Micromachined bonding surfaces and method of forming the same

A method of encapsulating a lead bonding pad region of an integrated circuit (such as a sensor used in an implantable medical device) is disclosed. The excapsulant (such as Teflon.TM.-TFE) is mechanically gripped on the surface of the circuit by anchor interlock portions which are held in undercut grooves, micromachined, in a predefined pattern, in the circuit substrate. The encapsulant is held down by the portions in the grooves, forms a tight mechanical seal with the substrate surface and with the insulation around an attached lead, and blocks intrusion of contaminants along the surfaces between these materials or through the encapsulant.

This invention relates to bonding a material to a surface. 
It is known that adhesion between a synthetic substrate and a metallized 
layer is improved by vertical and horizontal etching of a perforation 
pattern of random orientation, depth and undercutting to produce in the 
substrate recesses which have overhanging walls. A thin copper layer 
sputtered over the substrate fills the recesses and is thus mechanically 
locked onto the surface. 
It is also known that a material such as an epoxy, a urethane, a silicone 
elastomer, or a polyimide, for example, may be adhesively bonded to a 
surface of a structure such as an integrated circuit to strengthen, 
insulate, and protect from contamination and corrosion the connection 
between an electrical lead and a bonding pad of the circuit. Such 
integrated circuits are used in implantable medical devices. 
SUMMARY OF THE INVENTION 
In general, in one aspect, the invention features a lead bonding pad region 
exposed at a substrate surface that includes a pad for attaching an 
electrical lead and a predefined pattern of cavities in regions of the 
substrate adjacent the bonding pad; the openings of the cavities in the 
surface are smaller than portions of the cavities lying below the surface. 
In preferred embodiments, the substrate is crystalline (preferably (100) 
silicon), the cavities are substantially longer than their widths, the 
cavities in the pattern are aligned with a crystal plane in the substrate 
in order to take advantage of anisotropic etch technology, the cavities 
are parallel, the pattern is rectilinear around the pad, and at least two 
cavities have a center to center spacing of less than 20.mu. and 
preferably less than 10.mu.. 
In general, in another aspect, the invention features a method of 
encapsulating a lead bonding pad region with an encapsulant and also 
features the encapsulated lead bonding pad structure formed by the method. 
In preferred embodiments, the lead bonding pad region includes an attached 
lead. The method includes providing a predefined pattern of cavities in 
regions of the substrate surface adjacent the bonding pad, the openings of 
the cavities in the surface being smaller than portions of the cavities 
lying below the surface; applying an encapsulant to the bonding pad and 
the regions adjacent the bonding pad and permitting portions of the 
encapsulant to migrate into the cavities; and causing the encapsulant to 
harden to form an integral mass including anchor portions interlocked 
within the cavities connected to encapsulating portions lying on the 
bonding pad and the regions of the surface adjacent the bonding pad. 
In general, in another aspect, a similar method can be used to encapsulate 
any area on a substrate surface or to bond any coating material to such a 
surface. 
In preferred embodiments, a plurality of cavities (preferably in the form 
of grooves) is arranged in a rectilinear, symmetrical pattern and 
substantially surrounds the area being encapsulated. In another preferred 
embodiment, a plurality of small, rectangular cavities is arranged around 
the area being encapsulated in a pattern resembling a waffle. The 
encapsulated area, on a (100) silicon substrate, is part of an electrical 
connection of an integrated circuit in a medical device, implanted in a 
patient. 
In a related aspect, the invention features a method of forming a cavity of 
controlled geometry and controlled surface appearance in the surface of a 
silicon substrate and also features the isotropic etch composition 
suitable for producing the controlled etching; the etch composition 
includes concentrated nitric acid and concentrated hydrofluoric acid, 
without additional water. Preferred etch compositions contain ten percent 
or less concentrated hydrofluoric acid; more preferred compositions 
contain six percent, two to five percent, or one percent hydrofluoric 
acid. 
In another related aspect, the invention features a method of etching 
undercut cavities in a predefined pattern in the surface of a substrate 
structure that is covered with a layer of mask material. The method 
includes providing a predefined pattern for the cavities on the layer of 
mask material, etching the pattern in the mask material using a first 
etching chemical in a first etchant to which the substrate is resistant 
and etching the undercut cavities in the substrate using a second etching 
chemical in a second etchant to which the mask material is resistant. In 
the resultant structure, the openings of the cavities in the masking layer 
are smaller than portions of the cavities in the substrate. 
In preferred embodiments, the predefined pattern is provided by forming a 
layer of photosensitive polymer on the mask material, exposing the 
photosensitive polymer in the predefined pattern, and developing the 
photosensitive polymer to form the pattern. An individual undercut cavity 
is a groove whose width (preferably.ltoreq.20.mu.; most 
preferably.ltoreq.10.mu.) at the widest point in the crystalline substrate 
is greater than twice the width (preferably.ltoreq.10.mu.; most 
preferably.ltoreq.5.mu.) of the groove opening in the layer of mask 
material. The portion of the groove furthest removed from the groove 
opening is preferably rectangular or more preferably V-shaped. 
The microbonding (microlock) encapsulation method permits the secure 
attachment of a protective coating to the portion of a integrated circuit 
that is vulnerable to degradation caused by environmental contamination 
and permits the use of valuable protective coatings, such as Teflon.TM., 
that have no inherent adhesive properties. 
Protective encapsulants attached to integrated circuits by the microlock 
method have substantially increased surface electrical insulation and 
protective properties compared to adhesion bonded encapsulants. The 
microlock method permits the formation of a cavity or groove of precisely 
defined dimensions in a precisely defined location. The regularity of the 
groove patterns possible with this technique permits the grooves to be 
arrayed extremely close together, thus ensuring a large number of 
coating/substrate interlocks per unit area. A groove pattern can be 
oriented to maximize the barrier against outside leakage and 
contamination, and the groove geometry can be chosen to maximize the path 
(and hence time) a contaminating fluid must take to reach the protected 
region of the circuit and to maximize the strength of the 
coating/substrate bond. 
The microlock bond between the coating and the substrate is not adversely 
affected by water absorption by the coating, which, with traditional 
adhesively attached coatings, causes degredation of the adhesive chemical 
bonds at the substrate/coating interface, electrical current leakage, 
electrochemical damage, and eventual delamination of the coating. If the 
coating is chosen to have mismatched thermal expansion and water 
absorption properties compared to the substrate, differential swelling of 
the coating will provide even tighter bonding and substantially increased 
peel strength rather than delamination. Coatings attached by the 
micromachined bonding method are expected to have a lifetime of decades 
before degradation in contrast to the two to three month lifetime for 
coatings attached by adhesion. 
Sensors with lead bonding pad regions protected by the microbonding 
encapsulation method can be used not only as implantable medical devices, 
but also in similar hostile environments in military, space, 
communications, robotic, or automotive applications. In addition, for 
microchip structures not requiring an exposed electrical contact (such as 
sensors require), which are traditionally sealed from the external 
environment in a metal cannister package, the microlock encapsulation 
method, by completely protecting the vulnerable regions of the circuit for 
a long period of time, makes additional sealing with a cannister 
unnecessary and permits increased miniaturization of such structures. 
Alternatively, the microbonding method can be used to attach two surfaces 
together, with the coating material serving the function of an adhesive. 
Other features and advantages of the invention will be apparent from the 
following description of the preferred embodiment and from the claims.

STRUCTURE 
Referring to FIGS. 1-4, in an integrated circuit sensor 10 (such as that 
used in an implantable medical device) the electrical connection between a 
circuit element 14 and an attached lead 18, through lead bonding pad 16, 
is protected from the external environment by a Teflon.TM. encapsulant 40. 
Encapsulant 40 is mechanically gripped on the surface 26 of the circuit by 
anchor interlock portions 42 (FIG. 3) which are held in undercut grooves 
22, micromachined, in a predefined pattern, in the circuit substrate 28. 
The encapsulant is strong enough to be held down by the anchor portions in 
the grooves and forms a tight mechanical seal with the substrate surface 
and with the insulation around the attached lead and blocks intrusion of 
contaminants along the surfaces between these materials or through the 
encapsulant. 
Referring to FIGS. 1 and 2, integrated circuit sensor 10 contains sensing 
electrodes 12, thin film polycrystalline silicon interconnect elements 14, 
lead bonding pads 16 (100.mu..times.100.mu., made of gold and platinum 
over tantalum), and attached gold leads 18, secured to surface layer 26 of 
silicon (100) substrate chip 20. Surrounding each lead bonding pad, in a 
rectangular pattern, are grooves (represented by lines 22 in FIGS. 1 and 
2), micromachined into the chip in predefined locations. 
Referring to FIGS. 3 and 4, surface layer 26 of silicon chip 20 is made up 
of a silicon dioxide layer 30 (5386 A) and a silicon nitride layer 32 
(1660 A) and forms a masking layer overtop the base silicon layer 28. Each 
groove 22 undercuts surface layer 26 by 1.5.mu. on each side of opening 
24, forming overhangs 34, and is shaped (in base layer 28) generally like 
a V with the width 23 (5.mu.) of the groove at its widest point being 
wider than the opening 24 (2.mu. between overhangs 34) in surface 26. The 
groove depth 27 is 5.mu., and the spacing 29 between grooves is 7.mu. on 
center. The pattern of grooves 22 fills the regions 99 between lead 
bonding pads 16 (FIG. 1) and is designed to take advantage of the 
orientation of the (111) crystal planes of the (100) silicon substrate for 
ease of groove formation and to provide the maximum possible barrier to 
intrusion of contaminants (discussed in detail below). 
Thin film feedthrough interconnect element 14 connects sensor 12 with 
bonding pad 16 (consisting of 5000 A gold layer 76, 1000 A platinum layer 
74, and 4000 A base tantalum layer 72) along a narrow region 21 that 
interrupts the groove pattern. Feedthrouqh 14, 0.5.mu. thick and 3.mu. 
wide, is formed on surface 26 of the silicon chip and insulated with a 
thin coating of silicon dioxide 15 (0.5.mu.) and silicon nitride 17 
(0.15.mu.). The portion of feedthrouqh 14 beneath bonding pad 16 is 5.mu. 
wide and contains contact site 78, a small hole 3.mu. square in the 
silicon dioxide/silicon nitride insulation layers (to expose the 
polycrystalline silicon of the feedthrouqh). Contact site 78 is lined at 
the bottom with a diffused layer of platinum silicide 80. 
An encapsulant material (Teflon.TM.-TFE, E.I. duPont de Nemours & Co., 
Inc., Wilminqton, Del.) 40 forms an integral coating mass over feedthrouqh 
element 14, lead bonding pad 16, and attached lead 18, and over and in 
grooves 22. Encapsulant anchor portions 42 interlocked in grooves 22 by 
overhang shelves 34 form an interlocking structure which physically grip 
the coating onto the circuit surface. The Teflon.TM. encapsulant 40 forms 
a continuous structure with the Teflon.TM. insulation 35 on lead 18. 
FABRICATION 
Referring to FIGS. 1-4, first the desired integrated circuit is formed on 
the top of surface layer 26 of the (100) silicon substrate microchip 20. 
Next, in order to form the encapsulant anchoring groove pattern around 
lead bonding pads 16 and feed through elements 14, silicon chip 20 is 
coated with a standard photosensitive polymer "photoresist" used for 
photolithography in the semiconductor industry, and the planned groove 
pattern is aligned with the crystal (111) planes of the (100) silicon 
substrate appropriate for the anisotropic etch step. The photoresist is 
exposed through a standard glass mask imaged with the desired pattern for 
the grooves. The resultant image is developed by standard techniques. 
Etching of the groove pattern into the mask layer 26 is achieved by using, 
in sequence, sulphur hexafluoride plasma as a silicon nitride plasma etch 
step and buffered hydrofluoric acid solution as a silicon dioxide etch 
step. The photoresist is stripped from the surface of the chip with a 
sulphuric acid-hydrogen peroxide solution. 
Referring to FIG. 5, controlled geometry formation of grooves 22 is then 
produced, in two stages. The 1.5.mu. undercuts to form the interlock 
shelves 34 are etched using a concentrated hydrofluoric acid concentrated 
nitric acid isotropic etching system to form profile 56. An etchant of 1% 
concentrated hydrofluoric acid in concentrated nitric acid produces an 
undercut etch rate of about 1760 A per minute. 
The deep, V-shaped profile 58 of groove 22 is formed using a selective 
(50%) cesium hydroxide etch system described in U.S. patent application 
Ser. No. 360,370, filed June 2, 1989, assigned to the same assignee as the 
present application, incorporated by reference. The shape of a groove is 
given by: 
##EQU1## 
where W=the width of the groove at the surface of the silicon, D=the depth 
of the groove, and .alpha.=the angle of the V with the surface, 
54.7.degree.. (In the case of (110) silicon, the groove will be more 
rectangular because of the orientation of the crystal planes.) 
Referring again to FIG. 3, gold lead wire 18 is attached to bonding pad 16 
with ultrasonic ball bonding. A fluid emulsion of Teflon.TM.-TFE, applied 
to the bonding pad and groove regions of the chip, fills each groove 
cavity. The microchip is placed in a low temperature oven (80.degree. C.) 
to evaporate the water from the emulsion, and then on a hot plate where it 
is heated from the bottom. The Teflon.TM. powder, remaining from the dried 
emulsion, melts to form a liquid, and the Teflon.TM. coating on lead 18 
softens and melts into the encapsulative material. The hot plate is turned 
off and the encapsulated chip is allowed to cool. Because of the chip 
placement on the cooling hot plate, the substrate portion of the chip 
containing the grooves will cool after the Teflon.TM. encapsulant, and a 
cooling groove structure (in the substrate) will close down on a solid (i 
e., already cooled) Teflon.TM. locking element and thus compress the 
Teflon.TM. material in the groove. The resultant cured Teflon.TM. 
encapsulant physically grips the surface of the integrated circuit and 
forms a set of closely spaced concentric seals, with only a minimal break 
for a feedthrough element, thus preventing electrical leakage currents and 
water intrusion along the surface of the device. 
THEORETICAL CONSIDERATIONS FOR DETERMINING GROOVE GEOMETRY 
When a sensor with encapsulated lead bonding area is implanted in a site of 
use, exposure to the environment generates forces in the encapsulant 
anchor portions. These forces have many possible causes, e.g., absorption 
of water by the encapsulant, differential thermal expansion, swelling of 
the encapsulant due to polymerization, crystal formation, or molecular 
changes, and the forces may be sufficiently strong to rip the mask 
insulator from the substrate or to break off the anchor portions. The 
shape of the grooves (and thus of the anchor portions) can be tailored so 
as to minimize the effects of these forces and to reduce the tendency for 
the anchor portions to extrude through the groove openings. With groove 
geometry (e.g., size of undercut, shape of the groove bottom, groove 
depth, spacing, and groove opening width) based on the character of the 
encapsulant, swelling of the trapped coating during immersion of the 
microchip will exert desired sealing forces. 
The size of the undercut is chosen to represent a middle ground between the 
extremes where too small an undercut would result in a structure with 
little holding power while too large an undercut would result in an 
overhang which could easily deform. The appropriate geometrical 
configuration of the undercut needs to be redesigned for each variation in 
the masking layer dimensions. 
The shape of the bottom portion of the groove is important for providing 
maximum grip of the substrate on the encapsulant material. If the groove 
does not have a flat bottom, i.e., is deeper in the center than on the 
sides, it will be more difficult to deform the central portion of the 
encapsulant material, and thus more difficult to unlock the sides of the 
encapsulant and pull it out from under interlock shelf 34. In addition, 
the path length of any possible leakage currents along the interface would 
be maximized by etching the deepest grooves possible. 
The finer the groove pattern (the more closely spaced the grooves), the 
more connections will be possible between the encapsulant and the surface 
per unit area, and, therefore, the better the bonding. The increased chip 
surface area provided by a large number of grooves per unit area also 
takes better advantage of any adhesive and sealing properties of the 
encapsulant. 
The smaller the grooves, the more closely spaced the grooves can be. The 
only expected lower limit on the width of the groove opening is the 
effective size of an individual molecule of encapsulant which must flow 
into a groove. 
The attachment area typically is designed to surround the area to be 
protected and to present a long surface pathway between the lead bonding 
site and the external environment. Any break in the continuity of the 
attachment area (e.g., to leave space for a feedthrough) should be small 
in cross section. The encapsulant attachment must be tight all across the 
surface of the area, not just at the edges, to prevent entry of even a 
monolayer of water, and must be capable of withstanding thermal expansion 
and volume changes due to the absorption of water. 
Other embodiments are within the following claims. 
For example, this method is particularly useful for encapsulating materials 
with generally poor surface adhesion but other desired properties such as 
electrical insulating ability and encompasses any method of deposition 
appropriate for such materials. 
Any crystalline, polycrystalline, or non-crystalline material can be used 
as the microchip substrate. For crystalline substrates, greater variation 
in the dimensions of an individual groove is possible than with other 
materials. Any masking material which is substantially inert to the 
etching chemical used to form the undercut grooves is appropriate to 
protect the substrate during the etching procedure. For a two-layer 
silicon dioxide/silicon nitride mask, the dioxide layer can range from 100 
A to 1.mu. and the nitride layer from 500 A to 2000 A. 
Other possible cavity configurations can provide similar advantages. For 
example, referring to FIG. 8, instead of extended grooves in a concentric 
arrangement around a lead bonding pad, small, rectangular cavities 86 may 
be arranged in a waffle pattern. Typical groove dimensions are: surface 
opening, 0.1.mu.-10.mu.; mask thickness, 0.1.mu.-1.mu.; undercut, 
0.1.mu.-5.mu.; and the spacing from the edge of the one groove undercut 
edge to the edge of the adjacent groove opening, 3.mu.-20.mu.. The depth 
of the groove is primarily determined by the surface pattern (for crystal 
orientation dependent etches) and etchant, and the groove should be as 
deep as possible without weakening the substrate. 
Any isotropic etch composition of concentrated HF and concentrated 
HNO.sub.3 in which the percent of HF is under 10% and no water is added to 
dilute the concentrated acids yields acceptable undercut etch rates. The 
following table gives the etch rates at 21.degree. C. and the surface 
texture of the resulting groove for various HF concentrations: 
TABLE 
______________________________________ 
Isotropic Etch System 
HF--HNO.sub.3 
HF Concentration 
Etch Rate @ 21.degree. C. 
Texture 
______________________________________ 
1% 1760 A/min smooth 
2% 4150 A/min rough 
3% 5542 A/min rough 
4% 7604 A/min rough 
5% 13400 A/min rough 
6% 23100 A/min smooth 
______________________________________ 
Referring to FIG. 7, a graph of the isotropic etch data 50 shows an 
exponential least squares fit 52 with 95% confidence intervals. The 
correlation coefficient is 0.99. The percent concentration of concentrated 
HF in the composition is given on the abcissa. The ordinate shows the etch 
rate measured in angstroms per minute. 
With the proper choice of HF concentration, not only the etch rate but also 
the texture of the interior of the groove can be controlled. Surface 
roughening is observed at HF concentrations between 2-5% (roughness is 
also apparent at 10%). Surface roughness is useful in further increasing 
the effective path length for any potential surface leakage as well as 
increasing the surface area for improved bond strength from any 
contribution by adhesion. Smooth groove surfaces may be chosen when 
controllability and uniformity of the undercut surface itself are more 
important. This etch is very predictable, inexpensive, requires no special 
apparatus, forms the required structures, and etches the silicon dioxide 
layer only minimally. 
Any gas, liquid, or vapor isotropic etching technique is suitable for 
groove formation. Other categories of anisotropic etching include wet 
chemical, plasma, reactive ion, ion milling, or chemical etch stop 
techniques. Any method of forming micromachined undercut grooves of 
controlled geometry is suitable for the microlock technique. For example, 
isotropic or anisotropic etching could be used alone or the order of the 
isotropic and anisotropic etch steps could be reversed. 
An alternate, one step, etch system would be a less selective mixture of 
cesium hydroxide (about 45%) where the (111) etch rate of the sidewall at 
50.degree. C. would be 0.22.mu./hour. This would yield a 1.mu. undercut in 
about 4.5 hours. The vertical etch depth for (110) silicon would be 50.mu. 
while for (100) silicon, the etch depth would be defined by the final 
width of the groove according to the formula given above. 
Referring to FIG. 6, if an anisotropic etchant is used before an isotropic 
etchant on a (100) silicon substrate, the V-shaped, non-undercut profile 
60 produced can then be undercut on the sides and rounded on the bottom 62 
with the isotropic etchant system. 
For some uses, it may be desirable to form the groove pattern first before 
the circuit pattern is laid out on the surface of the chip. The 
feedthrouqhs could be oriented to follow the contour of a groove or to 
cross the groove pattern, dropping down into each groove and then 
returning to the surface of the substrate. 
Other possible configurations for the thin film feedthroughs are a zig zag 
pattern (FIG. 9) (i.e., a configuration designed to maximize the distance 
a feedthrough takes between the edge of the encapsulant and the lead 
bonding pad and thus maximize the distance any contaminating fluid must 
travel before reaching the bonding site) and a floating bridge (FIG. 10) 
(i.e., a configuration in which a groove on either side of a feedthrough 
is undercut toward the center of the feedthrough to such an extent that 
the grooves join and thus create a "floating bridge" structure supporting 
the feedthrough which can be entirely surrounded by encapsulant). The 
feedthroughs also may be buried under the surface using diffused lead 
technology.