Chapter 3 SU-8-Based Low-temperature Fabrication Process
3.4 An SU-8 Flexible Ribbon Cable 55
Recent research progress on the development of biomedical microsystems mainly focused on the design and fabrication of micromachined biomedical devices, such as silicon neural probes, ultrasonic transducers and so on [81,82].
However, it is also very critical to have flexible and reliable electrical interconnection from these micromachined devices to external instruments for better system performance, especially when the microsystems are used for in vivo monitoring. There are two kinds of interconnection technologies, i.e.
wireless and wire, for such applications. Wireless interconnection can also be called “active connection” requiring power to transmit and receive signal.
Although the wireless interconnecting system can offer many benefits in untethering the subjects from bulky external hardware [83], it is inevitable to have a complicate hardware system in the medical devices for obeying the federal wireless regulation [84]. In contrast, the wire interconnect scheme can provide a simple but secure and robust solution for the microsystem especially for in-situ operation applications. The only technical challenge in the
development of the wire interconnections is the design and fabrication of a flexible and scalable wire.
Several polymer cables and assembly methods have been developed for flexible interconnecting applications [85-89]. Nevertheless, complex process steps based on the simultaneous usage of polyimide and parylene and impracticality for the integration of various chips have led to a new exploratory regarding the flexible interconnecting scheme for biomedical microsystem applications. Thus, a flexible interconnecting scheme as shown in Figure 3.16 is presented by adopting a SU-8 flexible ribbon cable to integrate biomedical devices with a system-on-package (SOP) circuit system. SU-8, a negative photoresist (PR), has been widely used in BioMEMS fabrication owing to its superior material and process characteristics, such as photo-patternable, chemical stable, biocompatible and suitable for fabricating a feature with a high aspect ratio in terms of thickness control…etc., which make SU-8 suitable for the fabrication of flexible cable. Furthermore, the traditional flexible cables usually provide a short distance interconnection (only several cm) and have little stress relief capability easily resulting in cable breakage and failure during biomedical device handling. Previously, Huang et al. demonstrated a 3-D parylene coiled cable which is able to be stretched by 100% of its original length for prosthesis application [89]. In the coiled cable fabrication, since a 48 hours heat treatment at 200°C in vacuum is needed to form the coiled structure, it becomes impractical for mass production. Therefore, a new flexible cable structure is developed and implemented in the proposed integration scheme to provide a robust and reliable flexible interconnection in the work.
Via a wafer-level sacrificial release process for the SU-8 spiral ribbon fabrication and the previously developed low temperature bumpless Au-Au
thermocompressive bonding process [7,10], a low cost and reliable heterogeneous integration of biomedical microsystems, such as a Si neural probing system interfaced with a system-on-package (SOP) circuits, can be truly realized.
3.4.1 Flexible Interconnection Design and Fabrication
For the purpose of large actuation displacement, low driving voltage, small dimension and so on, structures, such as serpentine spring and spiral spring, have been widely used in MEMS sensors and actuators designs to replace straight line structure as a result of a low stiffness. The spring constant of a simple cantilever beam, kbeam, and the spiral spring, kspiral consisting of two semicircular beams can be calculated, respectively, as follows [90]:
3 of inertia, polar moment of inertia, length of cantilever beam, and average radius of the first and second semicircular beams, respectively. Therefore, for the same design of a 50μm in width and 50μm thick structure made of SU-8, the spring constants are 9.81×10-5 N/m and 5.76×10-5 N/m for a 4cm long cantilever spring and a spiral spring where the first and second beam radii are designed with 1.2 cm and 2 cm, respectively. Obviously, the spiral spring structure has a lower stiffness than the cantilever beam which makes itself suitable for being used in the structure design of a flexible ribbon cable.
A SU-8 spiral ribbon structure designed with 10 semicircular beams, 4 mm
in width, 82.6 cm in total length and 4 mm line spacing, respectively, is fabricated on a 4" silicon wafer for the demonstration of a long flexible electrical interconnect between a biomedical probe and an interface circuitry system which performs bio-signal processing. The three different metal line structures embedded in two layers of SU-8, whose line width/spacing/number of metal lines are designed with 200μm/100μm/12, 100μm/100μm/20, and 50μm/50μm/40, respectively, are then fabricated on the ribbon to demonstrate the feasibility for sensor array which needs a large number of interconnections.
The whole process sequence of the proposed flexible interconnecting scheme is listed as follows: a silicon handling substrate is first sputtered with 10nm/300nm thick Cr/Cu as a sacrificial layer followed by 26µm thick SU-8 (Gersteltec Sarl GM 1060) spin-coating. After photo-patterning the SU-8 with the aforementioned spiral structure, the SU-8 is hard-baked at 200°C for 2 hours in order to make a fully cross-linked SU-8, as shown in Figure 3.17(a), for having a higher glass transition temperature (Tg). Figure 3.17(b) shows that a 10nm/90nm Ti/Cu seeding layer is deposited on the SU-8 and followed by a photolithograph process using a 7μm thick AZ 4620 photo-resist to define the region for the metal lines. After electroplating a 5μm thick copper on that region, a serial of processes including 1μm electroless Ni and 0.4μm electroless Au plating are performed to metallize the Cu surface for later bonding as shown in Figure 3.17(c). Figure 3.17(d) shows that AZ 4620 and Ti/Cu seed layer are then removed using ACE, CR-7T, and then BOE, sequentially. Finally, the SU-8 ribbon cable is ready for bonding with a biomedical device after a secondary 26μm thick SU-8 spin-coating and patterning step to protect the Cu metal line. Once the biomedical device is assembled with the inner end of the ribbon cable using low temperature Au-Au TC bonding, the sacrificial Cr/Cu
layer is chemically etched away in Cu etchant (100:5:5 H2O:CH3COOH:H2O2) to release the SU-8 ribbon cable from the handling wafer as shown in Figure 3.17(e).
3.4.2 Results and Discussions
Figure 3.18 shows optical photographs of the as-fabricated SU-8 flexible ribbon with metal interconnection lines before sacrificial release. The enlarged optical photograph of the SU-8 flexible ribbons with 200μm, 100μm, and 50μm wide Au lines are shown in Figure 3.18(b), (c), and (d), respectively. Figure 3.19 shows the micrographs taken from one end of SU-8 flexible ribbon before and after sacrificial release in Cu etching solution. Due to the transparency characteristic of SU-8 under an optical microscope, the removal of Cu sacrificial layer underneath the SU-8 ribbon can be clearly observed to check whether the process is completed.
While handling a biomedical device with the ribbon, the low stiffness in the spiral structure design in comparison with that in a straight line cable can not only increase an out-of-plane displacement but also play the role of a spring to ease the stress built on the assembly joint between the device and cable. The stress release can effectively avoid possible bonding reliability issues during the device handling. Figure 3.20(a) and (b) show the comparison between the experimental and ANSYS simulation about a 22 cm long SU-8 flexible ribbon deformation under gravity load. About the same vertical displacement (3.5 cm) and similar deformation shape predictions indicate the correctness of the simulation. Figure 3.20(c) and (d) further show the simulated SU-8 ribbon deformation shape and stress distribution respectively for a 5 cm vertical
displacement at the outer end. The result indicates that the stress near the bonding area can be restrained for about 0.6 MPa.
The proposed heterogeneous integration scheme of biomedical microsystems is preliminarily demonstrated as shown in Figure 3.21(a). A micromachined Si probe with polysilicon resistance temperature detectors (RTDs) is bonded with a 2.4cm long SU-8 flexible ribbon using proposed low-temperature Au-Au TC bonding after the sacrificial release from the Si handling wafer. A push-pull test is performed to characterize shear bonding strength which is about 3.3 MPa and obviously larger than the aforementioned simulated stress, i.e. 0.6 MPa. Furthermore, partial Ni/Au bonding pad on the silicon probe is broken and pulled away after the push-pull test as shown in Figure 3.21(b). It indicates that the bonding strength of the proposed low temperature Au-Au TC bonding is as strong as the adhesion force of the Ti layer under the bond pad to silicon. Figure 3.22 shows the resistance measurements of the temperature probe before and after bonding with the SU-8/Cu flexible cable for the temperature coefficient of resistance (TCR) characterization [91].
The resistance versus temperature measurement validates the linear relationship and shows less than 5% TCR variation resulted by the integration of flexible interconnects. Thus, the SU-8 flexible ribbon cable reveals the potential of clinical applications like microsurgical tools for electrically connecting the in-situ microdevices to outside instruments.
3.5 Summary
In this chapter, two SU-8 based low-temperature process techniques, including a SU-8 micromachining process for RF MEMS passive fabrication
and a low temperature Au-Au TC bonding for chip integration on SU-8, are presented for RF SOP applications on flexible organic substrate. A low-temperature micromachining process for the fabrication of SU-8 RF MEMS series switches has been developed and demonstrated. Owing to the low processing temperature characteristics, which can be kept below 135°C, and the simple spin-coating process, the proposed micromachining process can provide an alternative for making other RF MEMS components such as tunable capacitor, inductor and antenna for flexible RF applications. Next, a flexible wireless microsystem fabrication scheme in regard to wafer-level sacrificial release process and low temperature Au-Au TC bonding has been successfully demonstrated with a good broadband electrical performance and reliable bonding strength within CMOS chips and polymer substrates. By adopting a surface cleaning step, bonding temperature of Au–Au bond can be lower than 200°C. Not only CMOS chips but also other heterogeneous chips, like MEMS and III-V chips, can be fully integrated with a flexible organic substrate using the proposed technology to form a high performance wireless microsystem.
Besides, a SU-8 flexible ribbon cable has been successfully fabricated using the proposed flexible microsystem fabrication scheme. In this work, low temperature Au-Au TC bonding is utilized for electrically and mechanically joining within various chips by the flexible cable. Such a SU-8 ribbon with the characteristics of simple process, low stress spiral structure, and high density metal interconnects has shown its potential application for the heterogeneous integration of biomedical microsystems.
Figure 3.1 The scheme of clamped-clamped SU-8 beam MEMS series switch.
Figure 3.2 The schematic process flow of proposed SU-8 serial MEMS switch.
CPW Driving Electrodes
Anchor Contact Area Clamped-Clamped SU-8 beam
10μm
320μm 1350μm 1000μm
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Si SiO2
Ti/Cu
Cu SU-8
JSR THB-120N AZ4620
Figure 3.3 Optical micrographs of the enlarged view on the area of driving electrode of the switch (a) before and (b) after sacrificial JSR layer removal.
Figure 3.4 The AFM 3D profile image and average roughness of electroplating Cu surface.
(a) (b)
Figure 3.5 High frequency measurement setup for the MEMS switch.
Figure 3.6 The measurement and simulation results of the SU-8 switch while the switch is operated at “on” and “off” states.
Figure 3.7 The percentage of contact resistance deviation of SU-8 MEMS switch at preliminary 2000 times cycling test.
Figure 3.8 Scheme of wafer-level chip scale flexible wireless microsystem fabrication.
Figure 3.9 Scheme of designed interconnection structure includes two coplanar waveguides (CPWs) on flexible substrate and microstrip on CMOS chip for characterization of the proposed integration technology.
CMOS Chip
Figure 3.10 Scheme of the integration process. (a) Deposition of a Cr/Cu sacrificial layer covered with a fully cured SU-8. (b) Ti/Cu seed layer deposition with PR patterning on the top. (c) Cu plating for the fabrication of CPW structure, followed by electroless Ni/Au plating for bonding. (d) PR and seed layer removal. (e) CMOS chip to SU-8 substrate bonding. (f) Si handle wafer detachment by sacrificial layer release in Cu etchant and then SU-8 film attachment on PDMS.
Figure 3.11 (a) Comparison of the RF measurements and HFSS simulations on SU-8/Si and on SU-8/PDMS, respectively, of the designed transition structure. (b) Comparison of the simulation results of the transition on SU-8/PDMS with CPW made by either Cu/Ni/Au or pure Au, respectively.
Figure 3.12 The optical photographs of (a) the detached SU-8 substrate, the enlarged inset photograph shows CMOS chip successfully bonded onto SU-8 substrate. (b) SU-8 attaches to thick PDMS substrate.
Figure 3.13 Enlarged SEM micrographs of two bonded substrates, CMOS and SU-8 respectively, after forcefully separating the compressive bond.
(a) The four alignment marks totally torn away from the SU-8 substrate, and (b) transferred onto the CMOS chip.
Figure 3.14 The optical photographs of the demonstration of wafer-level chip scale flexible microsystem fabrication. (a) A 4-inchs Si wafer is patterned with forty separated SU-8 which each size is 9.5×6mm2, (b) twelve chips are bonded to twelve flexible substrate, respectively, (c) the enlarged view on the area of twelve bonded flexible microsystem before sacrificial release, and (d) released flexible microsystem “die”.
Figure 3.15 The optical photographs of a SU-8 flexible substrate which is immersion into Cu etchant after (a) 1mins, (b) 10mins, (c) 20mins and (d) 25mins, respectively.
(a) (b)
(c) (d)
(a) (b)
(c) (d)
Table 3.1 Surface element content analysis by XPS before and after acidic surface cleaning
Surface Contents (at.%) Cleaning time
(seconds) C O Au
0 1.71 56.38 41.91
180 1.45 33.95 64.4
Table 3.2 Specific contact resistance vs. bonding temperature
Temperature(°C) 160 200 240
SCR(10-7 Ω·cm2) 5.65±1.86 4.74±1.69 2.84±1.03
Table 3.3 Comparisons between previously developed technologies and this work.
Ref. [69] Ref. [71] Ref. [72] Ref. [73] Ref. [74] This Work Method Direct
Fabrication Transfer
Printing Wafer Transfer Lateral
Interconnection Convectional
Flip Chip Bumpless Au-Au TC Bonding
Substrate Material PI PI/PDMS FR4 N/A PI or LCP SU-8/PDMS
Wafer-level Process Yes Yes Yes Yes No Yes
Process Complexity Simple Complex Complex Complex Simple Simple
Multi-Chip Module Incapable Incapable Incapable Capable Capable Capable Heterogeneous
Integration Incapable Incapable Incapable Capable Capable Capable High Frequency
Performance Poor Moderate Moderate Poor Moderate Good
Figure 3.16 Scheme of the heterogeneous integration of biomedical microsystems. A Si neural probe is interfaced with a system-on-package (SOP) circuit system via the SU-8 flexible ribbon cable using a low-temperature bumpless Au-Au thermocompressive bond.
Figure 3.17 The fabrication processes of the proposed SU-8 flexible ribbon cable. (a) Deposition of (10nm) Cr/ (300nm) Cu sacrificial layer and 26μm fully cured SU-8. (b) Deposition of (10nm) Ti/ (90nm) Cu seeding layer and spin coating 7μm AZ-4620 to define metal lines. (c) 5μm thick copper electroplating to make metal structure followed by a series of (1μm) electroless Ni and (0.4μm) Au plating. (d) PR and seeding layer removal. (e) Secondary SU-8 deposition and then etching sacrificial layer in Cu etchant to remove Si handle wafer.
Figure 3.18 Optical photographs of as-fabricated SU-8 flexible ribbon cable.
(a) The SU-8 is patterned as spiral structure for long electrical interconnection. Enlarge view on the one end of SU-8 ribbon, the width of metal lines are designed with (b) 200μm, (c) 100μm, and (d) 50μm, respectively.
Figure 3.19 Optical micrographs of the enlarged view on the area of the one end of SU-8 flexible ribbon (a) before and (a) after sacrificial layer removal.
Figure 3.20 (a) Experiment and (b) ANSYS simulation of SU-8 ribbon hang down under gravity force. ANSYS simulated (c) out-of-plane deformation and (d) stress distribution of the 3.5-turn, 22cm long, 4mm width and 52μm thick SU-8 spiral ribbon under the condition of 5cm vertical displacement in outer end.
Figure 3.21 (a) Optical photograph of micromachined silicon probe bonded with a 2.4cm SU-8 flexible ribbon cable with 12 interconnect line after sacrificial release from Si handle substrate and inset enlarged view on the area of bonding pads, and (b) SEM photograph of broken metal pad on silicon probe after the push-pull test.
Figure 3.22 Relative resistance variation of the temperature sensor before and after bonded with ribbon cable. The resistances are measured from about 15°C to 70°C and then normalized with the resistance at 30°C (R0). The upper right inset shows the measured result from 20°C to 40°C and the lower right inset shows the sensing accuracy is ~0.9°C.