Fabrication of a High-Density MCM-D for a Pixel Detector System using a BCB/Cu/PbSn Technology

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1 Fabrication of a High-Density MCM-D for a Pixel Detector System using a BCB/Cu/PbSn Technology M. Töpper, P. Gerlach*, L. Dietrich, S. Fehlberg, C. Karduck, C. Meinherz, J. Wolf, O. Ehrmann, K.-H. Becks*, and H. Reichl Technical University of Berlin and Fraunhofer Institute for Reliability and Microintegration Berlin, Germany *Physics Department, University of Wuppertal, Germany Gustav Meyer Allee 25 D Berlin Phone: Fax: toepper@izm.fhg.de Abstract The multichip module described in this paper is a prototype for a pixel detector system for the planned Large Hadron Collider LHC at CERN, Geneva. The research project is within the ATLAS experiment. It addresses the study of proton-proton interactions. A module consists of a sensor tile with an active area of 16.4mm x 60.4mm, 16 read-out chips, each serving 24 x 160 pixel unit cells, a module controller chip, an optical transceiver, a local signal interconnection, and power distribution busses. Two module concepts were designed for this detector; MCM-D and MCM-L. In this paper, the MCM-D type will be discussed. The extremely high wiring density, necessary to interconnect the read-out chips, was achieved using a thin film Copper/Photo-BCB process above the pixel array. The bumping of the read out chips was achieved using electroplating PbSn. All dice are then attached by Flip Chip assembly to the sensor diodes and the local busses. The focus of this paper is the detailed description of the technologies (thin film BCB/Cu and PbSn bumping) for the fabrication of this advanced MCM-D using high-density Flip Chip assembly. Key words: MCM-D, BCB, Flip Chip, and Pixel Detector. 1. Introduction Large array pixel detectors are planned for the ATLAS project at the European Lab for particle physics in Geneva 1-3. The AT- LAS experiment is being constructed by 1700 collaborators in 144 institutes around the world. It will study proton-proton interactions at the Large Hadron Collider LHC at the European Laboratory for Particle Physics CERN. The main purpose of this detector are as follows: Ø The search for the Higgs Boson, the last undiscovered particle in the Standard Model of elementary particles and their interactions. The existence of a Higgs Boson would provide insight in the origin of particle masses. Ø The search for extension to the Standard Model at mass scales up to 2 TeV, especially the search for Supersymmetry. Ø Study the decays of the top quark, which was discovered 1994, with high statistics. Ø Study CP-violation in B-meson decays. For the pixel detector, a modular system is needed which can be placed together to build this large detector system. While the diode-pixel-arrays have an active area of about 10cm 2, the readout chips are one order of magnitude smaller due to their higher complexity. In general, the diode-pixel-arrays (currently about 8 cm in length) and the read-out chips (currently about 1 cm²) can be fabricated in wafer size dimensions. 305

Intl. Journal of Microcircuits and Electronic Packaging The researchers have to deal with medium frequencies in the LHC environment.

In the higher frequency domain, the interconnections must be controlled-impedance transmission lines and should be terminated.

A structure which is favorable for this application is a microstrip line configuration where the x- and the y-signal lines are embedded into a dielectric material which reference metal planes on top

2 Intl. Journal of Microcircuits and Electronic Packaging The researchers have to deal with medium frequencies in the LHC environment. But, even for clock rates of 40 MHz, one has to take care that line capacitance and line resistance do not cause RC-line charging which may slow down signal speeds quite considerably. In the higher frequency domain, the interconnections must be controlled-impedance transmission lines and should be terminated. The line cross sections have to be large to decrease DC and AC resistive losses. The power and ground planes positioning in the layer stack becomes important. A structure which is favorable for this application is a microstrip line configuration where the x- and the y-signal lines are embedded into a dielectric material which reference metal planes on top and/or bottom of the structure. In such a MCM-D structure, it is possible to build a bus system with low crosstalk between two parallel lines with fixed line width, thickness, and line spacing Electronic Packaging Two module concepts were realized for prototyping; a flexhybrid MCM-L (Figure 1) and a MCM-D (Figure 2). The sensor substrate is smaller than the front-end (FE) read-out chips. These chips are wirebonded to a flex hybrid on which the routing is achieved. The Module Controller Chip (MCC) with 25µm PbSn bumps is Flip Chip bonded on the flex. On the detector substrate, the UBM TiW/Cu is deposited. The highest packaging efficiency for this advanced electronic system is to use the concept of MCM-Ds in combination with Flip Chip assembly (Figure 2). Figure 2. MCM-D concept. The silicon diode array is used as the substrate, that is, the basic building block for the detector system. The read-out chips are then Flip Chip bonded onto this module by wiring the data lines, control lines, and power distributions to the periphery of the module. The module is working at frequencies in the range of 40 MHz. A microstrip line configuration will take care that line capacitances and line resistances do not cause RC-line charging which may slow down signal speeds quite considerably. A combination of a low k dielectric (BCB) with electroplated copper is used to build-up this MCM-D multilayer part. The interconnection between read-out chips and sensor substrate is achieved using Flip Chip technique using electroplated solder bumps. The advantage of this approach is to achieve the highest integration density with best frequency behavior using a reliable process. 3. Design of the MCM-D Part A module consists of a sensor tile with an active area of 16.4mm x 60.4mm, 16 read-out ICs, each serving 24 x 160 pixel units, a module controller chip, an optical transceiver, a local signal interconnection, and power distribution busses over the non-active area of the detector substrate (Figure 3). Figure 1. Flex-hybrid concept. active detector area (16.4mm x 60.4 mm) with feed through elements (not shown) Figure 3. Design of the MCM-D concept. The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number4, Fourth Quarter 1999 (ISSN ) 306

Two pixel detector substrates were on each 4 wafer. A four layer thin film metallization is necessary for routing the power, the ground, and the signal layers.

3 Two pixel detector substrates were on each 4 wafer. A four layer thin film metallization is necessary for routing the power, the ground, and the signal layers. The signal lines were designed in microstrip line configuration. The signal layers are bus lines for the interconnection of the read-out chips. The link between the silicon pixel cells and the Flip Chip contact pads were realized using vias of staggered and staircase type. This was only achievable with a high-dense multilayer metallization. A maximum of 25 µm for the diameter of the vias was allowed by the design rules (Figure 4). commercially available thin film polymers having a low dielectric constant and a high thermal stability. Using photosensitive polymers requires even fewer processing steps for multilayer wiring than dry-etching non-photosensitive materials 5. The advantage of Photo-BCB (Cyclotene TM, Trademark of The Dow Chemical Company) compared to Photo-PI is its low dielectrical constant and dielectric losses, minimal moisture uptake during and after processing, and very good planarization. The first established Photo-BCB process 6,7 was based on the development system DS 2100 TM. A comprehensive overview is given in Reference 8. A batch development system which was used for this module was introduced for large area substrates, thick layers and high density, and small vias 9,10. BCB has a moderate curing temperature compared to PI which is important for the pixel detector substrate. A low electrical resistivity of the wiring system is achieved by electroplating copper. A sputtered layer of Ti:W/Cu (100/200 nm) serves as a plating base. A positive working photoresist is applied to create the plating mask. After metal deposition, the plating base is removed by a combination of wet and dry etching. In Figure 5, the thin film Copper/BCB process is summarized. Figure 4. Design of the feedthrough elements. On the other hand, a minimum of 3µm thick isolation between the metal layers was demanded. A photo polymer process was recommended due to the number of process steps affecting the cost. Photo-BCB was selected due to the matching of physical parameters and the process technology of the polymer. 3 µm Copper was electroplated for the metallization. For copper lines of width w = 20µm, thickness t = 2.2µm, with a line spacing s = 30µm, a BCB dielectric thickness h = 8µm, one can compute for a microstrip line configuration a typical line capacitance of 1.2pF/ cm with a time of flight of about 55 ps/cm. The characteristic impedance is around 50W and the voltage coupling to the neighbored line is estimated to be -20dB. For a 7cm long line, the signal attenuation is about 30%. For the ATLAS experiment 2228 robust, reliable and functional modules have to be built up. sputter diffusion barrier / plating base (Ti:W/Cu) photoresist processing electroplating (Cu) strip photoresist etch plating base spin on dielectric layer / prebake expose / develop cure descum Figure 5. Thin film Cu/BCB process. 4. Technology 4.1. Thin Film Multilayer The thin film process was achieved using the 4 /6 Fraunhofer IZM / TU-Berlin. Polymers are preferred for the dielectrical layers in the MCM-D technology due to their low dielectrical constant and the minimum loss tangent a allowing thinner insulating layers and a higher pitch for the electrical wiring. Polyimides (PI) and Benzocyclobutene (BCB) are the only 4.2. Bumping Different bumping technologies are Fraunhofer IZM / TU-Berlin. An optimum of yield versus bump height has to be achieved for a given pad pitch. Preformed solder balls are preferred for a pad pitch of 700µm and higher. Stencil printing can be used for a pitch between 200µm and 1mm. The electroplating technology can be used even for a pitch below 50µm. The pitch of the I/Os of the pixel detector is 50µm and redistribution to a larger pitch was not possible. Therefore, PbSn bumps were plated for the Flip Chip assembly of the read-out chips. 307

Intl. Journal of Microcircuits and Electronic Packaging Sixteen chips were assembled on the multilayer detector substrate. The detector wafers had aluminium alloy pads with an inorganic passivation.

A layer of 200nm Ti:W is then sputtered on the whole wafer as an adhesion layer and a diffusion barrier, followed by a second layer of 300nm copper which is used as the plating base.

to the Copper. The sputtering conditions are optimized to low stress deposition of the metals. avoiding under etching.

4 Intl. Journal of Microcircuits and Electronic Packaging Sixteen chips were assembled on the multilayer detector substrate. The detector wafers had aluminium alloy pads with an inorganic passivation. The bumping process flow of Fraunhofer IZM / TU-Berlin is shown in Figure 6. The first step is the backsputtering of the Al using Ar to remove the aluminium oxide. A layer of 200nm Ti:W is then sputtered on the whole wafer as an adhesion layer and a diffusion barrier, followed by a second layer of 300nm copper which is used as the plating base. The titanium-tungsten has to protect the pad metallization against the formation of intermetallics of Al and has a high thermal stability, low electrical contact resistance, and a very high adhesion to the Copper. The sputtering conditions are optimized to low stress deposition of the metals. avoiding under etching. Under etching for Ti:W is below 1µm on 6,, wafers using the process Fraunhofer IZM and TU-Belin. The ohmic resistance of the electroplated PbSn bumps is 2mW for 100µm x 100µm bumps. Figures 7 and 8 show the 25µm bumps with a pitch of 50µm before and after reflow in a fluxless process using an organic compound, respectively. Figure 7. Eutectic PbSn bumps on 5µm Cu pedestal with a pitch of 50µm after etching the plating base (SEM). Figure 8. Plated PbSn of Figure 3 after reflow (SEM). Figure 6. Process flow of Fraunhofer IZM/TU-Berlin for bumping by the electroplating PbSn. 5. Results The Figures 9 and 10 illustrate the design and parts of the High viscous photoresist is used to produce resist layers with multilayer Copper/BCB wiring system. As BCB is transparent, a thickness of 5µm up to nearly 50µm by one spin coating and one can see the feed-through elements. patterning process 10. For this high end application of the detector system, an optimization of the photoresist process was neces- Test structures for the evaluation of the electrical resistivity of the vias were measured. Vias chains with vias down to 10µm sary to achieve a yield of nearly 100%. Prior to the plating base diameter (mask dimensions) are possible (Figure 11). The electrical resistance of each via designed for this pixel detector mod- etching, the photoresist has to be removed without any residues. Essential for a wet etching process is that the etchant should ule is around 5mW per via. This attribute gives mw per erode the thin film plating base uniformly and completely, but The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number4, Fourth Quarter 1999 (ISSN ) 308

feedthrough element (10 mw/square for 2µm thick copper and around 5 mw/via). layer (component layer) are opened to allow the solder joining of the read-out chips.

In Figure 12, the 2µm thick and 20µm wide feedthrough elements are illustrated. For better visualization in the Scan Electron Microscopy (SEM), the BCB was etched by Reactive Ion Etching (RIE).

Cu feedthrough elements from the substrate to the top Flip Chip interconnection layer (BCB was etched by RIE for better visualization), (SEM). Figure 10.

10 The connections between pairs of the feedthrough elements are shown on the substrates which are connected by the Flip Chip assembled devices as shown in Figure 13.

5 feedthrough element (10 mw/square for 2µm thick copper and around 5 mw/via). layer (component layer) are opened to allow the solder joining of the read-out chips. Full scale modules with 16 read-out chips bump bonded to the substrate have been built. From more than 1.1. million monitored vias, a defect rate of less than 10-5 has been monitored. In Figure 12, the 2µm thick and 20µm wide feedthrough elements are illustrated. For better visualization in the Scan Electron Microscopy (SEM), the BCB was etched by Reactive Ion Etching (RIE). Figure 9. Top view of one of the four metal feedthrough configuration (video print). Figure 12. Cu feedthrough elements from the substrate to the top Flip Chip interconnection layer (BCB was etched by RIE for better visualization), (SEM). Figure 10. Detail of one of the four metal feedthrough configuration (video print). 10 The connections between pairs of the feedthrough elements are shown on the substrates which are connected by the Flip Chip assembled devices as shown in Figure 13. Figure 14 shows part of the detector substrate with the four layer Cu/BCB metallization. Thin high-density signal lines are close to the wide power planes, presenting a challenge for the Copper/BCB process. 8 Resistance [m Ω] Via diameter (mask) [µm] Figure 11. Via resistance versus via diameter (mask dimensions). Figure 13. Cross section through the solder balls and the four layer metallization (Cu/BCB). The results show that it is possible to build high density MCM- Ds with more than 6000 I/Os per cm 2 through a multilayer BCB/ Cu Si-substrate. Four via layers are needed for the feedthrough connections from the sensor pad to a pad in the uppermost Cu layer (to be used for the bump connection to the read-out chips). As there are more than 61,000 interconnection structures with nearly 250,000 vias in a single module, a test program has been set up to determine experimentally the via yield of the thin film multilayer and to study the procedure of the Flip Chip assembly onto the MCM layers. In this case, four 2 µm thick Cu layers separated by 5 µm thick Photo-BCB layers have been deposited onto dummy sensor substrates. The vias in the uppermost BCB Figure 14. Photograph of the detector substrate with the four layer Cu/BCB metallization. 309

6 Intl. Journal of Microcircuits and Electronic Packaging Figure 15 shows the demonstration module with the sensor chips fabricated by CiS Company (Germany) and Seiko (Japan). The packaging density allows additional passive components to be assembled which will be evaluated in the near future Only nine vias of 1.2 millions were not open. This MCM- D type module in combination with Flip Chip technique is considered therefore a promising candidate as a building block for the large area pixel detector since it combines high reliability with highest packaging density both most important for this highend application. Acknowledgment Figure 15. Photograph of the detector module with 16 Flip Chip bonded read-out chips. As the detector elements are located very near to the circulation beams, the MCM-D has to be radiation tolerant enough to survive the expected life time. The degradation of the BCB due to the irradiation has been examined analyzing the effective relative permittivity. Only changes in the range of 1.5 to 2.5% were measured. 6. Reliability Issue Thermal cycling ( -65 C / +155 C, 10 min dwell, 20 min ramp) of a two layer Cu-Photo-BCB-Cu test structure (via-chains) was achieved in the work cycles were passed without any degradation in the electrical resistivity of the vias. The same test structures also passed 2000 hours at 85 % humidity and 85 C. During thermal 150 C, the contact resistance of the PbSn bumps hardly changed, as only a slight increase of 0.2 mw was observed after 1500 hours 12. The reliability of the bump metallurgies under operation conditions was checked by MIL-STD-883 (AATC -55 C to +125 C). At a shear height of 8µm, which is well above the copper-tin intermetallics, solder ball shear was observed without any delamination. Over 4000 cycles were passed without any significant change in the shear force. 7. Conclusion 1. K.-H. Becks et al. A Multichip Module, the Basic Building Block for Large Area Pixel Detectors, Proceedings of The Multichip Module Conference, MCM 96, pg. 16, J. Wolf, P. Gerlach, E. Beyne, M. Töpper, L. Dietrich, K. H. Becks, N. Wermes, O. Ehrmann, and H. Reichl, High Density Pixel Detector Module using Flip Chip and Thin Film Technology, Proceedings of the International Symposium of Packaging System, ISPS 99, K. H. Becks, E. Beyne, O. Ehrmann, P. Gerlach, I. Gregor, P. Pieters, M. Töpper, C. Truzzi, and J. Wolf A MCM-D-type Module for the ATLAS Pixel Detector, Proceedings IEEE NSS, Toronto, Canada, A. Elshabini and F. D. Barlow, Thin Film Technology Handbook, McGraw-Hill, P. E. Garrou and I. Turlik, Multichip Module Technology Handbook, McGraw-Hill, 1998, USA. 6. G. Chmiel, M. Töpper, Ch. Jöhren, and A. Achen, Thin film MCMs, Process Development Using Photosensitive BCB, Proceedings of Microsystem Technologies `94, Berlin, October M. Töpper, J. Wolf, V. Glaw, K. Buschick, A. Dabek, L. Dietrich, O. Ehrmann, and H. Reichl, MCM-D With Embedded Active and Passive Components, Proceedings of The 1996 International Microelectronics Symposium, ISHM 96, Minneapolis, Minnesota, pp , A. J. G. Strandjord, Y. Ida, P. E. Garrou, W. B. Rogers, S. L. Cummings, S. R. Kisting, MCM-D Fabrication with Photosensitive BCB: Processing, Solder Bumping, System Assembly and Testing Proceedings of The International Symposium of Microelectronics, ISHM 95, Los Angeles, California, pp , M. Töpper, K. Scherpinski, R. Hahn, O. Ehrmann, Ch. Schmaus, F. Bechthold, and H. Reichl, Combination of MCM-C Technology with MCM-D Technology using Photo- It was demonstrated in this paper the MCM-D approach is an excellent combination of high integration density, very good frequency behavior, and high reliability for the pixel detector system illustration. Two copper layers separated by the low k BCB allow for fast signal transmission and low crosstalk between adjacent signal lines on the interconnection bus lines. The microstrip lines result in controlled impedance transmission signal lines. It was demonstrated by TUB / Fraunhofer IZM that the defect rate of such a high density four layer metallization was only 8.13 x sensitive Polymers, Proceedings of the 4th International The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number4, Fourth Quarter 1999 (ISSN ) 310 The authors wish to thank Ch. Kallmayer, C. Meinherz, C. Karduck and E. Busse for their technical support. References

7 Symposium on Advanced Packaging Materials, Braselton (USA), pp , March 15-18, M. Töpper, M. Schaldach, S. Fehlberg, C. Karduck, C. Meinherz, K. Heinricht, V. Bader, L. Hoster, P. Coskina, A. Kloeser, O. Ehrmann, and H. Reichl, Chip Size Package - The Option of Choice for Miniaturized Medical Devices, Proceedings of The International Microelectronics and Packaging Symposium, IMAPS 98, San Diego, California, pp , D. Tönnies, M. Töpper, J. Wolf, G. Engelmann, and H. Reichl, Mask Aligners in Advanced Packaging, Solid State Technology, pp. S13-S18, March L. Dietrich, J. Wolf, O. Ehrmann, H. Reichl, Wafer Bumping technique using Electrolplating for High-Dense Chip Packaging, Third International Symposium on Electronic Technology, Beijing, China, pp , August About the authors Michael Töpper (Dipl.-Chem) is Head of the Group Wafer Level Chip Size Package and Thin Film Technologies at the Fraunhofer IZM / TU-Berlin. Peter Gerlach (Dipl.-Ing.) is Senior Research Scientist at the University of Wuppertal. Lothar Dietrich (Dipl. Ing. FH) was Senior Research Scientist for electroplating at Fraunhofer IZM. Since 1999, he is with a German PCB-company. Simone Fehlberg (Dipl.-Ing. Chem. FH) is Senior Research Scientist for photolithography at Fraunhofer IZM / TU-Berlin. Claudia Karduck (CTA) is responsible for electroplating copper at Fraunhofer IZM / TU-Berlin. Claudia Meinherz (TAM) is responsible for sputtering copper at Fraunhofer IZM / TU-Berlin. Jürgen Wolf (Dipl.-Ing.) is Head of the Group RF-Modules and Microsystems at Fraunhofer IZM / TU-Berlin. Oswin Ehrmann (Dipl.-Phys.) is Head of the Wafer-Level- Packaging and High-Density Interconnection Department of Fraunhofer IZM / TU-Berlin. Karl-Heinz Becks (Prof. Dr.) is Professor at the University of Wuppertal. Herbert Reichl (Prof. Dr.-Ing. Dr. e.h.) is Director of the Fraunhofer IZM / TU-Berlin-Microperipherics. 311

Fabrication of a High-Density MCM-D for a Pixel Detector System using a BCB/Cu Technology

Fabrication of a High-Density MCM-D for a Pixel Detector System using a BCB/Cu Technology M. Töpper, L. Dietrich, G. Engelmann, S. Fehlberg, P. Gerlach*, J. Wolf, O. Ehrmann, K.-H. Becks*, H. Reichl Technical