Development of new assembly techniques for a silicon micro-vertex detector unit using the flip-chip bonding method

Nuclear Instruments and Methods in Physics Research A 342 (1994) 298-301 Ntirth-1 -tollitnd

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Development of new assembly techniques for a silicon micro-vertex detector unit using the flip-chip bonding method Y. Saitoh '", H. Takeuchi ", M. Mandai ", H. Kanazawa ", J . Yamanaka ", S. Miyahara M. Kamiya ", Y. Fujita Y. Higashi ", H . Ikeda ", M. Ikeda ", S. Koike ", T. Matsuda H. Ozaki M. Tanaka T. Tsuboyama ~, S. Avrillon ', S. Okuno ", J. Haba H. Hanai S. Mori K. Yusa C. Fukunaga f 1,

1,

" Seiko histnonents Inc., 563 Takatsuka Shinden, Afatsudo-4d, ClUba-ken 271, Japan KEK, National Labonitoryfor High Diem- Physics, Tviketba. lbaraki 30, Japan Apartment of Accelerator Science, The Graduate Universityfor Advanced Studies, Tvukuba, lbaraki 30.5, Japan [hywrtment of Pitysics, Osaka University, To.yonaka, Osaka 500, Japan histitute of Applied Physics, Tsukuba Univerfi~y, Tvukuba, lbaraki 305, Japan I'Department of Pikysics. Tokyo Metropolitan University, flachioji, Tokyo 192-03, Japan

Full-size models of a detector unit for a silicon micro-vcrtex detector were built for the KEK B factory . The Flip-Chip Bonding (FCB) method using a new type anisotropic conductive film was examined. The structure using the FCB method successfully provides a new architecture for the silicon micro-vertex detector unit.

1. Introduction

2. Fabrication procedure

The micro-vertex detector for the KEK B factory consis,, of three layers of double-sidcd, double-metal silicon microstrip detectors (DSSD) 111. The detector element of the vertex detector, a detector unit, consists of four to Nix DSSDs daisy-chained together with silicon end boards at each end of the unit. CMOS preamplifier-analog memory VLSIs, a CMOS control VLSI and other necessary electronic components are mounted on both sides of the end boards . Fig. I shows the unit, which consists of four DSSDs. To obtain the necessary electrical connection to strips between the DSSDs and the silicon end boards . the wire bonding method, which has been used for singlc-sided silicon strip detectors, can be applied . The application of the wire bonding method to the DSSD, however, requires skilled hands and careful handling of the detector unit. In order to study the new assembly method for detector units using Flip-Chir Bonding (FCB) methods, several model detector units were built [2] . We will report here on an FC13 method using a new type anisotropic conductive film (ACF) to achieve double-sidcd bonding of 640 strips at a pitch of 50 Rm .

The DSSDs were first aligned with their neighbors using a guiding tool on an alignment bench equipped with vacuum chucks . A silicon bonding bridge (4.22 mm X 33.18 mm) with the anisotropic conductive film (SONY chemical CP series, current type CP7131 and new type) on its bonding pads was mounted on the xyO alignment head of a robot arm, aligned to the two DSSDs, and glued onto them. After connecting four DSSDs and two silicon end boards on one sidc, the unit was cured at 150'C for 20 s with a pressure of 10 g/pad on the silicon bonding bridges . The dummy VLSI chips were mounted previously . On the other side, five bonding bridges and dummy chips were mounted in a similar way . Finally, the I-Beams (CFRP) were glued on the long edges of the unit using an epoxy resin (EPOTEC 302-2) at room temperature . -rtI tic fabrication configuration is shown in Frig. IL. Fig. r 3 shows a completed detector unit .

" Corresponding author .

3. Mechanical properties The detector unit was checked using a three-dimensional measurement machine . The strips on the four DSSDs were straight to better than ± 5 ~Lrn. Attaching the two ends of the detector unit on a precision sup-

0168-9002/94/$07 .00 K, 1994 - Elsevier Science 13N. All rights reserved SSDI 0168-90020ME0964-T

Y Saittlh et al /Narl Instr. and Meth . in Ph,ts. Rrx A 34Z ( 1 W4)

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Fig. 2. Fabricalion configuration .

Fig. 1 . A schematic of the configuration of the detector unit .

porting framc rigidly, the gravitational sag was measured to be about 2-5 lLm, as shown in Fig . 4. The natural deformation of the detector unit when the ends of the detector unit were placed on two supports freely was around 80 tLm. On the precision supporting frame, the deviation from a plane defined by the framc varied depending on how the unit was fixed on the frame. These values can be improved either by careful assembly or by minor modifications of the design . An accclcrated test for heat shock (-30"C to 80'C, 100 times) and a high-tcmpcraturc, high-hur-L4 1 1ty test (85*C/85'7"r . 500 hours) showed no significant. d-~-!~-ri-rnti-n -f the mechanical performance . 4. Electrical properties The anisotropic conductive film used here is made of an epoxy glue (25 ~Lrn thick) which contains 5 Rm

diameter Au- and Ni-plated plastic balls. The nominal , density of the balls was about 41W balls/mm in the current type and 160W balls/mm 2 in the new type. Fig . 5 shows the schematic cross section of the conncctcd pads. The balls between the Au bump.s were to provide an electrical connection between the facing bumps . The balls in the new type ACF were coated with a thin insulating layer. The balls held between the faced bumps provided an electrical connection because the insulating layer was crashed by the bumps.s. but unexpected connection with neighboring bumps was prevented in spite of high density. Table I shows the calculated and measured electrical connection failure rate between DSSDs. The calculated failure rate was derived from the ACF ball density and the bonding pad area (120 Rm X 60 gm pads arranged in double columns (i c. two pads per line) at a pitch of 100 jLm as shown in Fig . 6 using Poisson's distribution . The -source of the difference between the calculated and measured values would be the ball density variation (vender specifications. maximum and minimum) within the ACF . The ball dcnsltv ~'arlatlon for the new type has not been examined yet, thus is not yet guaranteed. Another factor would be the small voids which were found in the connected area. These voids might make

Fig. 3. A completed detector unit using the FCB method used in this study. 111 . COLLIDER APPLICATIONS

Y Saitoh et al. I Nucl. Instr. and Meth. in Phys. Res. A 342 (1994) 298-301

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100 80 M

60

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0 40 20

Fig. 4. The gravitational sag. The vertical scale is the deviation (Rm), and the horizontal scale is position (mm) along with the detector unit length .

Fig . 5. Schematic cross section of connected pads.

the measured failure rate higher than the calculated failure rate in the case of the current type ACF. These voids might be caused by a heater with insufficient capability used in the curing process . The electrical connection is also to be improved through pad size optimization. The contact resistance was measured to be 1.07 il on the average, and was small enough compared to the strip resistance . The isolation resistance between neighboring strips was measured to be more than 500 Mil for the new type. An accelerated test for heat shoc.% (-300C to 800C, 100 times) and an accelerated high-temperature, highhumidity test (85*C/85%, 500 hours) showed some increase in the connection failure as shown in Fig . 7. We fabricated six samples the same way . However, the ACF epoxy of samples 2 and 6 might not have been cured fully. This might be caused by insufficient heater capability (heater head 0.5 mm thickness stainless steel). An accelerated 500-hour test is equivalent to 10 years of normal operation . The curing problem will be improved by exchanging the heater head for one of 5.0 min thickness stainless steel. Radiation effects will be examined and a reliability endurance test will be done on assembled units having a structure similar to a genuine vertex detector. Fig . 8 shows a demonstration model of the micro-vertex detector assembled in this study with one layer of units. 5 . Conclusion

The FCB method using anisotropic conductive film was tried assuming use in the new architecture for a vertex detector unit using DSSD. The architecture has provided good mechanical performance for a detector unit in a silicon micro-vertex detector, and also pro-

Fig . 6.120 ~LmX60 ~Lrri pads arranged in double columns at a pitch of 100 ~Lm.

S-ple No I Smpk No 2 S.mpk No 3 S.mpk No 4 S-pic N. 5 Sarnp e No 6

Table I Electrical connection failure Ball density (balls/ MM2) (Vender specification) Calculated failure rate Double column M Single column M Measured failure rate Double column (%) Single column ( 114C)

New type 16000 -

Current type 4100 + 2600, - 2100)

0.01 1 .6

0.2 -

ave. 0.5

ave. 3.4 -

Endurance Sequence Fig. 7. Failure rite increasing along wi!h the endurance sequence . The vertical scale is the electrical connection fail ure (open) rate, and the horizontal scale is the endurance sequence .

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silicon end board, will provide flexibility in the line and pad layout and in turn in the design of readout VLSIs having the same thermal expansion coefficient. Acknowledgments The authors wish to thank Dr. P. Weilhammer, CERN/PPE and Dr. R. Horisberger, PSI, for providing us with the necessary mask design for the dummy detector. This work was done as a part of detector R&D for the KEK B factory . Fig. 8. A demonstration model of the micro-vertex detector assembled in this study with one layer of units. vided tolerable electrical connections and sufficient endurance reliability . The FCB method, assisted ".)y the anisotropic conductive film together with the use of the

References [1] C. Fukunaga et al., KEK preprint 93-115 (1993). [2] Y. Saitoh et al., IEEE Trans. Nucl. Sci . NS-40 (4) (1993) 552.

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