax janusz.borecki@itr.org.pl 368

ANALYSIS OF BLIND MICROVIAS FORMING PROCESS IN MULTILAYER PRINTED CIRCUIT BOARDS*

PL ISSNe 0209-0058 MATERIAŁY ELEKTRONICZNE T. 37-2009 NR 1 ANALYSIS OF BLIND MICROVIAS FORMING PROCESS IN MULTILAYER PRINTED CIRCUIT BOARDS* Janusz Borecki 1 ), Jan Felba 2), Artur Wymysłowski 2) The paper