A Hermetic Glass-silicon Micropackage With High-Density on-chip Feedthroughs for Sensors and Actuators - Ziaie, Arx - … Systems, Journal

166 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 5, NO. 3, SEPTEMBER 1996

A Hermetic Glass-Silicon Micropack h-Density On-Chip Feedthro

for Sensors and Actuators Babak Ziaie, Jeffrey A. Von Arx, Mehmet R. Dokmeci, and Khalil Najafi, Member, ZEEE

Abstract-This paper describes the development of a hermetic micropackage with high-density on-chip feedthroughs for sensor and actuator applications. The packaging technique uses low- temperature (320" C) electrostatic bonding of a custom-made glass capsule (Corning #7740,2 x 2 x 8 mm3) to fine grain polysilicon in order to form a hermetically sealed cavity. High-density on-chip multiple polysilicon feedthroughs (200 per millimeter) are used for connecting external sensors and actuators to the electronic circuitry inside the package. A high degree of planarity over feedthrough areas is obtained by using grid-shaped polysilicon feedthrough lines that are covered with phosphosilicate glass (PSG), which is subsequently reflown at 1100C in steam for 2 h. Saline and DI water soak tests at elevated temperatures (85 and 95" C) were performed to determine the reliability of the package. Preliminary results have shown a mean time to failure (MTTF) of 284 days and 118 days at 85 and 95"C, respectively, in DI water. An Arrhenius diffusion model for moisture penetration yields an expected lifetime of 116 years at body temperature (37C) for these packages. In vivo tests in guinea pigs and rats for periods ranging from one to two months have shown no sign of infection, inflammation, or tissue abnormality around the implanted package. [181]

I. INTRODUCTION ROPER PACKAGING of sensors and actuators along with their associated interface circuitry is one of the most

challenging problems that any sensor designer encounters. This is particularly formidable when the sensor has to operate in corrosive environments such as salt water, chemical tanks and containers, automobile engines, and biological tissue. Implantable biomedical devices pose the greatest challenge in this respect. Biological fluid is one of the most corrosive environments; it contains various organic and inorganic materials and cellular components like salts (NaC1, KCl, phosphates, carbonates, etc.), enzymes, hormones, proteins, and blood cells. Any implantable device should be protected from body fluids while providing proper access between the sensor and the body. In addition, the body also needs to be protected from the package materials, i.e., the materials used in the package should be noncarcinogenic, noninflammatory, and nonthrombogenic. The second problem is easier to solve, and a wide variety of biocompatible materials are available

Manuscript received October 3, 1995; revised April 16, 1996. Subject Edi- tor, S. D. Senturia. This work was supported by the Neural Prosthesis Program (NIH) under Contracts NIH-NINCDS-N01-NS-4-23 19, NIH-NINCDS-NOI- NS-1-23 14, and NLH-NINCDS-NOI-NS-8-23 12.

The authors are with the Center for Integrated Sensors and Circuits, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109-2122 USA.

Publisher Item Identifier S 1057-7157(96)06190-2.

[ 11. Protecting the sensor and circuitry from biological fluids, however, is more difficult. Implantable devices have to be protected from body fluids by a hermetic package. Depending on the device expected lifetime, a variety of materials and techniques have been employed to encapsulate implantable devices 121, [3]. More traditionally, polymers like silicone rubber 141, polyurethane [5] , and Parylene C 161 have been used where the implant life expectancy is not more than a few months. Hard shell titanium packages have provided several decades of lifetime for cardiac pacemakers [7], although their use is limited to applications that do not require radio-frequency (RF) power transfer to the implant. More recently, newer materials and techniques like silicon dioxide 181, polyimide [9], and anodic bonding [lo]-[ 131 have been investigated for hermetically sealing implantable devices.

In this paper, a hermetic packaging technique with multiple feedthroughs is described that has been developed for a single-channel microstimulator [ 141. This microdevice can be used to stimulate denervated muscle groups in paraplegic and quadriplegic patients suffering from spinal cord injuries. The microstimulator size allows implantation by expulsion from a gauge- 10 hypodermic needle, thus reducing the surgical risks and discomfort. This packaging technique can also be used in a variety of other applications where a sensor or circuit has to be protected from a hostile environment.

After a brief description of the package structure in Section 11, the fabrication process is described in Section 111. This is followed by a discussion on low-temperature electrostatic bonding (Section IV) and multiple feedthrough technology (Section V). Package hermeticity is then discussed in Section VI followed by a description of the test procedure in Section VII. Test results are presented in Section VI11 followed by a conclusion.

11. PACKAGE STRUCTURE Fig. 1 shows the microstimulator structure which consists

of: 1) a silicon substrate supporting a stimulating electrode at each end and providing multiple feedthroughs; 2) receiver circuitry along with its hybrid chip capacitor and receiver coil; and 3) a custom-made glass capsule that is electrostatically bonded to the substrate to protect the receiver circuitry and hybrid elements from body fluids. The microstimulator receives power and data extenally through an inductively coupled link, charges the hybrid chip capacitor, and delivers a constant amplitude current pulse into the muscle upon the reception of

1057-7157/96$05.00 0 1996 IEEE

ZIAIE et al.: GLASS-SILICON MICROPACKAGE WITH HIGH-DENSITY FEEDTHROUGHS 167

Fig. 1. The microstimulator structure showing the silicon substrate, receiver circuitry, and glass capsule.

the appropriate signal from an external transmitter [14]. The microstimulator overall dimensions must be 2 x 2 x 10mm3. A major requirement regarding the microstimulator, and for any other chronically implantable device, is proper packaging and encapsulation. In this application a hermetic package had to be developed to house the circuitry and hybrid elements and protect them from body fluids for a working period of at least 40 years.

Fig. 2 shows the package structure adopted for this device. As can be seen, a custom made glass capsule is electrostatically bonded to a polysilicon overlayer, thus providing a hermetic cavity for the circuitry and hybrid elements. Feedthrough lines for connecting the receiver circuitry to the stimulating

electrodes are provided using polysilicon conductors covered by dielectric layers for passivation. These lines can also be used to connect various other sensors (e.g., pH sensor, pressure sensor, etc.) to their associated interface circuitry inside the package in other applications. A more detailed description of various components and the fabrication technology of the package will follow in subsequent sections.

111. SUBSTRATE FABRICATION PROCESS Fig. 3 shows the cross section of the substrate fabrica-

tion process. The fabrication begins with a standard silicon substrate, over which -1 pm of thermal oxide is grown, followed by the deposition of 1-pm-thick LPCVD polysilicon


SUBSTRATE

POLYSILICON CONDUCTOR

FEEDTHROUGH)

ELECTRICS

Fig. 2. Microstimulator packaging structure with polysilicon overlayer

for feedthrough lines. The polysilicon is deposited at 570C by pyrolyzing silane (SiH4) and doped by diffusing phosphorus using POC13 liquid source at 950C for 30 min. This yields a low sheet resistance needed for feedthrough lines (R, ~5 to 10 Wsquare). This is very important since a low series resistance is desirable for the electrodes (

ZIAIE et al.: GLASSSILICON MICROPACKAGE WITH HIGH-DENSITY FEEDTHROUGHS 169

Substrate

(4

Poly lnterco

,-Si nne

1 Substrate I I (b)

Cross section of the microstimulator substrate fabrication process. Fig. 3.

bond with the silicon surface [12]. The nature of the bond is chemical and is due to the formation of a thin silicon dioxide layer at the interface. The bond is stronger than either of the two materials (i.e., silicon and glass), and any attempt to break the bond would result in breaking either glass or silicon. The quality of the bond depends on the surface roughness and cleanness of the silicon and glass in the areas where they come into contact. Any surface nonplanarity of more than a few hundred angstroms results in unbonded areas and eventual leakage. Therefore, it is very critical to thoroughly clean the bonding surfaces for a high yield process.

Fig. 4(a) shows a SEM photograph of a typical glass capsule used in the package structure which is 8 mm long, 2 mm wide, 2 mm high, and 250 pm thick. The glass capsules are fabricated by an external vendor that specializes in glass microworking and can be ordered in different sizes. This glass capsule is bonded to the substrate that supports the circuit chip and other hybrid elements (Figs. 1 and 2). Fig. 4(b) shows a photograph of a glass capsule bonded to a bare silicon substrate.

As was mentioned before, polysilicon feedthrough lines are used to connect the sensors to the circuitry inside the package. These feedthrough lines have to be insulated from the body fluids on top in order to prevent electrical shorts. This is done by depositing dielectric layers of SiOZ/SiSN4/SiOz on top of the feedthroughs (Section 111). In order to bond the glass capsule to the dielectric sandwich layers, one needs to raise the temperature to at least 430C. This is mainly due to the greater difficulty of bonding between the glass capsule and silicon dioxide. The reason for this is the voltage drop across the dielectric layer, although it seems that the decreased availability of silicon atoms necessary for bonding

I I Substrate

I I Substrate

I Substrate I

at the silicon dioxide interface can also be a contributing factor. Raising the temperature to above 4OOOC increases the conductivity in the dielectric sandwich layer and increases the voltage drop at the interface. At this temperature, however, most hybrid components would1 normally be destroyed. This problem was overcome by adding a polysilicon overlayer on top of the dielectric sandwich and bonding the glass capsule to this polysilicon layer, as illustrated in Fig. 2. Bonding to a polysilicon overlayer effectively reduces the bonding temperature to the range tolerable by the hybrid components (-320C). This is mainly because the voltage is now applied across the polysilicon-glass system (Fig. 2) and the thickness of the underlying dielectric does not affect the bond temperature.

The polysilicon overlayer used for bonding must have a very smooth surface if a good bond is to be achieved. Polysilicon microstructure is strongly influenced by dopants, impurities, deposition temperature, and post-deposition heat cycles [ 181. Polysilicon deposited below 575C is fine-grain and has a smooth surface, whereas the polysilicon deposited above 625C is coarse-grain and has columnar structure [19], [20]. In order to achieve a smooth surface for electrostatic bonding of the glass capsule, polysilicon deposited at 570C was chosen in this work [21]. Fig. 5 shows SEM photographs of the two undoped polysilicon films deposited at 570 and 625C. As shown, there is no detectable surface roughness in fine grain polysilicon.

The polysilicon was doped in order to reduce its sheet resistance (this reduces the voltage drop in the polysilicon conductor and improves the bond quality). In our lab, liquid source (Phosphorus Oxychloride, POC13, a liquid at room temperature) is used for doping the polysilicon with phos-

170 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 5 , NO. 3, SEPTEMBER 1996

(b)

Fig. 4. (a) SEM photograph of a custom-made glass capsule used to encapsulate the microstimulator receiver circuitry. (b) SEM photograph of a glass capsule bonded to a silicon substrate.

phorus (ion-implantation can also be used to dope the top polysilicon layer if it does not increase the surface roughness). In order to bring the POC13 into the furnace, a carrier gas (usually nitrogen) is passed through a bubbler and brings the vapor into the diffusion furnace. The gas stream also contains oxygen, and PzO5 glass is deposited on the surface of the wafers. It was noticed that doping fine-grain polysilicon by the diffusion of phosphorous at 900-950C with a POC13 flow of 400 sccm for 30 min (sheet resistance -10 Ohquare) increases the surface roughness. The surface roughness of the POCl3-doped fine-grain polysilicon is related to phosphorus concentration [22]. Therefore, we decided to preserve the surface quality by simply exposing the wafer to residual impurities in a phosphorus diffusion tube at 950C for 30 min without actually running POClz in the tube. This reduces the sheet resistance to about 80 Rlsquare, which is adequate for a good bond while maintaining the surface quality. The polysilicon surface roughness was also measured using Atomic Force Microscopy (AFM). Table I shows the results of these measurements for doped and undoped fine-grain and coarse- grain polysilicon. As can be seen, the lightly doped fine-grain

(b)

Fig. 5. (b) a coarse-grain polysilicon film.

SEM photographs of the surface of (a) an undoped fine-grain and

TABLE I COMPARISON OF SURFACE ROUGHNESS OF VARIOUS

POLYSLICON FILMS AND CRYSTALLWE SILICON (THESE WERE MEASURED USING ATOMIC FORCE MICROSCOPY)

Roughness (Angstrom rms) Polished crystalline silicon

Fine-grain polysilicon, lightly doped

Coarse-grain polysilicon, undoped 200

polysilicon has a very smooth surface (surface roughness of POCl3 doped coarse-grain polysilicon was not measured in this study but it is expected to be more than undoped coarse- grain polysilicon which is -200 A).

v. MULTIPLE FEEDTHROUGH TECHNOLOGY The electrostatic bonding of glass to polysilicon that was

described in the previous section provides a means to hermetically seal the circuitry and hybrid elements. The package must also be able to provide feedthrough lines for connecting the circuitry inside the package to the sensors outside. This lead transfer should be done without disturbing the surface


Fig. 6. layer.

SEM photograph of two polysilicon feedthrough lines with top PSG

over the feedthrough lines. Any nonplanarity (more than 100 A) over these areas compromises the glass capsule to the silicon substrate bond in that region [12]. Therefore, a lead transfer technique had to be adopted to enable us to transfer feedthrough lines from inside the package to the outside while allowing the creation of a planar surface.

A technique was developed for this purpose that utilized the deposition of phosphosilicate glass (PSG) films to fill the gaps between closely spaced feedthrough conductor lines and high-temperature annealing to reflow the PSG and planarize the top bonding surface [18]. Our approach to the planarization problem was to use a well known trench-refilling technique [23]. This method uses LPCVD deposited thin- films to refill trenches in deep trench isolation technology. Various LPCVD thin-films like polysilicon, phosphosilicate glass (PSG), and low-temperature oxide (LTO) have been used for trench refilling; LPCVD phosphosilicate glass (PSG) was chosen for planarization of the feedthroughs in this application. It is known that LPCVD phosphosilicate glass (PSG) can reflow and fill surface nonuniformities if subjected to high temperature in steam [ 181. Therefore, if the polysilicon feedthrough lines are spaced closely, a thick PSG layer can be used to fill the space between these lines and planarize the surface. This requires that the sidewalls of the feedthrough conductor lines produced after pattern transfer using dry etching be vertical, and that these lines be spaced closely to allow adequate filling by the PSG. The vertical sidewalls are required because of the conformal nature of PSG deposition. With the sloped sidewalls one needs more PSG to planarize the surface. Since the PSG deposition is conformal, the thickness of PSG that is required to completely fill the trench is half the trench width. Therefore, closer-spaced polysilicon lines require less PSG for planarization. Fig. 6 shows an SEM photograph of two polysilicon feedthrough lines with top PSG layer.

As can be seen from Fig. 6 the planarization is not perfect and a small dimple (a few hundred A high) is noticeable on the surface. In order to reduce the height of this nonplanarity and to achieve a more planar surface over the feedthrough lines a grid structure was used. Fig. 7 shows the basic idea behind the grid feedthrough structure. The fact that using grid feedthrough lines improves planarity can be explained

as follows. There is a certain distance (dl) that the top PSG layer has to cover in order to reach its final height (Ah). By using grid-shaped lines, one actually creates an interference region, i.e., the PSG layer does not have enough space (d2 ) to reach its final height, and an overlap between the lines creates a more planar surface. As was mentioned previously, the amount of PSG required for the planarization depends on the spacing between the lines; therefore, it is advantageous to use thick (to reduce the series resistance) and closely space polysilicon lines. Fig. 8(a) shows grid feedthrough lines ( 3 pm lines with 2 pm spacing resulting in 200 feedthrough lines per millimeter) after PSG deposition and before any reflow. This photograph shows that PSG deposition is conformal to the grid feedthroughs. A reflow of at least two hours at 1100C in steam is required to planarize the grid feedthrough lines. Fig. 8(b)-(d) shows the grid feedthrough lines after 30 min, 1 and 2 h of reflow. Surface dimples of ~ 0 . 7 p m , and 0.3 pm can be seen in these photographs after 30 min and 1 h of planarization as compared to the near perfect planarization after 2 h. Fig. 9 is a SEM photograph of the cross section of the grid feedthrough structure showing the high degree of planarity over polysilicon feedthrough lines.

It should be mentioned that in the final microstimulator structure only two feedthrough lines are required to connect the stimulating electrodes to the receiver circuitry inside the package. Therefore, one has the option of connecting all the parallel polysilicon lines together or leaving some of them floating. In other applications that might require more feedthrough lines, various other configurations can be used to connect the inside of the package to the outside. This has to be done in a way that the grid pattern in between the lines under the bonding areas is not disturbed, i.e., polysilicon lines of minimum spacing are required in these areas (at the extreme ends of the substrate nonplanarity does not cause any problem due to its separation ffrom the bonding areas). Two important parameters in designing multiple feedthrough lines are line resistance and parasitic capacitances (line-line and line-substrate). The resistance depends on the first polysilicon layer sheet resistance and dimensions. In our process the first poly is doped heavily to reduce the sheet resistance (-10 Wsquare) and unless the lines are very narrow and long this does not cause any problem (connecting lines in parallel can be used to further reduce the resistance). The line-line and line-substrate capacitances can be calculated to be 15 and 220 fF/mm, respectively, for a 2 pm wide line with 2 pm separation to the adjacent line both over 1 pm silicon dioxide and covered with 1 pm PSG [24]. Generally speaking, the amount of crosstalk between two adjacent lines depends on the Cline-linelCline-substrate ratio (smaller ratio results in reduced crosstalk) [25]. Although crosstalk is not a problem in the microstimulator application, the ability to ground every other line or connecting many lines in parallel provides an easy way to reduce the crosstalk wherever it is found to be necessary.

Using the aforementioned multiple feedthrough technique and fine-grain polysilicon overlayer, the microstimulator substrates were fabricated and electrostatically bonded to the glass capsules. The packages were then soaked in DI water and saline to determine their hermeticity.


Fig. 7. Gnd feedthrough technique with closely spaced polysilicon lines and top PSG layer.

Fig. 8. dimples of 0.3 and 0.7 pm after 30 min and 1 h of reflow compared to the flat surface after 2 h of reflow.

(a) SEM photograph of the grid feedthrough lines with the top PSG before reflow and after (b) 30 miu, (c) 1 h, and (d) 2 h of reflow. Note surface

VI. PACKAGE HEMETICITY

As was mentioned in Section 11, the microstimulator package must operate in the body for a period of 40 years. This requires the ability to withstand the harsh body environment without permitting water to penetrate inside. Moisture is a major cause of failure in nonhermetic packaging, and, together with temperature, is responsible for over 50% of microelectronics device failures [26]. The common causes of failure due to moisture are: 1) charge separation and surface inversion in MOS devices; 2) corrosion of the wire bonds, wire bond pads,

and metal runs; and 3) gold and/or silver migration between conducting paths [27]. In the microstimulator application, water penetration inside the package can be a major source of failure (bonding pads, metal runs, and bond wires are the most susceptible parts to moisture). Five possible sources of water vapor inside the package are: 1) moisture leakage through holes and cracks at the silicon-glass interface; 2) moisture diffusion through the glass; 3) moisture diffusion through thin-films used in the substrate fabrication; 4) water outgassing from glass or substrate during high-temperature bonding; and 5) moisture adsorption on the internal surfaces


Fig. 9. ture.

SEM photograph of the cross section of the grid feedthrough struc-

of the package and components. The first three sources of moisture penetration into the package are the most important and need to be monitored over time. The last two sources are somewhat less important and can be significantly reduced by baking the components at a vacuum level of a few Torr at 200C for 24 h prior to electrostatic bonding 1271.

Moisture diffusion through the silicon-glass bond area depends on the quality and uniformity of the bond and can be stopped if the glass-polysilicon bond is uniform all around the glass edge. The permeation of moisture through the glass capsule is very slow (water diffusivity and solubility in commer- cial borosilicate glass such as #7740 is -4.8 x cm2/sec and 0.011 gm/cc, respectively, at body temperature [28]) and the time required for enough moisture to diffuse through the glass capsule to cause condensation inside can be calculated. Based on the diffusivity and solubility values given above, it takes -130 years before there will be condensation inside the glass capsule (thickness -0.2 mm) due to diffusion at body temperature [29]. The diffusion of water through various thin films used in the packaging structure is different for nitride, oxide, and polysilicon. LPCVD silicon nitride is an excellent moisture barrier, with a diffusion rate of practically zero, even at temperatures of up to 1100C used in silicon processing 1301. Polysilicon is a good moisture barrier, but not as good as Si3N4. Diffusion in polysilicon occurs predominantly along grain boundaries. Silicon dioxide and especially PSG can absorb moisture very fast 1301. The diffusion constant of water in amorphous Si02 at 300C is 1.1 x cm2/sec, and for PSG the diffusion constant is appreciably higher [30]. As was mentioned sidewall passivation blocks the exposure of PSG and oxide to the body fluids and prevents the diffusion of moisture through these areas.

In order to test for hermeticity, the environment inside the package should be monitored for moisture condensation. In this work a dew point sensor was incorporated on the substrate, and hermeticity evaluation was done without the actual receiver circuitry and hybrid elements inside the package 1311. The dew-point sensor is based on an interdigitated structure formed by the top metal layer added to the substrate (Section 111). Any condensation of water on the surface causes a

decrease in the impedance between the two interdigitated electrodes, which can be detected outside of the package through the feedthrough lines. This structure is simple, compatible with our circuit fabrication technology, has good sensitivity and can be added to the layout as part of the receiver circuitry.

Since the package should operate inside the body for a period of 40 years, accelerated testing schemes must be used in order to determine the reliability of the package during this working period [27], [32]. Different variables can be used for accelerated tests depending on the failure mechanisms. Penetration of moisture inside the package occurs through diffusion and permeation, both of which can be accelerated by temperature and humidity. Since the microstimulator will ultimately be implanted inside the body, which is essentially a liquid environment, humidity is not a suitable accelerating parameter. Therefore, elevated temperatures were chosen in this study to accelerate leakage. Temperature is an easy variable to control and because moisture diffusion is an exponential function of temperature, acceleration factors of well over 100 can be easily obtained.

VII. TEST mOCEDURE

In these tests the package substrates with dew-point sensors on them and the glass capsules were first carefully cleaned in acetone and IPA to remove any particulate and residues due to handling. This is necessary, because even a small particle on the surface will cause a nonuniform bond and eventual package failure. Then, the glass capsules were electrostatically bonded to the substrates at 320C by applying 2000 V for 10 min. The bonds were performed on a digitally controlled hot plate in a clean room. The bonded substrates were immersed in a saline bath and deionized water at elevated temperatures of 85 and 95C (temperature-accelerated soak tests have been performed primarily in DI water).

The substrates were tested every three days for any room temperature moisture condensation. This was done by pulling the substrates out of the soak bath, rinsing them with DI water, inspecting for condensation visually, probing the pads connected to the dew-point sensors inside the package, and measuring the dew-point sensor impedance with an impedance meter. Any condensation of moisture on the surface of the dew-point sensor was detected as a decrease in the impedance magnitude and phase and the package was considered failed at that point. Fig. 10 shows the impedance and phase (measured at 5 KHz) of a dew-point sensor in a package that failed after soaking for eight days in saline solution at 95C. Initially the phase is around -89", which shows an almost purely capacitive impedance due to the capacitance between the dew- point sensor interdigitated lines. A change toward the more positive phase is an indication of moisture penetration and is due to the contribution of more resistive elements associated with moisture condensation. Generally, we are able to measure condensation electrically a few days before we can observe it visually. For the lifetime studies reported here, we define package failure as condensation at room temperature, which corresponds to


'"73

= Impedance

- Phase

-30

40

-50

a

-60 E L4 2

-70

-80

-90 0 2 4 6 8 10 12

Days Fig. 10. Impedance and phase of a dew-point sensor at 5 KHz in a package that failed after eight days of soaking at 95OC

of 85C [33]. The failed substrates were pulled apart after the soak test to inspect the bond area for uniformity. Fig. ll(a) shows a photograph of a bonded substrate that was pulled apart in order to observe the bond quality. The breakage of the polysilicon overlayer or glass capsule are signs of a good bond. As can be seen, the bond is uniform all around the perimeter. Fig. 1 l(b) is a SEM photograph of the bonded area from the same substrate showing a glass piece and polysilicon feedthrough lines.

VIII. TEST RESULTS Over the past few years, over a hundred of these packages

have been bonded and tested. The yield initially was about 20% and the high failure rate was mainly due to insufficient pre-bond cleaning, poor bonding alignment, lithography and glass capsule defects, nonuniform bonding electric field, and stress due to bonding temperature nonuniformities. Recently, we have improved our yield dramatically and it is now about 85%. Yield is defined as the percentage of packages that have no internal condensation after more than 24 h soaking at 95C. Generally either packages fail within 24 h, or they last many months at the accelerated temperatures (the equivalent of many years at body temperature). The yield was increased by considering the following factors:

1) The glass and silicon substrate should be extensively cleaned before bonding. Standard solvent cleaning in hot acetone (3 min followed by 30 s ultrasound) and hot IPA (3 min) followed by a DI water rinse is sufficient.

2) In some instances it helps if the silicon substrate is thinned to N 100 p m . This makes the substrate more conformal to any curvature on the glass capsule. It should be noted that this is a problem when the glass capsules are individually fabricated using either a molding technique or by manually working and polishing them. One way to circumvent this problem is to ultrasonically machine the capsules from thick wafers of glass. In this process, a 2-mm-thick #7740 glass wafer is ultrasonically machined to create cavities inside of it. The wafer is then diced to separate the individual cavities. This technique has several advantages, including the use of glass wafer with much better surface planarity and polish, lower cost, and compatibility with batch fabrication and wafer level encapsulation. Fig. 12 shows a SEM photograph of a glass capsule fabricated using this technique.

3) The temperature must be uniform during bonding. Since the glass package is rather thick, steps need to be taken to maintain a constant temperature across the glass-silicon sandwich. This can be achieved by placing the glass-silicon sandwich in an enclosure to prevent any heat loss due to


(b)

Fig. 11. SEM photograph of the bonded area on the same substrate.

(a) Photograph of a broken package showing the bond areas. (b)

convection. Note that after bonding, the bonded package should be cooled slowly (about 10 min to room temperature) to avoid generating large stresses.

4) The electric field distribution around the bonding area must be uniform. This can be done by sputtering aluminum on the top surface of the glass. The aluminum coating is removed after bonding by a wet etch. Uniform hermetic bonds are obtained in 10 min using this method. It should also be mentioned that for the 2-mm-high glass capsules used in this device a bonding voltage of -2000 V was required for obtaining consistently strong bonds at 320C. This is due to the fact that part of the applied voltage drops across the 2-mm- high glass, which can be reduced in applications that require shorter glass capsules.

Figs. 13 and 14 summarize the results from long-term soak tests to date, although the tests are still on-going for some of the packages [31].

Several points should be made with regard to these data. First, the failure of the packages that leak after a few days is due to misalignment of the glass capsule and/or defects on

Fig. 12. SEM photograph of an ultrasonically machined glass capsule.

the surface of the glass or silicon substrate. Second, many of the packages that failed in both of these tests were lost due to excessive handling and not because of the failure of the package itself (at least three packages in the 85OC test and two in the 95C test are known to have leaked immediately after being dropped). Third, we have repeatedly observed in saline-soaked high-temperature tests that the silicon substrate dissolves away slowly causing a failure. Dissolution rates of up to a few microns per day at 95C in saline have been measured. The dissolution rate of silicon in DI water is much smaller and does not affect the package significantly, and it has allowed us to obtain the above results. It should be noted that the dissolution of silicon in saline is much slower at body temperature and will not be a major issue in biomedical applications. Very thin pieces of silicon ( N 2 pm) have been soaked in saline for over three years at room temperature and have been implanted in guinea pigs for over 11 months with no observed dissolution of the silicon in either case [34]. Fourth, it should also be noted that corrosion of the thin films used can be a cause for failure, although we have not observed any significant corrosion effects in these packages yet further tests have to be done to verify this. Finally, the continuous cooling and heating of the packages when they are pulled out of the soaking solution for testing causes an unnecessary stress on the package, which may cause premature failure. In spite of these problems, the above data shows that these packages have lasted for a very long time in DI water.

Moisture penetration into packages is an Arrhenius process and the mean time to failure (MTTF) can be modeled as MTTF = Aexp (-Q/lcT) [27], [31]. In order to predict the lifetime of the package, one needs to determine MTTF and the activation energy (Q). Although the soak tests of this package are ongoing, we can extrapolate a Q of 0.997 eV from the MTTFs of the packages so far. This activation energy gives an acceleration factor for the 85C soak tests of 149, and for the 95C soak tests of 358.8. These acceleration factors along with the MTTFs so far (see Figs. 13 and 14) give an average lifetime of 116 years at body temperature (37C). We caution, however, that the sample size in this study is small, and a much larger study is needed to more accurately predict the package lifetime. Also, since a few of the packages in these tests have


Actual Data

Curve Fit 1 Dropped

and Broke Here

0 5 0 1 0 0 150 2 0 0 2 5 0 300 350

Days of Soak Testing at 95" C

(a)

MTTF) . I Packages lost due to mishandline 1 2

Longest lasting packages so far in this study

attributed to mishandlin

Fig. 13. Summary of the results for 95OC soak tests in DI water.

not failed yet, we expect a slightly higher Q and an even longer average lifetime at body temperature. It should be noted that the activation energy of 0.997 eV is rather conservative and is lower than that measured for some plastic packages used by the IC industry [27].

Soak tests on these packages are also being performed in saline at room temperature, as summarized in Table 11. So far, out of six packages tested at room temperature, one leaked within 24 h (infant mortality), one leaked after 160 days of soaking, and none of the remaining four have shown any sign of moisture penetration after an average of 285 days of soaking.

In addition to in vitro tests, six of these packages have been implanted in guinea pigs and rats for periods ranging from one to two months. Four of these devices were placed in the head of guinea pigs on top of the dura, and two of these devices were placed in a subcutaneous pocket in the dorsum of rats. The four devices that were implanted in guinea pigs were harvested after two months. Three of these packages showed no signs of leakage and one had fluid inside of it, probably due to damage in handling. In all four cases, healthy tissue had regrown up around the glass package. A more thorough histology was performed on the two devices that

TABLE I1 SUMMARY OF THE RESULTS FOR ROOM nMPERATUI(E SALINE SOAK TESTS

were implanted in rats. There was no evidence of tissue inflammation, edema, or infection indicative of package rejection macroscopically. At a microscopic level, hematoxylin-eosin- stained tissue in direct opposition to the implanted device appeared normal and showed no sign of rejection. There was no evidence of edema or inflammatory reaction as suggested by macrophage or polymorphonucleocyte (PMN) infiltration of any tissue component including epidermis, hair follicle, muscle, or connective tissue. These results clearly show the biocompatibility of the materials used in the package structure.

ZIAIE et al.: GLASS-SILICON MICROPACKAGE WITH HIGH-DENSITY FEEDTHROUGHS

W T F ) Packages lost due to mishandling

Longest lasting packages so far in this study

177

366

g 1 0 z 9 cn

0

Packages still under tests with no measurable room temperature condensation inside

Average lifetime to date (MTTF) including losses attributed to mishandling

Average lifetime to date (MTTF) not including losses attributed to mishandling 1

c n 7

' 5 6 - E $ 5

4 v)

r

O 3

days 4

187.4 d a y s 284

d a y s I

I Actual Data

2 Failed Prematurely n- Here 1 Dropped and kW Broke Here I Curve Fit

1 Broke Here Due to Probing Accident 0

1 Dropped 1 ani:;;ke I I 0 5 0 100 1 5 0 200 250 300 350

Days of Soak Testing at 85" C

(a)

Number of packages in this study I 10 Failed within 24 hours (not included in I 2

(b)

Fig. 14. Summary of the results for 85OC soak tests in DI water.

IX. CONCLUSION failure (MTTF) of 284 days and 118 days at 85 and 95C We have developed a hermetic micropackage with high-

density multiple feedthroughs for sensor and actuator applications. Although this package was developed for an implantable biomedical device, it can be used in other applications as well. These might include chemical and biological sensors, vacuum sensors, and, in general, all kinds of smart sensors in which either the sensor or its associated detection circuitry has to be protected from a hostile environment. This packaging technique uses electrostatic bonding of a custom-made glass capsule (Corning #7740, 2 x 2 x 8mm3) to a fine-grain polysilicon overlayer. Electrostatic bonding to the fine-grain polysilicon overlayer reduces the bonding temperature to -32OoC, a value tolerable by most hybrid chip elements. Multiple polysilicon feedthrough lines (200 lines per millimeter) planarized by phosphosilicate glass (PSG) reflow (2 h in steam at llOOC) provide a way to transfer multiple leads from inside of the package to the outside. The package hermeticity was tested by monitoring for room temperature condensation inside the package during elevated temperature (85 and 95C) soak tests in saline and DI water. In order to monitor for condensation a dew-point sensor was used. Preliminary results have shown a mean time to

respectively, in DI water. An Arrhenius diffusion model for moisture penetration yields an expected lifetime of 116 years at body temperature (37OC) for these packages. In vivo tests in guinea pigs and rats for periods ranging from one to two months have shown no sign of infection, inflammation, or tissue abnormality around the implanted package. This clearly demonstrates the biocompatibility of the materials used in the package structure, which are in contact with body fluids.

ACKNOWLEDGMENT

The authors would like to thank Dr. F.T. Hambrecht and Dr. W. Heetderks of the Neural Prosthesis Program (NINDS) for their encouragement and support throughout this work. They would also like to thank Dr. M. W. Putty and the General Motors Research and Development Center, Warren, MI, for help in PSG and LTO deposition. The assistance of Mr. J. Wiler, Ms. J. Hetke, and Mr. P. Finger with in vivo testing in guinea pigs and Mr. D. Wise for evaluating electrostatic bonds is also greatly appreciated. In vivo testing and histology in rats were performed at Vanderbilt University under supervision of Professor D. Zealear.


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Babak Ziaie received the B.S.E.E. degree from Tehran University, Tehran, Iran, in 1986 and the M.S. and Ph.D. degrees from the University of Michigan, Ann Arbor, in 1992 and 1994, respectively. His Ph.D. thesis concentrated on the design and development of a single-channel mcrostimula- tor for functional neuromuscular stimulabon.

In 1994-95, he was a Postdoctoral Research Fellow at the Cardm Rhythm Management Lab- oratory, University of Alabama at B m n g h a m , where he was involved in developing high-density

recording electrode arrays for mapping cardiac arrhytbma. Smce 1995 he has been employed as a Research Scientist at the Center for Integrated Sensors and Circuits, Department of Electncal Engineering and Computer Science, Urnversity of Michgan, where he has been involved in research and development of integrated sensors and actuators, mcrostructures. mcrotelemetry systems for biomedical applications, and packaging and encapsulation of implantable sensors. His major areas of interest include solid-state integrated sensors, mcromachining technologies, VLSI and custom integrated c~cu i t s , instrumentation, and biotelemetry.

Dr Ziaie is a member of Tau Beta Pi.

Jeffrey A. Von Arx received the B.S.E.E. degree summa cum laude from Tufts University, Boston, MA, in 1991 and the M.S. degree in electrical engineenng from the University of Michgan, Ann Arbor, in 1993, where he is currently a Ph.D. candidate.

From 1989 to 1991 he worked on an apphcation- specific neural network at Tufts University. In 1991 he began research at the University of Michigan on implantable solid-state systems for functional neuromuscular stimulation. His research interests include

mxed mode integrated circuit design, hermetic packaging, RF telemetry, and thin-film electrodes.

Mr Von Arx won the B. G . Brown award for outstanding scientific research at Tufts University for h s work on an application-specific neural network He is a member of Tau Beta Pi and Eta Kappa Nu.

Mehmet R. Dokmeci received the B.S.E.E. (with distinction) and M.S.E.E. degrees, both from the University of Minnesota, Twin Cities, in 1989 and 1992, respectively.

He is currently a Ph.D. student at the Univer- sity of Michigan, where his research interests are concentrated in the areas of bioimplantable sensors, micromachining and its applications to biomedical devices, and packaging for implantable sensing devices.

ZIAIE et al.: GLASS-SILICON MICROPACKAGE WITH HIGH-DENSITY FEEDTHROUGHS

Khalil Najafi (S84-M86) was bom in 1958 in Iran. He received the B.S.E.E degree in 1980 and the M.S.E.E. degree in 1981 both from the University of Michigan, Ann Arbor. He received the Ph.D. degree in electrical engineering from the University of Michigan in 1986.

From 1986 to 1988 he was employed as a Re- search Fellow, from 1988 to 1990 as an Assistant Research Scientist, from 1990 to 1993 as an As- sistant Professor, and since September 1993 as an Associate Professor in the Center for Integrated

Sensors and Circuits, Department of Electrical Engineering and Computer Science, University of Michigan. His research interests include the development and design of solid-state integrated sensors and microactuators; analog and digital integrated circuits; implantable microtelemetry systems and transducers for biomedical applications; technologies and structures for micro electromechanical systems and microstructures; and packaging techniques for microtransducers.

Dr. Najafi was awarded a National Science Foundation Young Investigator Award from 1992-1997, was the recipient of the Beatrice Winner Award for Editorial Excellence at the 1986 Intemational Solid-state Circuits Conference and the Paul Rappaport Award for co-authoring the Best Paper published in the IEEE TRANSACTIONS ON ELECTRON DEVICES. In 1994 he received the University of Michigans Henry Russel Award for outstanding achievement and scholarship and was selected by students in the Electrical Engineering and Computer Science Department as the Professor of the Year in 1993. He is an Associate Editor for the Joumal of Micromechanics and Microengineerwag.

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