3-D, Self-aligned, Micro-assembled, Electrical Interconnects for Heterogeneous Integration

Transcription

1 3-D, Self-aligned, Micro-assembled, Electrical Interconnects for Heterogeneous Integration Trent Huang, Erik Nilsen, Matt Ellis, Kabseog Kim, Ken Tsui, George Skidmore Zyvex Corporation, Richardson, TX Chuck Goldsmith MEMtronics Corporation, Plano, TX Arunkumar Nallani, J. B. Lee University of Texas at Dallas, Department of Electrical Engineering, Dallas, TX ABSTRACT It is of great interest to develop an efficient and reliable manufacturing approach that allows for the integration of microdevices each of which is optimally fabricated using a different process. We present a new method to achieve electrical and mechanical interconnects for use in heterogeneous integration. This method combines metal reflow and a self-aligned, 3-D microassembly approach. The results obtained so far include a self-aligned, 3-D assembly of MEMS to MEMS, post-processing which selectively deposited indium on 50 µm-thick MEMS structures, and reflow tests of indium-on-gold samples demonstrating mω resistances for contact areas ranging from 100 to 625 µm 2. 3-D microassembly coupled with metal reflow allows for the batch processing of a large number of heterogeneous devices into one system without sacrificing performance. In addition, its 3-D nature adds a new degree of freedom in system design space. Downward scalability of the method is also discussed. 1. Introduction Systems that significantly benefit from the integration of heterogeneous devices include those in the RF/microwave, optical, and bio-sensing arenas [1-9]. For example, RF transceiver architectures rely on off-chip passives to achieve the high quality factors demanded by many applications, optical systems and sub-systems are comprised of lenses, filters and active components, and some bio-sensors capable of detecting and transducing a wide variety of in vivo changes into electrical signals are best made by assembling sensors to microelectronic circuitry [7-9]. Existing methods for heterogeneous interconnect include monolithic process integration, flip-chip bonding, and wire bonding. Each of these approaches typically results in device or system performance compromises. Specifically, monolithic process integration is limited by the adaptability of the chosen fabrication processes and is often associated with performance losses due to the less than optimal processes employed to facilitate integration [22]; flip-chip bonding cannot be used to easily integrate more than two heterogeneous devices into one system [14]; wire bonding introduces parasitics that are undesirable in RF/microwave applications [1-3]. 3-D micro-assembly [6,15-21], accomplished typically by pick-and-place with an external robotic system, or self-assembly, provides solutions to some of these performance compromises. However, a robust, reliable, and inexpensive 3-D micro-assembly has yet to be demonstrated. One difficulty in establishing such an assembly method is achieving electrical interconnects. General requirements for these interconnects include low contact resistance (<0.1 Ω), immunity to vibration and temperature variations, and compatibility with the forces and alignment accuracies of 3-D assembly. In order to ensure effective electrical interconnects, a post-assembly contact reflow technique has been established. Our technique uses assembly to establish a temporary interconnect, which is later improved by reflowing a low-temperature metal throughout the contact area. The approach begins with the post-processing of micro-fabricated parts by selectively depositing metal at contact areas for the assembly; adhesives, such as epoxy, can be utilized to stabilize the connection, but this approach is not ideal because it does not offer low contact resistance and it is not compatible with wafer-level processing [6]. Subsequently, we use a microelectromechanical (MEM) gripper that is

2 attached to a five degree-of-freedom robotic system to assemble the parts together. Compliant mechanisms on the parts are used to obtain micron-scale alignment accuracy (Zyvex patents pending) in addition to providing the necessary contact pressure without the use of an external force. After the assembly is completed, we utilize metal reflow to achieve both mechanically stable and low-resistance electrical interconnects. Although this approach will work for parts made using a wide range of processes, this study focused on the design and fabrication of MEMS parts that are able to mate with other MEMS parts. 2. METHOD 2.1. Parts to be Assembled Two types of structures, namely connectors and receptacles, were designed, fabricated, and prepared for assembly. The connectors are picked up by MEM grippers and the receptacles are the parts that the connectors are assembled onto. The MEMS processing was conducted by Honeywell D&SS Redmond and the indium post-processing of the MEMS was performed by the University of Texas at Dallas. An overview of the process sequences for both the connector and receptacle are given in Figure 1. Figure 1. Processing sequences for making an assembled interconnect

3 Both processes start with SOI wafers (handle wafer thickness = 400µm, device layer thickness = 50µm, p-type, single crystal silicon, sheet resistance = 0.01Ωcm), onto which a 0.5µm thick layer of gold is deposited and patterned with wet-etch atop 150Å of chromium (adhesion layer). Afterwards, a layer of resist is spun on, patterned, and used as a mask to etch through the device layer. Following removal of the photoresist, the processing sequence diverges for the connectors versus the receptacles. For the connectors, a 40-minute etch in 10:1 buffered hydrofluoric acid (BHF) is used to undercut the oxide. A shadow mask, an etched-through silicon wafer, is then aligned and subsequently attached through wafer bonding. 0.5µm thick gold is sputtered to conformally coat the sidewall of the contact areas on the connectors. The gold on the sidewall serves as one of the mating surfaces for the assembly. After removing the shadow mask, the wafer is etched in HF (49%) to release the connectors. Following the HF release, the connectors are still connected to the substrate through thin silicon beams that serve as breakable tethers. Conversely, the receptacles goes through a lift-off process (at the University of Texas at Dallas) employing 25 um thick SJR 5740 photoresist with 1.0 um of indium being deposited. To ensure proper resist lift off, it is important for the resist to exhibit a re-entrant sidewall profile after UV exposure. One way to guarantee this type of profile is through treatment of the resist surface with chlorobenzene prior to UV exposure (Figure 2) [10]. Indium was chosen as the reflowable metal for its relatively low melting point, good wetting properties on gold, and high resistance to hydrofluoric acid (HF) (Figure 3) [14]. Because indium readily wets gold and our parts are able to exert the necessary contact pressure to ensure proper reflow, it is only necessary to deposit indium onto one of the mating surfaces. The areas with indium on top serve as the other mating surface for the assembled electrical interconnect. After removing the photoresist, the MEMS structures were released in HF (49%). Figure 2. Re-entrant resist sidewall profile created by post-exposure surface modification with chlorobenzene. Desired Property Low melting point Controllable reflow Adhesion to gold Resistance to HF Indium 156 deg. C Yes Excellent Excellent Figure 3. Indium was identified as a candidate low melting point, interconnect material

4 2.2. Metal Reflow for Interconnects In fabricate addition to the MEMS connectors and receptacles, simple electrical interconnect test samples were also d using a resist lift-off process similar to that used for the MEMS receptacles. As shown in Figure 4, the indium patterns were designed with 100µm 2, 250µm 2 and 625µm 2 ohmic contact areas. The indium-on-gold samples were made using standard 4, p-type, 525µm thick silicon wafers. After dicing, 10 pairs of samples were gathered and assembled manually utilizing a microscope for aid in alignment; it was estimated that the actual contact area ratio was, on average, 14% larger than the designed. The assembled structures were placed in a nitrogen filled gas chamber at 200 o C for 1.5 minutes. After cooling the reflowed structures, two-point resistance measurements were performed. The number of unique r -point measurements that can be obtained from a set of n unique measurement pads is equal to n!, (1) ( n r)! r! w hich, for our case of r = 2 and n = 4, equaled 6; for this simple case, the combinatorial relationship is trivial, but it is useful in cases where the n umber of pads is large. In addition to performing DC resistance measurements, several of the assemblies were cross-sectioned thereby allowing scanning electron microscope (SEM) analysis of the gold/indium interface. A (Bottom) 1 2 indium 25 um 10 um 25 um 10 um B (Top) 3 A (Top) B (Top) 4 gold a) b) Figure 4. a) Interconnect test sample pair each of which contains 10 um, 25 um wide lines of 1.0 um thick indium on a 0.5 um thick gold base layer b) shows the arrangement of the final assembly D, self-aligned, microassembly Our microassembly platform is a modified version of a standard pick-and-place tool that relies upon multiple DOF, high-precision, robotic systems [15-17]. By attaching MEM end-effectors (e.g., grippers) to the robot arm, we are able to pick and place specialized MEMS parts. These parts have designed compliances that allow for the self-alignment of the assembled parts (Figure 5). Figure 6 shows the principle of the self-alignment mechanism in one dimension of a connector/receptacle assembly.

5 Figure 5. MEMS connectors with compliant mechanisms assembled onto receptacles. Figure 6. Self-alignment of a one DOF connector assembly

6 First, the MEM gripper that is attached to the external robotic system locates the connector and picks it up by compressing the compliant springs. Second, the connector is moved to the proximity of its destination (i.e., the center of the mating receptacle). Third, the compliant springs on the connector are released and subsequently make contact with the mating receptacle. Fourth, the gripper is removed. The self-alignment of the assembled system occurs as the contact forces between the springs and the receptacle align and secure the relative position of the connector with respect to the receptacle. The symmetry of the compliant springs ensures the centering of the part independent of the initial gripper placement. The aforementioned mechanism can be extended to include other dimensions, thereby achieving selfalignment with multiple DOF. An example of a 2 DOF self-alignment mechanism is shown in Figure 7. Figure 7: Connector-receptacle assembled with 2 DOF A major advantage of this approach is that the precision of the assembly is not limited to the external robotic system s accuracy. In addition, the relaxation of the required placement accuracy of the robot lends itself well to scaling to a parallel assembly operation. 3. RESULTS 3.1. Processing of MEMS Parts The MEMS structures from Honeywell were fabricated with a minimum feature size of 5µm. Sidewall angle was controlled to be under 1 o. A small amount (<~2µm depth) of footing lateral etch that occurs after the deep etch reaches the buried oxide layer was found (Figure 8). Figure 9 shows indium on MEM gripper pads and on the top surface of the connector receptacle. An annealing step at 170 o C for 10 minutes was performed to prepare the gold surface prior to indium deposition. This step solved the problem of indium peeling in HF (Figure 10) which we believe is due to hydrocarbon contamination on the Au surface. Figure 8. Underside of fabrication MEMS structures shows good verticality and small amount of footing Indium Figure 9. Indium deposited on MEMS structures using lift-off

7 Evaporated, lift-off indium/gold Before HF dip After HF dip 30 minutes Figure 10. Without annealing Au surface, indium was found peeling off after 30 minutes dip in HF (49%) 3.2. Metal Reflow Characterization For each assembly, 6 two-point resistance (R) measurements were obtained. Each measured value was scaled by the number of assembled contacts in the measurement path to provide the average resistance of a single contact; for example, the measured resistance between pads 1 and 3, R(1,3), was scaled by 2 because there were two contacts in its path. Because pair-wise comparisons of the 6 estimates revealed that the estimates were, in fact, statistically indistinguishable from one another, an average of the estimates was used as the value of contact resistance for the respective sample. As shown in Figure 11, the average contact resistances were mω for contact areas ranging from 100 to 625µm 2. mω Mean = 33.1 Standard Deviation = 8.7 Sample Number Figure 11: Contact resistance measured from interconnect test samples In addition to resistance measurements, the quality of the bond was further investigated through SEM analysis. A SEM of a cross-section of the indium/gold interface is shown in Figure 12. The holes' in the indium evidence the quality of indium/gold bond because they suggest that the weakest part of the assembled structure was the indium itself.

8 A (Bottom) Indium-gold interface 60 um B (Top) 25 um line (before reflow) ~60 um line (after reflow) Figure 12. Cross section of debonded sample. Holes in indium are due to debonding (during the cross-sectioning) step D Microassembly The assembly of the MEMS connector to the substrate contact pads uses a 5-DOF robotic system MEMbler, composed of high-precision Newport stages, as the development platform for a pick and place micro-assembly approach. MEMS end-effectors are packaged and mounted onto the MEMbler arm. The silicon parts to be assembled were first de-tethered so that they were fully released from the substrate. MEM grippers were used to pick up a MEMS connector which was then assembled perpendicular to the substrate over the contact pads. Two batches of connector/receptacles were fabricated. Structures in batch 1 do not have sidewall metals and were designed for MEMS to MEMS assembly test only; Structures in batch 2 have gold on the contacting sidewall surfaces of connectors and Indium post-processed on the contact pads of the receptacles. Batch 1 was successfully tested for assembly, as shown in Figure 13. Some design errors were found in batch 2 (Figure 14-15), which prevented the gripper from properly release the connector without compromising contact forces. As a result, metal reflow was not performed. At this stage, the 3-D assembled parts are held together only by friction. This connection is known to be less than optimal with regard to stability and reliability, both mechanically and electrically. Figure 13. Left: a MEMS to MEMS self-aligned micro-assembly; Right: compliant mechanisms for self-alignment.

9 Figure 14. Component (connector) picked up by MEM gripper and assembled onto receptacle Figure 15. Left: connector with Au over the etch; Right: connector inserted onto receptacle (gripper still attached) 4. DISCUSSION 4.1. Assembly Alignment Errors Alignment errors in the assembly are related to the following factors: 1) Dimension error caused by imperfection in fabrication; this can be significantly reduced by implementing symmetry in designs 2) Surface roughness induced inter-locking; currently this is on the order of 100 nm, mainly from the deep etch. Process improvements and surface coating will help minimizing that error. 3) Friction; Not well understood. Minimizing contact area and application of surface coating will reduce friction.

10 4) Material plasticity; Material dependant. Single crystal materials display less plasticity. This will become much more important as structure sizes scale down to 10 s of nm, when the effect of surfaces on the overall elasticity of the structure can no longer be ignored [11-13]. 5) Pick and place accuracy; external robotic systems need to provide a pick and place accuracy beyond a threshold value. However, through proper design, the final alignment accuracy can be significantly better than and nearly independent of the pick and place accuracy, once that threshold is reached. The post-assembly state of the system, or, the relative locations of the connector and the receptacle, is determined by the equilibrium between the restoring force and friction. As illustrated in Figure 6, the linear alignment error is nearly independent of the external robotic system. To assess the range of the angular error, we use the following simplified design in Figure 16. Figure 16: Post-assembly, angular alignment error (h: height of receptacle; d: size of opening on receptacle; F c : contact force; f: friction; φ: angular error) Here the directions of the friction forces are along those that maximize the alignment errors. The equilibrium equations can be written as fd cosφ = F h / cosφ (2) F c c = k( l d cosφ) (3), where l is the size of the compliant springs (which fits inside the receptacle) before deformation and k is the stiffness of the compliant springs. Assuming the worst case, when f=f c µ s, where µ s is the friction coefficient. The maximum angular error is determined by using the design dimensions and the friction coefficient. If µ h d, the friction is too small to cause an angular error, or, φ =0. s / If µ > h d, φ = cos 1 ( h / µ d ) (4) s / max s As an example, for d = 100 µm, h = 50 µm and an estimated µ s of ~ 0.6 [23], the maximum angular alignment error is: φ max =24.10 o. In other words, if the external robotic system places the connector with an angular error φ > φ max, the connector will self-align to the receptacle and the angular error will reduce to φ max =24.10 o. If φ < φ max, the selfalignment mechanism will not help reduce the error and the final assembly will have an alignment error φ. Implementation of this self-alignment mechanism with 2 DOF will reduce φ max Stress during metal reflow Stress increases during the cooling of the reflowed metal [14]. However, the effect of this stress on the alignment error can be minimized through careful design. The 3-D nature of this assembly technology enables the flexibility in design which allows for a larger tolerance of experimental conditions that are difficult to predict and control. Using a connector design with a 2 DOF self-alignment, one can be confident that the final mechanical configuration is nearly independent of the stress generated by the reflowed indium, as shown in Figure 17; this is because the designed compliances are able to counteract any forces exerted upon the assembly due to the reflow step.

11 Figure 17: Schematic of assembled electrical interconnect after indium reflow The in-situ contact pressure caused by the restoring force in the compliant springs helps maintain the stability of the assembly during the reflow. The range of this force is controllable by the mechanical design. For a part 100µm to 1mm in size, this pressure can be accurately controlled to be from 0 to 10 s of mn. We have designed devices that are able to generate a selectable contact force around 10 mn. The contact force between the assembled parts, when scaled up to a 1cm x 1cm chip area, is equivalent in pressure to 10 2 N, which is greater than the typical force used for solderreflow processes where the solder thickness is on the order of µm. The small (1 µm) thickness of the reflowable metal layer helps further securing the planarity and, hence, the positional accuracy in the direction normal to the contact areas Scalability As the minimum feature size of the microfabrication processes decreases, it is important to assess how our assembly and interconnect technologies scale with size. By assuming a constancy of material strength, or Young s modulus, surface forces dominated by friction and a constant friction coefficient µ, the fundamental characteristic properties of the assembly can be evaluated. Assuming that scaling occurs uniformly in all three dimensions, the stiffness of a flexure, k, scales linearly k moment _ of _ inertia L 3 L (5) The contact force due to the deformation in the compliant springs of the connectors scales accordingly F c 2 kl L (6) The friction between the contact areas, f, has the same dependence on the scale f 2 µ Fc L (7) Angular alignment error, φ, as shown in Eq. 4, is independent of the scale. Linear alignment errors, which are approximately a product of the angular errors and the length of the part, are proportional to the length. As the length scale approaches ~nm, surface/volume ratio increases to a point where the aforementioned assumptions no longer hold. Surface forces, such as friction, can no longer be considered area independent. In addition, the elastic modulus of the bulk material no longer describes the mechanical properties of the system accurately [11-13]. 5. CONCLUSION We have proposed a new method to provide electrical interconnects between micro-components that is based upon 3-D microassembly and metal reflow. Using parts with designed compliances, the final assembly precision can exceed the precision of the external robotic system used for the pick-and-place assembly. The low contact resistance, the

12 minimal area and volume required for the reflowed material, and the stable, accurate mechanical configuration make this method compatible with a wide range of applications that significantly benefit from heterogeneous assembly. Experimentally, we have successfully demonstrated 3-D, self-aligned assembly. Metal reflow characterization using a test structure showed a contact resistance of ~ mω. The next step of our work is to demonstrate this method on 3- D assembled structures. We will also conduct a more detailed characterization of this method and research other reflowable materials. ACKNOWLEDGEMENTS We would like to thank John Randall and Jim Von Ehr for insightful discussions. We also thank Honeywell D&SS Redmond for providing the fabrication. This work was performed under the support of the U.S. Department of Commerce, National Institute of Standards and Technology, Advanced Technology Program, Cooperative Agreement Number 70NANB1H3021. REFERENCES: [1] H. Patterson, Analysis of Ground Bond Wire Arrays for RFICs, IEEE, Vol. 21, No. 3, September [2] G. Straub, W. Menzel, Millimeter-Wave Monolithic Integrated Circuit Interconnects using Electromagnetic Field Coupling, IEEE Trans. CPMT, Part B, Vol. 19, No. 2, pp [3] C. J. Stanghan, Electrical Characterization of Packages for High-Speed Integrated Circuits, IEEE Trans. on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-8, No. 4, Dec. 1985, pp. [4] S. Kaneko, Novel Fiber Alignment Method Using a Partially Metal-Coated Fiber in a Silicon V-Groove, IEEE Photonics Technology Letters, Vol. 12, No. 6, Jun. 2000, pp [5] S. Jang, Automation Manufacturing Systems Technology for Opto-electronic Device Packaging, Electronic Components and Technology Conference, 2000, pp [6] J. D. Berger, Y. Zhang, J. D. Grade, H. Lee, S. Hrinya and H. Jerman, Optical Fiber Conference (OFC 2001), paper TuJ2. [7] R. Eberhardt, T. Scheller, Automated Assembly of Microoptical Components, SPIE Proc. Microrobotics and Microsystem Fabrication, Vol. 3202, pp , [8] J.A. Ferguson,, T.C. Boles, C.P. Adams, D.R. Walt, "A Fiber-Optic DNA Biosensor Microarray for the Analysis of Gene Expression, Nature Biotechnology, 1996, 14, [9] Johnson, S.R., Sutter, J.M., Engelhardt, H.L., Jurs, P.C., White, J, Kauer, J.S., Dickinson, T.A., Walt, D.R, "Identification of Multiple Analytes Using an Optical Sensor Array and Pattern Recognition Neural Networks"., Anal. Chem., 1997, 69, [10] W. Moreau, Semiconductor Lithography-Principles, Practices, and Materials, Plenum Press, 1988, New York. [11] H.G. Craighead, Naoelectromechnaical systems, Science, vol. 290, 24 Nov 2000, pp [12] S.R. Quake and A. Scherer, From Micro- to Nanofabrication with soft materials, Science, vol. 290, 24 Nov 2000, pp

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