STRUCTURE AND METHOD OF USING BENZOCYCLOBUTENE AS A BIOCOMPATIBLE MATERIAL Statement Regarding Federally-Sponsored Research or Development [0001] The U. S. Government has a paid-up license in the present invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided by the terms of Defense Advanced Research Projects Agency (DARPA) Grant No. MDA9720010027 awarded by the Department of Defense.

Claim to Domestic Priority [0002] The present non-provisional patent application claims priority to provisional application serial no.

60/445,156 entitled"Benzocyclobutene (BCB) as a Biocompatible Material", filed on February 4,2003, by Jiping He et al.

Field of the Invention [00031 The present invention relates in general to biocompatible materials and, more particularly, to a structure and method of using benzocyclobutene as a biocompatible material.

Background of the Invention [0004] Many materials find applications in more than one industry. Photosensitive benzocyclobutene (BCB) is a polymer material widely and commonly used in the semiconductor industry in the manufacture of electronic

circuits. The manufacturing of a semiconductor device typically involves growing a cylindrical-shaped silicon (or other base semiconductive material) ingot. The ingot is sliced into circular flat wafers. Through a number of thermal, chemical, and physical manufacturing processes, active semiconductor devices and passive devices are formed on one or both surfaces of the wafer. The wafer is cut into individual rectangular semiconductor die which are then mounted and attached to a leadframe, encapsulated, and packaged as discrete or integrated circuits. The packaged discrete and integrated circuits are mounted to a printed circuit board and interconnected to perform the desired electrical function.

[0005] BCB resin is the material of choice for many applications in the semiconductor and microelectronic industry because of its low dielectric constant, low electrical loss factor at high frequencies, low moisture absorption, low cure temperature, high degree of planarization due to the low viscosity, low level of ionic contaminant, optical clarity, good thermal stability, and chemical resistance. These properties have lead to the application of BCB in multi-layer interconnect for packaging, stress buffer and passivation, flat panel displays, silicon and gallium arsenide interlayer dielectrics, bumping and redistribution, micro-machines and optical interconnects.

Summary of the Invention [0006] In one embodiment, the present invention is a method of making a biocompatible device comprising the steps of dispensing benzocyclobutene (BCB) resin onto a silicon wafer, spinning the silicon wafer to distribute the BCB resin, curing the BCB resin on the silicon wafer

to form a BCB thin film layer, patterning the biocompatible device in the BCB thin film layer on the silicon wafer, removing BCB residue from the silicon wafer, performing a final cure of the BCB thin film, and removing the biocompatible device from the silicon wafer.

[0007] In another embodiment, the present invention is a biosensor for implanting in live tissue comprising a thin film substrate including benzocyclobutene (BCB) material. A transducer is disposed in the thin film substrate for converting biophysical phenomenon to an electrical signal. A conductor is coupled to the transducer and routed along the thin film substrate for transmitting the electrical signal.

[0008] In another embodiment, the present invention is a biocompatible device comprising a substrate including benzocyclobutene material which is suitable for implant into living tissue.

[0009] In another embodiment, the present invention is a method of using benzocyclobutene material in a biocompatible device, comprising the step of forming a substrate from the benzocyclobutene material so that the substrate is suitable for implant into living tissue.

Brief Description of the Drawings [00010] FIG. 1 illustrates a biosensor suitable for implant into live tissue ; FIG. 2 illustrates the steps involved in making the biosensor of FIG. 1; and FIG. 3 illustrates UV-VIS spectra of BCB thin film at various stages of processing.

Detailed Description of the Drawings [00011] Benzocyclobutene (BCB) in its base form is a polymer liquid or resin available under the tradename of Cyclotene 4026. The resin contains 46 wt% B-staged divinylsiloxane-bis-benzocyclobutene in a mesitylene carrier solvent, along with trace amounts of polymerized 1, 2-dihydro-2,2, 4-trimethylquinoline, 2, 6-bis { (4- azidophenyl) methylene}-4-ethylcyclohexanone, and 1-1'- (1- methylethylidene) bis {4- (4-azidophenoxy) benzene}. BCB is a photosensitive, colorless, and high viscosity material.

[00012] In the present invention, BCB can be converted to and used as a biocompatible material in many applications such as biosensors, catheters, pacemakers, tissue replacement, medication dispensers, and other medical devices implanted in the body. BCB has many desirable electro-physical-chemical properties, including bio-compatibility and reliability for implantable devices. In order to assess and confirm the effectiveness of BCB as a biocompatible material, a number of studies and tests have been performed. The cytotoxicity and cell adhesion behavior of Cyclotene 4026 coatings exposed to monolayers of glial and fibroblast cells in vitro has been evaluated. The studies have confirmed BCB films deposited on silicon wafers using micro-fabrication processes have not adversely affected standard tests such as 3T3 fibroblast and T98-G glial cell function in vitro.

[00013] In FIG. 1, a biocompatible device is shown made from BCB material. In the present example, the biocompatible device is biosensor 10. Biosensor 10 includes a flexible substrate 12 suitable for implant into living tissue. Flexible substrate 12 is made with

biocompatible BCB material. Recording sites 14 are provided in substrate 12. Recording sites 14 operate as transducers to convert electro-chemical and physical reactions and biophysical phenomena present in the living tissue to electrical signals. Metal conductors 16 are disposed in substrate 12 to route the electrical signals from recording sites 12 to connector 18. Metal conductor 16 may be disposed on the surface of substrate 12, or sandwiched between first and second layers of substrate 12. Connector 18 provides an interface to other conductors to transmit the electrical signals to measurement instrumentation (not shown). Biosensor 10 is intended for insertion into living tissue, for example, as a neural implant.

[00014] FIG. 2 illustrates the steps of making a biosensor 10 from BCB resin which can be implanted in vitro. The photosensitive BCB resin stored at-20°C in a light-protected container, i. e. , in the dark, will maintain a shelf life of about one year. At 4°C, the shelf life of BCB is reduced to one or two months, and at room temperature the shelf life is only one or two weeks.

Processing is performed in a class 100 clean room. A 15 milliliter (mL) dropper bottle is pre-rinsed in distilled water to remove particles and then allowed to dry. Fresh BCB is taken from the-20°C stock and transferred to the clean dropper bottle. BCB resin is allowed to equilibrate to room temperature for at least 3 hours before use. A 4"diameter silicon wafer or other suitable substrate is selected for application of BCB.

The silicon wafer is cleaned in a reactive ion etcher for 5 minutes at 50 watts in 50 standard cubic centimeter/meter (sccm) 02 flow at 100 millitorr total pressure to remove organic contaminants.

[00015] After cleaning, the silicon wafer is placed

into a programmable spin coater and an adhesion promoter is dispensed onto the middle of the wafer surface to promote adhesion of the BCB resin, as described in step 20. The adhesion promoter contains greater than 98 wt% 1-methoxy-2-propanol, less than 1 wt% water, and other trace elements. The programmable spin coater is fitted with a 2"diameter vacuum chuck to reduce backside contamination. The bowl is lined with cleanroom wipes along the bottom and sides in order to make it easier to keep the bowl clean and further to attenuate wind currents inside the bowl and to catch any solidified BCB strands that form during spinning.

[00016] For adhesion promoter application, the bowl cover is left off to facilitate evaporation of the adhesion promoter solvent. Enough adhesion promoter is applied to cover the entire wafer surface, typically about 1-5 mL for the 4"silicon wafer. The spin coater is spun at 800 rpm for 30 seconds to distribute the adhesion promoter over the wafer surface, followed by a linear ramp to 2000 rpm over 10 seconds. The wafer is dried at 2000 rpm for 30 seconds. The longer the time, the thinner the final coat of adhesion promoter. The spin coater is spun down to zero in 10 seconds.

[00017] After adhesion promoter application, about 1-5 mL of BCB resin is dispensed from the dropper bottle onto the center of the silicon wafer surface, as described in step 22. The spin coater ramps up to 800 rpm over 10 seconds in a linear fashion. The BCB spreads for 10 seconds at 800 rpm. The spin coater then linearly ramps up to 2000 rpm over 10 seconds and spins at 2000 rpm for 30 seconds to distribute an even thin film layer. The spin coater ramps down to zero over 10 seconds. The result is a thin film of BCB material evenly distributed over the wafer surface with a thickness of about 13

micrometers.

[00018] The spin coater bowl cover must be in place during spin coating of the BCB to keep the bowl saturated with mesitylene vapor and to retard the formation of solidified BCB strands. These strands are formed when mesitylene rapidly evaporates from the BCB resin. The strands have an appearance similar to spun sugar or spider webs and tend to contaminate the wafer if they should flop back onto the wafer surface after being formed at the periphery of the chuck. In addition, the bowl cover alters the velocity profile of the atmosphere inside the bowl, which redirects any solidified BCB strands that form away from the wafer surface.

[00019] In some applications, the thickness of the BCB thin film layer is controlled by the amount of BCB resin dispensed onto the wafer surface or by controlling the spin rate and duration of the programmable spin coater.

Alternatively, a second layer of BCB material is formed on the first layer of BCB material for additional thickness in the resulting BCB thin film material. The second layer is formed as described for the first layer of BCB material, i. e. , by dispensing an adhesion promoter, spinning the wafer to evenly distribute the adhesion promoter, dispensing BCB resin, and spinning the wafer to evenly distribute the BCB resin. Metal conductors 16 in biosensor 10 can be routed between the first and second BCB layers.

[00020] The wafer containing the thin film layer of BCB material is removed from the spin coater with wafer tongs and allowed to cool to room temperature before placing in a convection oven to soft-bake, as per step 24. The spinner chuck may be cleaned with an acetone-soaked cleanroom wipe. The convection oven containing the silicon wafer is heated to 70-80°C and purged with tri-

nitrogen to soft-bake the wafer for 20 minutes. The soft-bake process removes residual mesitylene. After the soft-bake process, the thin film layer of BCB material, as prepared on the silicon wafer surface, is about 10 um in thickness.

[00021] The post-soft-bake silicon wafer is cooled to room temperature for 5 minutes and then loaded onto a contact aligner. The contact aligner uses a photolithographic process to form the biocompatible device in the thin film layer of BCB material. In the present example, a mask having the form of a plurality of biosensors 10 is placed in the contact aligner over the silicon wafer, as per step 26. The contact aligner uses a 350-watt mercury arc lamp with G-line (436 nm), H-line (405 nm), and I-line (356 nm) wavelengths. The exposure reliability is about 3%. Depending on whether the first or second layer of BCB is applied, the appropriate dark- field emulsion mask is loaded into the contact aligner and the silicon wafer is aligned to the mask alignment structures. The gap between the top surface of the wafer and the underside of the mask is adjusted during loading to maintain a just-contact position during the exposure so that lateral UV light scattering does not occur.

[00022] After proper alignment, the mask with underlying silicon wafer is exposed to an ultra-violet (UV) light source to develop the BCB material and pattern the plurality of biosensors 10. The BCB-coated wafers <BR> <BR> are exposed using all three wavelengths, i. e. , H-line, I- line, and G-line, with the power intensity measured at the I-line wavelength. An optical filter is attached to perform a broadband exposure and provide a good patterning of the BCB thin films. The recommended time of exposure calculation is based on delivering an exposure dose of 60 millijoules/CM2/PM to the BCB thin

film as measured at the I-line wavelength. Since the power intensity measurement is based on H-line radiation, the time-of-exposure calculation may need to be modified slightly to account for the wavelength-dependent power reading. For example, an exposure time of 3 minutes with power densities 4.0-4. 5 mW/cm~2 should be sufficient to obtain the desired development of 10 um post-soft-bake BCB thin film material and patterning of the plurality of biosensors 10.

[00023] In FIG. 3, the UV-VIS spectra of BCB thin film at various stages of processing is shown. Note that at 405 nm (H-line), the post-soft-bake BCB thin film is nearly transparent to the radiation. The H-line wavelength alone would most likely result in transmission of the radiation all the way through the thin film to the wafer surface, where it can reflect into the areas under the mask intended to be shielded from the radiation. At the 365 nm I-line wavelength, the thin film has a much higher absorbance. The I-line wavelength results in much less reflection of the H-line radiation off the underlying wafer surface, as much more cross-linking of the photosensitizers in the thin film occur during a 3- minute exposure versus similar exposure of only H-line radiation. One can see that the thin film absorbance increases over the course of the exposure from an initially low value at the H-line wavelength to a value comparable to the final I-line wavelength value. At the I-line wavelength, the initial absorbance is high, but decreases to a final value that is still much higher than the initial H-line wavelength value.

[00024] Following UV-exposure, the silicon wafer is placed into a 10 cm diameter by 8 cm tall glass container and about 5 mL of room temperature puddle developer is added, sufficient to cover the surface of the wafer, as

per step 28. The unexposed BCB material is dissolved by the puddle developer. An endpoint, defined as the time to dissolve through the entire layer of unexposed BCB material, is observed by the disappearance of a colored interference fringe pattern. For 10 um post-soft-bake BCB thin film material, the endpoint varies from about 1 minute 20 seconds to 2 minutes. The variation is likely due to the temperature variation of the soft-bake, with hotter soft-bake temperatures leading to longer observed endpoints. Development continued an additional 30% to 100% after observing the endpoint. For example, 50% past a 1: 30 endpoint gives a 2: 15 total develop time.

[00025] After puddle development, the silicon wafer is rinsed for 10 seconds in another beaker with 5 mL of fresh, clean puddle developer, as per step 30. The silicon wafer is then immediately dried with a stream of dry nitrogen. Additional rinses in fresh puddle developer may be required to produce a clean and smooth wafer surface. The silicon wafer is baked again in the convention oven at 75 15°C for 60 seconds to remove residual puddle developer.

[00026] The silicon wafer with the developed and patterned BCB material undergoes a final cure process to create a BCB polymer structure, as per step 32. The silicon wafer is placed in a furnace. The furnace is purged with nitrogen at room temperature for one hour to remove any residual oxygen, which is necessary to prevent oxidation of the BCB thin film during curing. After the one-hour purge, the silicon wafer is cured in the inert atmosphere by rapidly raising the temperature to 210°C for 40 minutes as a partial cure for the first BCB layer.

The cure temperature and time are 250°C for 60 minutes for full cure of the second BCB layer, if applicable.

After the required cure time, the furnace is turned off

and the silicon wafer is cooled for several hours to room temperature while still in the inert atmosphere.

[00027] During the final cure process, a thermally activated cyclobutene ring opening occurs in the BCB monomer. The reaction forms an o-quinodimethane intermediate, which serves as the diene. The intermediate reacts with one of the many dieneophiles, i. e. , a single double bond, in the BCB thin film material, and a highly cross-linked tetrahydronaphthalene structure is formed as the final product. Because there are no gaseous products formed in the BCB thin film during final curing, the BCB material can be cured as rapidly as desired without delamination concerns.

[00028] The silicon wafer is processed in a reactive ion etching chamber to clean and descum any residual BCB material, as per step 34. Partially-cured BCB thin films, which are softer and less resistant to the plasma than fully-cured thin films, are descummed with the chamber operating at 80 seem 02 and 20 sccm CF4 at 100 millitorr with 50 watts for 5 minutes. The harder and more plasma-resistant fully cured thin films are etched for 8 minutes using the same parameters. The silicon wafer is removed from the plasma chamber and visually inspected under a microscope. The reactive ion etching process is repeated until the residue is removed or until a dense series of nearly black spots appear on the BCB thin film. The black spots are pillars or pins of SiF or F+ metal that act as an etch mask.

[00029] In some applications, cleanly-developed BCB thin films, in particular, fully opened vias and recording sites, could not be achieved with only H-line exposure, even with the post-develop plasma descum. For this reason, an extra processing step is performed to clean the vias for one layer BCB or recording sites in

two layer BCB. A photoresist is applied to the silicon wafer using a manual spinner at 4000 rpm for 30 seconds with rapid acceleration/deceleration. The photoresist film is soft-baked in a nitrogen purged convection oven for 10 minutes at 80°C, followed by a 5-minute cool-down period. The wafer is then exposed on the contact aligner for 3 minutes using the complementary light-field mask.

The exposed thin film is developed for 2 minutes 20 seconds in a deionized H20 developer solution at room temperature. After development the patterned photoresist is hard-baked at 80°C for 10 minutes. After applying the soft mask, the wafer is treated in an 02/CF4 plasma in reactive ion etch mode using a 40 seem 02/10 sccm CF4 mixture at 100 millitorr and 100 watts for 5 minutes. A typical DC bias of 250 volts and a reflected power of 5 watts are used during this process step.

[00030] In forming metal conductors 16, metallic traces are added to the planar electrode structure after the first BCB layer is applied. The metal traces are composed of a 20 nm layer of chromium, followed by a 200 nm layer of gold. The process flow for adding these metal traces includes depositing chromium followed by gold, patterning with photoresist, etching away the gold, then chromium, and finally stripping away the photoresist. Conformal layers of chromium and gold are deposited using a thermal evaporator.

[00031] In step 36, the plurality of biosensors 10 are removed from the silicon wafer. Biosensor 10 are suitable for insertion into living tissue, for example, as a neural implant. The BCB material forming the substrate of biosensor 10 helps ensure the biocompatibility and reliability of the device when implanted in vitro. The BCB material is suitable for implant in living tissue because it has flexibility,

biocompability, a high degree of planarization, and low dielectric constant. The BCB material exhibits low moisture absorption and prevents bacteria infection.

[00032] To confirm the biocompatibility of BCB, the processed BCB thin films are subject to cytotoxicity and cell adhesion tests. Prior to cytotoxicity and cell adhesion tests, BCB covered silicon wafers are cleaned by (1) immersing in acetone in ultrasonic bath for two minutes, (2) rinsing with 95% ethanol and immersing in 95% ethanol in an ultrasonic bath for 20 minutes, (3) rinsing with deionized water (DI), immersing in a detergent solution in an ultrasonic bath for 20 minutes, and (4) extensively rinsing with deionized water, with one final immersion under ultrasound for 15 minutes. The wafers are placed on a sheet of aluminum foil and dried under the cell culture sterilized hood overnight. The wafers are wrapped in the same aluminum foil and autoclaved for about half an hour at 100°C.

[00033] All solutions utilized for dextran coating are filter sterilized. Dextran is immobilized to BCB thin films to modulate cell adhesion. Aminated BCB surfaces are prepared by immersion in 0. 01% aqueous Poly-L-Lysine (PLL) solution and incubated overnight.

Periodate-oxidized dextran is dissolved in 0.2 M sodium phosphate buffer containing pH 9 and 0. 02 g/ml.

Immediately following surface amination procedures in the cleanroom, oxidized dextran solution of 2 mL is added to sterile six-well multi-well dishes containing surface- aminated substrates. The substrates are allowed to incubate at room temperature for 16 hours on a rocker platform which is protected from light. Following incubation, the reaction mixture is decanted from the culture wells, and replaced by fresh 0. 1M solution of sodium borohydride (NaBH4) to reduce Schiff bases formed

and to quench any free unreacted aldehyde groups present on the oxidized dextran chain. The substrates are allowed to incubate for 2 hours on the rocker platform.

The NaBH4 solution is then decanted and the substrates are rinsed gently several times with deionized water to remove unbound dextran.

[00034] The 6-well culture plates are initially coated with a 0. 5% pHEMA in 95% ethanol solution to reduce cell attachment to well surfaces. Following thorough air drying of pHEMA-coated culture plates under the sterile hood, cleaned and sterile BCB materials are placed in each well. Approximately 2 mL of cell suspension in media with 15,000 cells/ml are added to each well of the culture dish. The cultures plates are then incubated at 37°C, 5% C02 for 24 hours.

[00035] Glial cell and fibroblast cytotoxicity are evaluated using a Live/Dead Viability/Cytotoxicity Kit.

Cells are seeded into BCB material wells. Stained BCB material is examined at 100X magnification via epi- fluorescence microscope to visualize both viable fluorescein filter set and non-viable rhodamine filter set cells. The percentage of the image covered by live cells is calculated using image analysis software. The percentage values from the independent experiment are compared between each run, and then combined. The groups of BCB materials are compared between each other.

[00036] The 3T3 and T98-G cells are seeded into BCB material wells and incubated for 24 hrs. Following incubation, samples are fixed in 3. 8% formaldehyde in PBS for 5 min and stained with 0. 1% aqueous toluidine blue for 5 min. Stained cells are examined using phase contrast or stereomicroscopy at 100x magnification.

Three random 100x fields are selected for each substrate for analysis. The extent of cell adhesion is determined

for each captured digital image by calculating a percentage of cell area coverage using digital image analysis software. Final data is presented as a percentage of control adhesion. The percentage of control cell area is calculated by multiplying the ratio of % area coverage on all substrates to % cell area coverage on tissue culture plastic. The average percentage of control adhesion is determined from duplicate independent experiments.

[00037] The percent viability values for glial cells and fibroblasts cultured on BCB-coated substrates are calculated from experimental data that is collected using the cytotoxicity assay. The results indicate that 3T3 and T98-G cell viability, with 3T3 of 99. 04. 0% and T98-G of 102. 06. 0%, is not significantly different from <BR> <BR> positive control values, i. e. , p &lt; 0.05. Thus, BCB thin film is considered non-toxic for cultured glial cells and fibroblasts.

[00038] Cell adhesion and spreading is determined on all substrates and expressed as a percentage of control cell area coverage on tissue culture plastic reference substrate. Morphology of adherent 3T3 fibroblasts on BCB films is similar to cells routinely cultured on tissue culture plastic. The 3T3 cell adhesion and spreading on BCB substrates is also comparable to tissue culture plastic, see FIG. 3; 118. 115. 2% control cell area coverage on BCB, 100. 417. 0% control on tissue culture plastic. These results further indicate that BCB thin films do not adversely affect 3T3 fibroblast adhesion, spreading, and function in comparison to normal culture conditions on tissue culture plastic. Surface immobilization of dextran on BCB thin films significantly <BR> <BR> reduced 3T3 cell adhesion and spreading, i. e. , p &lt; 0.001.

[00039] Morphology of adherent T98-G glial cells on BCB

films is similar to cells routinely cultured on tissue culture plastic. T98-G cell adhesion and spreading on BCB substrates is also comparable to tissue culture plastic; 101. 114. 3% control cell area coverage on BCB, 100. 017. 0% control on tissue culture plastic. These results further indicate that BCB films do not adversely affect T98-G glial cell adhesion, spreading, and function in comparison to normal culture conditions on tissue culture plastic. Surface immobilization of dextran on BCB films significantly reduced T98-G cell adhesion and spreading, i. e. , p &lt; 0.001.

[00040] The study of the cytotoxicity of BCB films on silicon wafers supports the use of BCB material for microelectronic neural implant applications. The methods utilized to deposit BCB films on silicon wafers are directly applicable to processes for the microfabrication of prototype BCB-based microelectrode neural implants.

The fibroblast and glial cell lines are representative of cells that are encountered in the neural implant environment. From these cell viability and adhesion studies, it can be concluded that BCB films do not adversely affect 3T3 fibroblast and T98-G glial cell function in vitro. The BCB thin films are non-adhesive with surface immobilized dextran using methods developed for other biomaterials and applications. These results demonstrate that BCB thin films can be used for dextran-based bioactive, cell-selective coatings.

[00041] A person skilled in the art will recognize that changes can be made in form and detail, and equivalents may be substituted for elements of the invention without departing from the scope and spirit of the invention.

The present description is therefore considered in all respects to be illustrative and not restrictive, the scope of the invention being determined by the following claims and their equivalents as supported by the above disclosure and drawings.