lEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 35, NO. 6, JUNE 1988 787

Micromachined Packaging for Chemical Microsensors

Abstract-A review of the critical issues involved in chemical micro- sensor packaging and encapsulation is made, and a hybrid solution is presented. In the design approach, the microsensor is divided into two principal physical parts, an electrode and electronics-containing sub- strate, and a micromachined membrane package. The fabrication and, hence, the resultant performance of each part is independently opti- mizeable. The final microsensor is constructed by, first, binding the two parts together at the wafer level, followed by die separation, and then lead attachment. The micromachined membrane holders are then filled with liquid membranes to yield functioning sensors. A calcium ion sensor fabricated by this method is demonstrated.

I. INTRODUCTION PECIFIC, and often independently addressed, areas of S chemical microsensor packaging and encapsulation

that require the attention of the designer are: 1) electronic isolation of active devices from solution, 2) lead attach- ment and encapsulation, and 3 ) membrane attachment and isolation. In addition to these, there remains the reference electrode. Several solutions have been found for each in- dividual problem area, but these often result in added complexity to the FET fabrication process, making the process noncompatible with standard IC processing tech- niques. This means that the addition of circuitry to achieve the ever sought after multisensor chip becomes exorbitant in development and manufacturing cost. This then re- moves the low-cost disposable feature that would make this device desirable.

A brief review of the techniques and materials that have been employed in CHEMFET and other chemical micro- sensor packaging and encapsulation is given below. A combination of some of these techniques with silicon mi- cromachining is then described, which presents a hybrid solution to microchemical sensor packaging and encap- sulation, and which requires minimal deviation from stan- dard processing techniques for the FET’s or any associ- ated circuitry.

11. REVIEW A . Electronic Isolation

The electrodes, FET’s, and any other electronic de- vices that are on the same chemical sensor substrate must be electrically isolated from the surrounding conductive solution in order to operate properly. The CHEMFET ex- emplifies the requirement for electronic isolation. The ba- sis of operation of any FET is that a field is induced in

Manuscript received October 13, 1987; revised January 21, 1988. R . L. Smith was with the Massachusetts Institute of Technology, Cam-

IEEE Log Number 8820664. bridge, MA 02139.

the channel region that controls the conductivity between the source and drain. Since the gate of a CHEMFET in- cludes the surrounding solution, isolation from the solu- tion is as important to proper functioning as is the isola- tion of MOS components in circuits, from one another and from the environment. In addition, the surrounding solution is of changing chemistry, which is sensed by a change in gate potential. All interfaces with the solution will also have a characteristic potential that can alter in- terfacial processes, i.e., exchange currents, that occur there. Since FET fabrication is most readily accomplished using planar technologies, CHEMFET fabrication pro- cesses have traditionally employed solid-state coatings with low water and ion permeability, such as silicon ni- tride and aluminum oxide, on the uppermost surface. Chemical vapor deposition (CVD) and patterning tech- niques for high-quality materials have been developed and perfected by the IC industry. However, after separation of the individual die from the silicon wafers, the substrate becomes exposed at the sides of the die. This poses an encapsulation problem for the sensor package design.

Isolation of the exposed substrate can be accomplished by encapsulation with an insulating organic, such as epoxy resin [1]-[4]. One approach, developed by Matsuo [5], [ 6 ] , was to micromachine the sensor wafer into needles, attached together at one end like the teeth of a comb. These structures were then coated on all sides by CVD silicon nitride. Processing of these devices is severely complicated by the nonplanar and very fragile structure. More recently, the application of diode isolation and SO1 techniques to the problem of substrate isolation have been employed [ 7 ] - [ 9 ] . These techniques have been developed for circuits and therefore exist as standard processes. The combination of one of these isolation technologies and a top surface barrier coating appears to be the most suc- cessful and the most promising method of electronic iso- lation and enables a part of the packaging to be accom- plished on wafer, by solid-state materials and methods.

B. Inputloutput Lead Wires Chemical microsensors, especially active devices such

as the CHEMFET, require electrical signal and power supplying leads, which communicate between the device in solution and the outer “dry” data acquisition environ- ment. Therefore, a means by which leads can be attached to the chip and their electrical isolation from one another are necessary. In addition, encapsulation of the leads from water and ions in the solution is required to prevent del- eterious corrosion. Several approaches to this problem

0018-9383/88/0600-0787$01 .OO O 1988 IEEE

788 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 35. NO. 6, JUNE 1988

have been explored. Most often, commercially available bonding techniques have been employed such as wire bonding [1]-[7] and tape automated bonding (TAB) [lo], [ 111. The physical bonding of the leads to the chip has not been a significant problem. Rather, the difficulty lies in the geometrical puzzle of how to encapsulate the bonds and lead wires without covering the sensitive gate region. The bonding pads for CHEMFET’s are most conveniently placed along one edge of the chip, as far as possible from the active gate area. With this configuration, the bonds can be coated with an epoxy or other viscous liquid coat- ing, which is subsequently cured, without coating the gates.

There are no suitable commercially available chip car- riers or cables for these devices. Printed circuit cards and dual lumen catheters are among the hand-fashioned chip carriers and cabling that have been employed. TAB bond- ing can reduce this problem; however, failure of the adhe- sion layer with long-term exposure to ionic aqueous so- lutions makes commercially available Kapton@ tapes inadequate for encapsulation [ 101.

Special methods of lead attachment and encapsulation have been proposed and tested. The gate regions can be protected with photolithographically patterned materials, such as Riston’@ [lo] , prior to wire bonding and encap- sulation, and later removed. This technique requires some special processing techniques, such as mechanical grind- ing, but they can be performed at the wafer level. A to- tally different approach is to create “back-side contacts, ” which involves the etching of wafer via holes [12], dif- fusing dopants through the entire substrate by thermal gradient [13], [14], or employing SO1 fabrication tech- niques [9]. These methods place the lead attachment and encapsulation problem on the back of the chip, which need not be exposed to solution if a flow cell is employed [9], [ 151-a very attractive alternative. However, this method has significant limitations: 1) combination of back-side contacts and on-chip electronics (substrate) isolation in- volves very complex processing; 2) the number and place- ment of i / o leads is limited by the large space required per contact and/or poor spacial resolution capabilities; 3) the difficulties encountered in making IC fabrication com- patible with back-side contact formation; and 4) it does not solve the problem for in vivo sensors where chip and leads are immersed in electrolyte.

It is the authors’ opinion that the method of choice for lead attachment for a multichemical sensor chip will be one of the more standard methods, either wire or TAB bonding, with custom-made chip carriers and custom “tape” materials and monolithic cable fabrication.

C. Membranes Generally speaking, electro-chemical microsensors

either employ selectively sensitive materials as trans- ducers of chemical energy to a sensed potential, or they measure the rate of a reaction, represented by the current. CHEMFET’s are made chemically sensitive to a specific ion or other chemical species by attaching a sensing mem-

brane material in series with the gate insulator. The chem- ically established membrane potential is effectively in se- ries with any applied gate bias and is therefore sensed in the same manner as a change in the gate voltage of the FET. Several solid-state membrane materials exist, such as LaF3, AgC1, and Si3N4, which establish potentials se- lectively to fluoride, chloride, and hydrogen ions, respec- tively. Many of the solid-state materials that are used as hydrogen-ion-sensitive membranes are also insulators and excellent diffusion barriers to water and ions. They are often incorporated as the uppermost layer of the FET gate insulator and as an encapsulant. They can be integrated into the FET fabrication at the wafer level. However, for sensing most other chemical species, organic membranes are employed.

There exists a host of different organic membrane sys- tems, selective to a wide variety of ions [l] , [4], [15], [ 161. These materials are attached to the FET gate insu- lator by physio-chemical adhesion. Any electrical shunt path, either vertical or horizontal, through or around the membrane-solution potential generating interface, will diminish the potential sensed by the FET. Horizontal shunts between membrane-covered FET’s will create mu- tually dependent sensors and diminish their sensitivity. Therefore, the membrane integrity (no holes), adhesion, and isolation from other sensing gates are crucial to proper operation. Many solutions to this problem have been pre- sented. Hand-painted epoxy wells and silanization of sur- faces was the original approach [4]. It soon became evi- dent that this was not a commercially viable technique. The Riston’@ masking technique described earlier for lead encapsulation has also been applied to membrane-well fabrication [ 101.

Microfabricated meshes [ 161 have been made in spun- cast polyimide films, which improved the adhesion of subsequently solvent cast polymeric membrane materials. Alternatively, membranes may be formed by spin casting [ 171 a plasticized polymeric matrix onto a wafer contain- ing FET’s, or thin-film electrodes, and locally doping the organic film with an ion-sensitive material, e.g., iono- phores.

All of the techniques mentioned so far preclude the use of liquid-membrane materials and the possible fabrication of the classical ISE in miniature, i.e., membrane/filling solution/redox couple/metal/amplifier. This structure is desirable because each interface in this electrochemical system is thermodynamically well defined and therefore can be fabricated (presumably) in such a way that it is “well behaved” with respect to drift and reproducibility. This is not the case for the CHEMFET, which has a semiconductor-insulator-ionic solution or semiconduc- tor-insulator-membrane-ionic solution gate structure. The coupling between ionic and electronic conduction in these systems is unknown. The insulator-membrane in- terface is blocked [18], Le., impermeable to charge trans- fer, and therefore is not thermodynamically well defined. It is possible that instabilities in CHEMFET behavior, i.e., drift, emanate from this interface. Modification of

SMITH AND COLLINS. MICROMACHINED PACKAGING FOR CHEMICAL MICROSENSORS 789

the CHEMFET structure to include an inner reference so- lution and redox couple between the membrane and gate insulator would result in a highly improved chemical mi- crosensor.

This approach has recently been pursued by several in- vestigators. For example, a device very similar in concept was fabricated nearly a decade ago by Come and Janata, who laboriously pasted individual capillary tips over the gate regions of CHEMFET’s, epoxied them in place, and then filled them with a pH buffer solution, entirely by hand, to create a reference FET [3]. Prohaska [19], [20] has surface micromachined silicon nitride microchambers to form thin-film nearly planar electrochemical cells.

Micromachining of chambers in Pyrex glass plates, by etching and laser drilling [21]-[23] and then bonding these structures to the sensor-containing substrate is a more re- cent approach. Pyrex has been chosen as the membrane- holding material because it can be hermetically sealed to silicon substrates by field-assisted bonding. The process of field-assisted bonding employs high fields and temper- ature in order that a sufficiently large anodizing current may flow across the silicon substrate-glass interface to chemically bond the glass to the substrate [24], [25]. The intention is to bond these structures over thin-film elec- trodes, e.g, silver/silver chloride, which will provide a stable redox couple to a filling solution containing chlo- ride. The advantages of micromachined membrane hold- ers are:

1) They can be fabricated independently from the elec- trodes and any electronics, and the two joined together at the wafer level.

2) These and other structures can be micromachined into three-dimensional forms of great dimensional and functional variety, e.g. , to include flow and fill channels.

3) The stacking of the micromachined substrate over the sensor substrate adds significant vertical dimension to the sensing region such that wire bond encapsulation is no longer a difficulty.

There are, however, pitfalls in the to-date proposed methods of miromachined membrane-holder fabrication and attachment. The use of field-assisted bonding to at- tach these structures to the sensor substrate containing thin-film electrodes and/or FET’s poses the following technical difficulties:

1) The high fields and temperatures required for bond- ing are very detrimental to thin-film electrode materials such as silver and to MOSFET devices without gate pro- tection.

2) The bonding technique requires a conductive plane on the substrate surface that comes in direct contact with the glass. This conductive layer (doped polysilicon [21]- [23]) will short together adjacent sensors, unless those re- gions that are left exposed are somehow isolated.

3) The bondin5 technique requires an extremely planar surface, < 1000-A steps, which is more planar than a sil- icon wafer surface after the fabrication of integrated cir- cuits by standard processing techniques (which include planarization steps). This means that the electrodes and

any other devices or circuits on the substrates need be fabricated by other than traditional means. For this reason and because of the high field requirements, these struc- tures have not yet successfully employed FET’s at the sensing site.

Anodic bonding is not the only means of attachment and hermetic sealing of the sensing surface is not neces- sary if that surface has been coated with a moisture bar- rier, solid-state film such as silicon nitride. Also, her- metic sealing of the surface does not provide encapsulation of bonded leads nor of the substrate and its electronics. It does have the advantage of providing excellently adherent parts. Micromachined substrates can be bonded to silicon substrates by other means, including the attachment by organic adhesives, thermoplastics, poly imide film, glass frits, and reflow oxides. These methods do not require high temperatures or high fields, and do not require highly planarized surfaces. Therefore, all the advantages of the micromachined cavities can be utilized, and in ad- dition, 1) FET’s can be placed at the sensing site for at site impedance transformation and resultant improved sig- nal-to-noise ratio and 2) the use of materials other than Pyrex glass as the micromachined substrate can be em- ployed.

111. THE MICROMACHINED PACKAGE The most attractive micromachinable material for mi-

crosensor packaging is silicon [26]. Fine geometrical con- trol in all three dimensions is possible with the use of anisotropic etchants. Etch-stopping techniques exist for anisotropic and isotropic etchants. The structural possi- bilities tease the imagination. For example, one can com- bine porous silicon membranes with anisotropically etched cavities to produce a micro reference electrode [7]. Flow channels and valves can also be incorporated into the ma- chined substrates to aid in filling and for flow analysis. Therefore, silicon was the material of choice for the mi- cromachined package described here.

The design presented here was meant to incorporate what the authors believe have been the most successful techniques applied to the previously described problem areas of chemical microsensor packaging and encapsula- tion, with a multisensor chip application in mind. One important aspect of this design is that the fabrication of the sensor and electronics are sufficiently standard that foundry services can be utilized, at least up to the final contact hole via formation. Electronic isolation is achieved by diode isolation. Either the sensing FET’s and all other on-chip circuitry are of a single type, i.e., nMOS or PMOS, and placed in the opposite-type well, or a twin- tub CMOS technology can be employed. The FET’s can have polysilicon gates, as long as the gates are or can be electrically floated [27], [28]. The top surface of the sen- sor chip is completely coated with silicon nitride, or other encapsulating, solid-state layer, except for the bonding pads. This layer and final metallization represent a devia- tion from most VLSI processes and require some process development to ensure desired electronic operating char-

790 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 35, NO. 6. J U N E 1988

acteristics. The addition of thin-film silver electrodes over the gate regions of the FET’s would be a last additional step in order to make the microfabricated ISE-plus-am- plifier described earlier.

A . Fabrication The packaging of the sensor began with a 4-in wafer

containing approximately 2500, 1.46 X 1.87 mm die. Every third die contains a single n-MOS CHEMFET in a p-well and an aluminum-gate MOSFET of identical struc- ture. The wafers were processed commercially (Solid State Scientific) following a metal-gate CMOS process, withoddual dielectric over the gates, comprised of 500 L- 50 A of silicon dioxide and 1 100 f lOOA of LPCVD silicon nitride. The detailed design, fabrication, and test- ing of the CHEMFET’s used here has been previously reported [7]. The silicon nitride proved to be an excellent passivation layer, with leakage current less than 20 pA over an exposed area greater than 1 mm2. The mask lay- out for the micromachined package was designed, and di- mensions were assigned in accordance with the sensor wafer layout. The cavity and bonding area patterns were photolithographically transferred onto both sides of an ox- idized (100)-orientation 2-in-diameter double-side pol- ished silicon wafer with the aid of an infrared aligner. The oxide was removed from the patterned areas in hydro- fluoric acid. The wafer was then placed in KOH at 60°C where exposed silicon was anisotropically removed from both sides of the wafer. Etching was terminated when the pyramidal pits forming on either side met one another ap- proximately midway through the wafer. A sketch of the resultant structure, taken in cross section through the CHEMFET region, is shown in Fig. 1.

The micromachined cavities were positioned over the CHEMFET gate and the large bonding openings were po- sitioned over the bonding pad area. The latter were posi- tioned such that the borders of the opening were just at the edge of the scribe lanes. With this configuration, after the attachment of the micromachined substrate, individual sensor die-plus-membrane-holders could be separated with a diamond diesaw.

The under-side of the micromachined wafer, which would be attached to the sensor wafer, was then coated with epoxy. This was accomplished by applying a thin film of epoxy (Shell Epon 825 and Jeffamine D-230) onto a glass slide, placing the under-side of the machined wafer onto the epoxy-coated slide and then gently pulling the wafer and slide apart. Although this method worked well, more controllable techniques such as screen printing, spray coating or photopatterning, and other materials could be employed. The machined 2-in wafer and 1 / 4 of the 4-in sensor wafer were then aligned with respect to one another under a microscope with an x-y-z positioning stage and a vacuum pickup arm. A manual contact-type wafer aligner can readily be used for this procedure, with the machined wafer replacing what is normally the mask.

When aligned, the two wafers were brought into con- tact and left at room temperature to partially cure for 12

P P J n substrate

Fig. 1. A cross-sectional sketch of the micromachined packagr: the posi- tioning of the membrane chambers with respect to the underlying CHEMFET gates, and approximate dimensions.

h, and then completely cured at 80°C for another 8 h. A photomicrograph of the adhered machined wafer and the sensor wafer is shown in Fig. 2, which also clearly shows the bonding area and membrane cavities. The sandwiched structure is then diced, individual die are glued to a printed circuit card, and aluminum wires were wedge bonded to the bonding pads on the chip and to the copper leads of the PC card. The wires were then coated in epoxy, which was dispensed from a needle. The machined substrate is approximately 300 pm thick and provides an excellent barrier to the flow of epoxy into the gate region. This sec- ond application of epoxy was then fully cured.

B. Electrochemical Testing The microsensors were loaded with a liquid ion ex-

changer for serum calcium ion (Orion membrane number 9825). The membrane choice was dictated by its avail- ability and clinical interest. The chamber was filled with the liquid membrane by positioning the liquid over the chamber opening while applying vacuum to evacuate the air inside the chamber. Upon release of the vacuum, the liquid fills the chamber. An Orion barrel electrode was charged with the same membrane material and tested along with the microsensor for comparison. Both sensors were titrated with CaCI2 in a constant background elec- trolyte of 0 .2 M KCI. All potentials are referenced to a Saturated Calomel Electrode (SCE). The CHEMFET was operated in a feedback mode [ l ] , [7] and all measure- ments were made at room temperature, 24°C.

C. Results and Discussion The responses of several microsensors and the Orion

macro electrode to calcium ion concentration is shown in Fig. 3. The Orion electrode gives a linear response to Ca+ + in the range from 0.01 to 0.1 M, with a slope equal to the theoretical value of 30 mVlpH. Although the mi- crosensors show a slightly lower sensitivity (27 mVlpH) than the Orion macro electrode, their response is very re- producible. The microsensors gave an identical response to repeated titration after 24-h immersion in a solution containing 0. I-M CaCI2 and 0.2-M KCI.

The microsensor drift reached a steady-state value of less than 0.1 mV/h after the first hour of exposure to

SMITH

Fig. 2 .

AND COLLINS: MICROMACHINED PACKAGING FOR CHEMICAL MICROSENSORS 79 I

A photomicrograph of the aligned and epoxied together, micro- machined package and CHEMFET wafer, with exposed bonding pad re- gions. Bonding pads 1 and 5 are the source and drain of the CHEMFET and pad 2 is the p-well contact. Bonding pads 3, 4 , and 5 are the source, gate, and drain of the MOSFET, respectively.

\ y e o r e t i c a l Slope 30 mV A e Orion Electrode I 60

into solution, or had otherwise developed an electrolytic shunt.

It was noted that these microsensors were sensitive to rigorous movement, such as shaking, but their gate po- tentials always returned to their original value (within l mV ), even afrer repeated removal from solution and bias- ing. This is highly unusual for unshielded FET sensors and may mean that the silicon package provides some electrostatic shielding. The conductivity of the package may also explain the slightly lower sensitivity of the mi- crosensor relative to the macroelectrode, i.e., an electri- cal shunt path across the membrane through the conduc- tive silicon package may exist. This is possible since the package demonstrated here has no insulating layer, other than native oxide, on the inner walls of the membrane chamber. This can be remedied by oxidation and/or the application of LPCVD silicon nitride after micromachin- ing the package.

A micro ion sensor with micromachined membrane containing cavities attached to and positioned over a CHEMFET has been demonstrated. Several aspects of this design, combined together, differentiate it from previ- ously described microsensor structures: 1) The membrane isolating and positioning cavities are micromachined in silicon and as such can be made of smaller dimensions and with greater three dimensional flexibility than can be presently achieved in glass or in thin films. 2) Attachment of the micromachined cavity containing wafer to the FET containing wafer is achieved without anodic bonding and hence without the need to planarize the substrate nor elec- trostatically protect the FET’s. 3) The FET is placed di- rectly below the membrane and thereby provides at site impedance transformation and consequently improved signal-to-noise ratio.

Further development in this direction is the construc- tion of the micro-miniature ISE configuration described earlier, over a FET. The assembly of this package is the same, with the following differences: a thin film of Ag is deposited and patterned over the CHEMFET gate, which are consequently chloridized, and the cavities are filled with a chloride-ion-saturated hydrogel prior to the addi- tion of a polymeric ion selective membrane. These struc- tures are currently being fabricated. They are expected to perform even better with respect to stability and sensitiv- ity than the membrane-coated CHEMFET presented here.

-1 -2 -3 -4 Log [ Ca++]

Fig. 3. The potential response of the Orion macro electrode (filled sym- bols) and the microsensor (open symbols) to calcium ion titration. Each symbol represents a single titration.

solution. This level of drift was maintained for the next 5 days, during which time it continued to respond to addi- tions of CaClz. The device sensitivity was not checked by titration after 30 h of operation. After 5 days, the device no longer responded to CaCl, and a sudden shift in gate potential of approximately 10 mV was noted. It is as- sumed that the membrane material had either dissolved

ACKNOWLEDGMENT

The authors would like to thank M. Schmidt of the De- partment of Electrical Engineering and Computer Sci- ence, MIT, for so generously volunteering to assist us in mask generation and fabrication of the micromachined package.

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792 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 35. NO. 6, JUNE 1988

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*

R. L. Smith received the B.S. degree in biomedical electronics engineer- ing from the University of Rhode Island, Kingston, in 1977, and the M.S. and Ph.D. degrees in bioengineering from the University of Utah, Salt Lake City, in 1979 and 1982, respectively.

From 1982 to 1984, she was an Assistant Professor in the Department of Electrical Engineering at Drexel University, Philadelphia. The follow- ing two years were spent at the Centre Suisse d’Electronique et Microtech- nique, Neuchatel, Switzerland, as a Visiting Scientist in the Chemical Sen- sor Department. She is currently the Sinclair Visiting Assistant Professor in the Department of Electrical Engineering and Computer Science, MIT. Her research interests include solid-state sensors, microelectronics, and their biomedical applications.

*

S. D. Collins, photograph and biography not available at the time of pub- lication.