IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2. FEBRUARY 1992
202
Communications Temperature Selective Deposition of Parylene-C E. M . Charlson, E. J. Charlson, and Roya Sabeti Akfract-A new method for selectively depositing conformal, biocompatible coatings of Parylene-C on implantable integrated circuit structures is described. The technique consists of using thin 61m or diffused resistors to electrically heat the area where an opening in the Parylene is desired. Theoretically predicted and actual Parylene-C thickness are compared.
I. INTRODUCTION Recent advances in solid-state technology have brought the ability to fabricate complex transducer circuitry directly onto miniature implantable probes and sensors. This has opened up a wide range of possibilities for biological applications such as neural stimulation and control [ 11-[5]. Considerations involving the protection of these active implanted devices in the hostile environment of extracellular fluids (ECF) are of primary importance. Conventional integrated circuit (IC) packages traditionally used to ensure hermetic seals, although small considering the number of devices which they contain, cannot be used for many potential applications because of their size compared to the volume available in which to implant them. A thin conformal coating is required which can be applied to the device itself and will afford the protection normally provided by bulky packages [4], [6]. There are multiple demands on this thin layer of insulating coating. it must be compatible with the environment in the body in which it is implanted, as well as with the device which it coats. The insulating layer must function as a good dielectric while preventing or minimizing the passage of ions and liquid water. It must adhere well to the implanted circuit over long periods of time. Under typical integrated circuit operating conditions, loss of adhesion at the metal conductors in the presence of liquid and an electrical signal will result in corrosion of many commonly used metals and will promote the occurrence of electrochemical reactions, accompanied by the evolution of gas. The electrochemical activity will result in catastrophic detachment of the protective coating within a short period of time. It is also necessary that there be windows in the coating for particular applications, such as sensing and stimulation [ 5 ] . Parylene-C has been shown to satisfy many of the above mentioned requirements [7]-[9]; however, the problem of producing openings in thin insulating coatings continues to be a significant one. Efforts to date on Parylene-C have concentrated on selectively removing the insulating coating using an oxygen plasma and conventional photolithography [IO]-[12]. Since the etch rates of the photoresist mask and Parylene-C are about the same, Parylene Manuscript received February 22, 1991; revised August 6, 1991. This work was supported in part by contract DHHS-NIH-N01-NS-4-2374funded by the National Institutes of Health and by National Science Foundation Grant NSFECS 8908644. The authors are with the Department of Electrical and Computer EngiColumbia, MO 9104916.
thickness must not exceed that of the photoresist. This limits the usefulness of the technique for application to Parylene which will be used as a protective coating for implantable integrated circuits, since thicknesses of at least 10 pm are required. In addition, pattern resolution degrades on films whose thickness exceeds one micron. Finally, based on the reported etch rate of 0.14 pmlmin [lo], the time required to etch a 10 p n film would be prohibitively long, more than 70 min. Even short term exposure of Parylene-C to oxygen plasma is known to result in modification of the bulk polymer through chlorine loss [ 131. Laser ablation of Parylene has also been reported in the literature [14]; however, this method is still in the preliminary stages and small diameter openings have not yet been reported. We have devised a technique for making openings in ParyleneC by temperature selective application, taking advantage of its varying deposition rate on substrates of different temperature [15], [16]. We will show in this communication that openings in the coating can be created by selectively heating areas of an implant device using thin film or diffused resistors which are made as a normal part of the integrated circuit fabrication process. Also reported is the closed form solution for the temperature distribution near the edge of a thin film resistor which is used to provide the temperature differential. 11. METHOD There are a variety of possible circuit implementations in which to use temperature selective deposition for the formation of openings in a Parylene-C outer coating. One possibility will be described here to illustrate the principles involved and to establish the boundary conditions for the theory. Fig. 1 shows a cross section and top view of a sample used to create a heated area by the application of a voltage to a thin film resistor pattern. The thin metal film is deposited on a silicon substrate which is coated with one micron of silicon dioxide grown by thermal oxidation. This pattern is designed to localize the heat at what will become the stimulating or sensing electrode. This exposed area is connected to on-chip circuitry by the conducting metal line. The relatively large pads are located where the current for heating the selective deposition resistor will be applied and are accessed by circuitry which is independent of the circuit being protected. The resistor is configured for uniform power density Q:
P A
Q = -
where P is the dc power applied, and A is the area of the resistor. It will be shown subsequently that Q will determine the temperature profile and hence deposition asymmetry. Power supply requirements depend on the terminal resistance, R, such that
R = nR,
(2)
where R, is the sheet resistance of the thin film resistor, and n is the aspect ratio of resistor (lengthlwidth). Typically, R, is chosen to be 100 Q per square. During the Parylene deposition, voltage is applied to the resistor, causing a very localized hot zone at the small area where power
0018-9294/92$03.00 0 1992 IEEE
( 8 .
,
203
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992 TO ON CHIP CIRCUITRY t
U
HEAT SINK
METAL
SI02
SILICON
150 nm
1 Irm
500 I r m
TI (r.0) - T o TII (r.0)
-
TI (0.x)
---
-
a lI(r,x)
To
TII (a.x)
all,
at r - a
ar
ar
Fig. 2. Circular heat sources on infinite substrate.
Fig. 1 . Cross section and top view of resistor used to create hot spot (not to scale).
density is very high. Higher temperature in this area locally prevents deposition there and results in an exposed stimulating or sensing point. The implementation of temperature selective deposition described above can be modeled with the resistor approximated by a hot circular area on glass (silicon dioxide) which conducts heat down through the substrate. Convection loss of heat can be ignored since the deposition occurs in a vacuum of approximately 10 pm. Radiation heat loss, while present, is ineffective, compared to conduction, in removing heat because of the relatively low temperatures involved. A key issue in this theory is the temperature variation at the edge of the heated circle. The abruptness of this change determines the variation of thickness with distance and hence the inherent resolution of the selective deposition process.
180-
130-
'.'.'.-.
30
-7n
-1
0
(n
To solve for the theoretical variation of temperature with distance, we will assume a circularly symmetric, constant power density (Q) heated zone of radius a, conducting into a uniform substrate whose bottom side is at constant temperature. Typically, the back side of the substrate or device is clamped to a cooling block during Parylene deposition to provide predictable temperatures and deposition rates. The geometry appropriate to this problem is shown in Fig. 2 . Cylindrical coordinates will be used with height x measured above the heat sink which is held at temperature To. Variable r is the radius from the center of the circular hot spot. The substrate is assumed to extend to infinity in the r direction. Zero heat flux out of the top surface (x = L, r > a) will also be assumed. Because there are no sources of heat within the domain of the solution, the appropriate heat equation is =
0.
(3)
This equation was solved using separation of variables with boundary conditions as stated in Fig. 2 . The solutions for Regions I and I1 were found to be =
C an
2aG Kl(a,p)Io(a,,r)sin ( a n x ) + Gx LCY,
(- 1)""
+ To (4)
and
300
Fig. 3. Temperature versus distance from center of hot spot (theoretical).
A. General Solution [17]
T,(r, x)
200
100
DISTANCE FROM CENTER OF HOT SPOT (pm)
where
111. THEORY
V2T(r, x)
(r,x)
a, =
~
+ L
,
The remaining relationship required for a specific temperature distribution involves that of the constant G in (4) to the heat flux into the circular area. Using the following boundary condition: (7)
G can be related to the thermal flux Q using
where k is the thermal conductivity of the substrate. The thermal flux Q is related to the input power to the resistor P by (1). Using this theory and the fact that a temperature of 80°C is needed at the edge of the hot spot to prevent Parylene-C deposition, it was found that an input power of 0.8 W was required. A theoretical plot of temperature versus distance near the edge of a thin film resistor disspating 0.8 W is shown in Fig. 3. IV. EXPERIMENTAL The three layers which make up the coating are deposited in succession, without breaking vacuum in the reactor, which is shown in Fig. 4. Before deposition, oxygen gas is fed slowly into the vacuum chamber surrounded by an RF coil which sustains a plasma within the chamber. This results in a short etch of the chamber and sample and produces a cleaner environment for the depositions which follow. Methane gas is then fed into the chamber and is plasma polymerized on the sample. This methane primer layer has
204
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992
Fig. 4. Polymer deposition reactor.
been shown to adhere tenaciously to common substrate materials 1181. During methane polymerization, a known amount of Parylene-C dimer is heated to 60°C in the sublimation portion of the reactor. The resulting dimer vapor passes through the pyrolysis tube where it is heated to 650°C. Pyrolysis of the dimer yields monomeric Parylene-C radicals. These reactive groups polymerize on all available surfaces in the deposition chamber, which is typically at room temperature during conventional Parylene deposition procedures. For the first 5 min of the Parylene-C deposition, the RF plasma is maintained, resulting in an intermediate plasma deposited layer of Parylene between the plasma polymerized methane primer layer and the bulk Parylene protective coating. A . Results To test the feasibility of the implementation suggested in the theory section, a sample was fabricated and the deposition was carried out as has been described. Fig. 5 shows a scanning electron micrograph of the resulting opening in the coating. The metal conductor which had the opening on it was contacted by a probe at a point some distance away from the opening to create one terminal for resistance measurements. A small amount of saline solution was poured onto the coating at the opening, making sure that it did not touch the conductor at any point which was not protected by the coating film. A graphite rod was placed in the saline solution to be the other terminal for the measurements. Even a thin film, 1-2 pm, of Parylene-C at the opening produced a capacitive reading, typically in the megohm range with a phase angle of - 88". This was the same as was measured on areas where the coating was 10 pm thick. When the opening was free of Parylene, resistance measurements were on the order of tens of thousands of ohms with only 3-5" phase angles. To predict the thickness gradient at the edge of the resistor from the theoretical temperature distribution, data on deposition rate versus substrate temperature is required. Fig. 6 shows this data for our system in the temperature range from -60°C to 100°C on oxidized silicon substrates. Using the results of temperature versus distance calculations shown in Fig. 3, and assuming To to be
Fig. 5. Scanning electron micrograph of a selectively deposited opening in a 40 pm thick Parylene-C film.
Z
0.800-
0.700v
w
0.600-
2
0.500-
t
.
0.100
0.000 -60
-40
-20
0
20
40
60
80
SUBSTRATE TEMPERATURE ("C)
Fig. 6. Experimentally determined deposition rate versus substrate temperature on an oxidized silicon substrate.
205
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992
75 0-0
A
-A
THEORY EXPERlMENl
/ LINE SPACING
’.IA,
0 4 =-•
0
100
200
300
4
I
DISTANCE FROM CENTER OF HOT SPOT ( p m )
Fig. 7. Theoretical and experimental plot of thickness versus distance from center of hot spot.
- I O T , curves of thickness versus distance were calculated for the sample whose scanning electron micrograph is shown in Fig. 5. These data are shown in Fig. 7 along with measured values taken using a Tencor Alpha Step 200 profilometer. These coatings were tested for adhesion using a modified version of the ASTM standard adhesion test D3359-83 (“Measuring adhesion by tape test,”--1984 ASTM Standards). This test involves scribing a pattern of regularly spaced lines in the coating on the sample prior to boiling the sample in simulated extracellular fluid (ECF), which is a 0.9% saline solution. The adhesion of the coating is evaluated according to the number of squares which detach from the substrate when adhesive tape is applied and subsequently removed. The tape test is performed and the rating recorded after regular one hour intervals of boiling the sample in ECF, which is a form of accelerated life testing. A typical set of satisfactory coatings was found to have adhesion ratings in the two highest ranges after one hour of boiling in ECF. Coatings were also deposited on a set of interdigitated comb patterns, shown in Fig. 8. Since the comb pattems have 130 pm width and 130 pm spacings, the line width to line spacing ratio is 1. The leakage current between combs was monitored while the samples were under soak in ECF at room temperature under continuous 3 V bias. Fig. 9 shows a comparison between the leakage currents of three typical comb pattern samples. One sample was coated with an opening created at a distance of 130 pm from one of the comb fingers. Another sample was coated with no opening created in the coating, and a third sample was coated with sufficient power supplied to the resistor to permit deposition of a thin film of Parylene-C on the heated area. Transistors have also been coated and tested over time while soaking in ECF under the same conditions as the comb patterns. A continuous 3 V bias was applied between the source and drain of the transistors. Preliminary results indicate that typical coatings, with openings created in the coating using the selective deposition technique, can protect the transistors for periods similar to the comb pattern samples, and the change in the leakage current and threshold voltage of the devices is within 15% of the values before soaking was started.
V. CONCLUSION
A method has been presented for selectively depositing Parylene-C using a thin film structure fabricated as part of the normal processing for implantable monolithic integrated probes and sen-
MEASUREMENT IN CENTIMETERS
Fig. 8. Abbreviated drawing of interdigitated comb test pattern (not to scale).
&--I
0
WITHOUT OPENING
10
20
30
40
DAYS
Fig. 9. Leakage current ( A ) versus time under soak (days) for typical comb patterns.
sors. Data from a planar resistor implementation of the method on an oxidized silicon substrate show a sharp gradient of thickness versus distance and ability to withstand soak under bias in simulated extracellular fluid with low leakage current. The theory presented should allow prediction of first order results of applying thermal selective deposition to a variety of integrated circuit technologies. Modified versions of this technique could be used in various applications. External heat sources, such as lasers, could be used instead of the thin film resistors provided substrates are clamped to heat sinks, such as the Peltier coolers we use, so that temperature rise is confined to the area of the beam. Also, a gradual decrease in the thickness of the Parylene-C coating over the length of a neural probe could be achieved if the tip of the probe is heated and the other end is on a heat sink. This protective coating would fulfill the requirements proposed by some workers [19].
206
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992 ACKNOWLEDGMENT
The authors are grateful t o R. Faup o f t h e UMC College o f Agriculture Electron Microscope Facility for taking t h e scanning electron micrograph. REFERENCES [ l ] J. T. Mortimer, “Electrical excitability: the basis for applied neural control,” Eng. Med. Biol., vol. 2, pp. 12-13, Sept. 1983. [2] G. A. May, S. A. Shamma, and R. L. White. “A tantalum-on-saDphire microelectrode array,” IEEE Trans. Electron. D e v . , vol. ED26, pp. 1932-1939, Dec. 1979. S. L. BeMent, K. D. Wise, D. J. Anderson, K. Najafi, andK. Drake, “Solid-state electrodes for multichannel multiplexed intracortical neuronal recording,” IEEE Trans. Biomed. Eng., vol. BME-33, pp. 230-241, Feb., 1986. P. E. Crago, H. J. Chizeck, M. R. Neuman, and F. T. Hambrecht, “Sensors for use with functional neuromuscular stimulation,” IEEE Trans. Biomed. Eng., vol. BME-3, pp. 256-268, Feb. 1986. W. J. Heetderks and F. T. Hambrecht, “Applied neural control in the 1990s,” Proc. IEEE, vol. 76, no. 7, pp. 1115-1121, Sept. 1988. B. Smith, P. Peckham, M. Keith, and D. Roscoe, “An externally powered multichannel, implantable stimulator for versatile control of paralyzed muscle,” IEEE Trans. Biomed. Eng., vol. BME-34, pp. 499-508, July 1987. E. M. Schmidt, J. S. McIntosh, and M. J. Bak, “Long-term implants of Parylene-C coated microelectrodes.” Med. B i d . Eng. Comput., vol. 26, pp. 96-101, 1988. E. J. Charlson, E. M. Charlson, A. K. Sharma, and H. K. Yasuda, “Electrical properties of glow-discharge polymers, Parylenes, and composite films,” J. Appl. Polym. Sci: Appl. Polym. Symp., vol. 38, pp. 137-148, 1984. M. F. Nichols, A. W. Hahn, W. J. James, A. K. Sharma, a n d H . K. Yasuda, “Evaluating the adhesion characteristics of glow-discharge plasma-polymerized films by a novel voltage cycling technique,” J. Appl. Polym. Sci: Appl. Polym. Symp., vol. 38, pp. 21-33, 1984. J. T. C. Yeh and K. R. Grebe, “Patterning of poly-para-xylylenes by reactive ion etching,” J. Vac. Sei. Technol. A, vol. 1, pp. 604608, Apr.-June, 1983. B. P. Levy, S. L. Campbell, and T. L. Rose, “Definition of the geometric area of a microelectrode tip by plasma etching of Parylene,” IEEE Trans. Biomed. Eng., vol. BME-33, pp. 1046-1049, Nov. 1986.
1121 Union Carbide, Bound Brook, NJ. [I31 A. Dilks and A. Van Laken, Physiochemical Aspects of Polymer Surfaces, vol. 2, New York: Plenum, 1983, p. 749. [14] Y.-Y. J. Yang, S.-J. Lee, and S. D. Allen, “Carbon dioxide and excimer laser ablation of parylene,” Summ. Pap. Presented Conf. Laser Electro O p t . , Apr. 1989, pp. 264-265. 1151 P. Kramer, A. K. Sharma, E. E. Henneke, and H. Yasuda, “Polymerization of Para-Xylylene Derivatives (Parylene Polymerization). I. Deposition Kinetics for Parylene N and Parylene C,” J. Polym. Sci., vol. 22, pp. 475-491, 1984. [16] M. Gazicki, G. Surendran, W. James, and H. Yasuda, “Polymerization of Para-Xylylene Derivatives (Parylene polymerization). 11. Heat effects during deposition of Parylene C at different temperatures,” J. Polym. Sei., vol. 23, pp. 2255-2277, 1985. [I71 Nat. Inst. Health Contract Nol-NS-4-2374, Quarterly Progr. Rep. 9 , 1986. [IS] A. H. Hahn, H. K. Yasuda, W. J. James, M. F. Nichols, R. K. Sadhir, A. K. Sharma, 0. A. Pringle, D.H. York, and E. J. Charlson, “Glow discharge polymers as coatings for implanted devices,” in Proc. 18th Rocky Mountain Bioeng. Symp. Laramie, WY, April 20-22, 1981, pp. 109-113. [19] K. Najafi and K. D. Wise, “An implantable multielectrode array with on-chip signal processing,” IEEEJ. Solid Stare Circuits, vol. SC-21, pp. 1035-1044, Dec. 1986.
Correction to “Laser Doppler Velocimetry Stabilized in One Dimension” M. T. Milbocker, G. T. F e k e , a n d D.G. Goger T h e byline o f t h e a b o v e paper was published incorrectly. Please note t h e c h a n g e regarding t h e authorship o f t h e communication. Manuscript received November 15, 1991. The authors are with the Eye Research Institute, Boston, MA 021 14. IEEE Log Number 9 105635. M. T. Milbocker et a l . , IEEE Trans. Biomed. Eng., vol. 38, pp. 928930, 1991.
202
Communications Temperature Selective Deposition of Parylene-C E. M . Charlson, E. J. Charlson, and Roya Sabeti Akfract-A new method for selectively depositing conformal, biocompatible coatings of Parylene-C on implantable integrated circuit structures is described. The technique consists of using thin 61m or diffused resistors to electrically heat the area where an opening in the Parylene is desired. Theoretically predicted and actual Parylene-C thickness are compared.
I. INTRODUCTION Recent advances in solid-state technology have brought the ability to fabricate complex transducer circuitry directly onto miniature implantable probes and sensors. This has opened up a wide range of possibilities for biological applications such as neural stimulation and control [ 11-[5]. Considerations involving the protection of these active implanted devices in the hostile environment of extracellular fluids (ECF) are of primary importance. Conventional integrated circuit (IC) packages traditionally used to ensure hermetic seals, although small considering the number of devices which they contain, cannot be used for many potential applications because of their size compared to the volume available in which to implant them. A thin conformal coating is required which can be applied to the device itself and will afford the protection normally provided by bulky packages [4], [6]. There are multiple demands on this thin layer of insulating coating. it must be compatible with the environment in the body in which it is implanted, as well as with the device which it coats. The insulating layer must function as a good dielectric while preventing or minimizing the passage of ions and liquid water. It must adhere well to the implanted circuit over long periods of time. Under typical integrated circuit operating conditions, loss of adhesion at the metal conductors in the presence of liquid and an electrical signal will result in corrosion of many commonly used metals and will promote the occurrence of electrochemical reactions, accompanied by the evolution of gas. The electrochemical activity will result in catastrophic detachment of the protective coating within a short period of time. It is also necessary that there be windows in the coating for particular applications, such as sensing and stimulation [ 5 ] . Parylene-C has been shown to satisfy many of the above mentioned requirements [7]-[9]; however, the problem of producing openings in thin insulating coatings continues to be a significant one. Efforts to date on Parylene-C have concentrated on selectively removing the insulating coating using an oxygen plasma and conventional photolithography [IO]-[12]. Since the etch rates of the photoresist mask and Parylene-C are about the same, Parylene Manuscript received February 22, 1991; revised August 6, 1991. This work was supported in part by contract DHHS-NIH-N01-NS-4-2374funded by the National Institutes of Health and by National Science Foundation Grant NSFECS 8908644. The authors are with the Department of Electrical and Computer EngiColumbia, MO 9104916.
thickness must not exceed that of the photoresist. This limits the usefulness of the technique for application to Parylene which will be used as a protective coating for implantable integrated circuits, since thicknesses of at least 10 pm are required. In addition, pattern resolution degrades on films whose thickness exceeds one micron. Finally, based on the reported etch rate of 0.14 pmlmin [lo], the time required to etch a 10 p n film would be prohibitively long, more than 70 min. Even short term exposure of Parylene-C to oxygen plasma is known to result in modification of the bulk polymer through chlorine loss [ 131. Laser ablation of Parylene has also been reported in the literature [14]; however, this method is still in the preliminary stages and small diameter openings have not yet been reported. We have devised a technique for making openings in ParyleneC by temperature selective application, taking advantage of its varying deposition rate on substrates of different temperature [15], [16]. We will show in this communication that openings in the coating can be created by selectively heating areas of an implant device using thin film or diffused resistors which are made as a normal part of the integrated circuit fabrication process. Also reported is the closed form solution for the temperature distribution near the edge of a thin film resistor which is used to provide the temperature differential. 11. METHOD There are a variety of possible circuit implementations in which to use temperature selective deposition for the formation of openings in a Parylene-C outer coating. One possibility will be described here to illustrate the principles involved and to establish the boundary conditions for the theory. Fig. 1 shows a cross section and top view of a sample used to create a heated area by the application of a voltage to a thin film resistor pattern. The thin metal film is deposited on a silicon substrate which is coated with one micron of silicon dioxide grown by thermal oxidation. This pattern is designed to localize the heat at what will become the stimulating or sensing electrode. This exposed area is connected to on-chip circuitry by the conducting metal line. The relatively large pads are located where the current for heating the selective deposition resistor will be applied and are accessed by circuitry which is independent of the circuit being protected. The resistor is configured for uniform power density Q:
P A
Q = -
where P is the dc power applied, and A is the area of the resistor. It will be shown subsequently that Q will determine the temperature profile and hence deposition asymmetry. Power supply requirements depend on the terminal resistance, R, such that
R = nR,
(2)
where R, is the sheet resistance of the thin film resistor, and n is the aspect ratio of resistor (lengthlwidth). Typically, R, is chosen to be 100 Q per square. During the Parylene deposition, voltage is applied to the resistor, causing a very localized hot zone at the small area where power
0018-9294/92$03.00 0 1992 IEEE
( 8 .
,
203
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992 TO ON CHIP CIRCUITRY t
U
HEAT SINK
METAL
SI02
SILICON
150 nm
1 Irm
500 I r m
TI (r.0) - T o TII (r.0)
-
TI (0.x)
---
-
a lI(r,x)
To
TII (a.x)
all,
at r - a
ar
ar
Fig. 2. Circular heat sources on infinite substrate.
Fig. 1 . Cross section and top view of resistor used to create hot spot (not to scale).
density is very high. Higher temperature in this area locally prevents deposition there and results in an exposed stimulating or sensing point. The implementation of temperature selective deposition described above can be modeled with the resistor approximated by a hot circular area on glass (silicon dioxide) which conducts heat down through the substrate. Convection loss of heat can be ignored since the deposition occurs in a vacuum of approximately 10 pm. Radiation heat loss, while present, is ineffective, compared to conduction, in removing heat because of the relatively low temperatures involved. A key issue in this theory is the temperature variation at the edge of the heated circle. The abruptness of this change determines the variation of thickness with distance and hence the inherent resolution of the selective deposition process.
180-
130-
'.'.'.-.
30
-7n
-1
0
(n
To solve for the theoretical variation of temperature with distance, we will assume a circularly symmetric, constant power density (Q) heated zone of radius a, conducting into a uniform substrate whose bottom side is at constant temperature. Typically, the back side of the substrate or device is clamped to a cooling block during Parylene deposition to provide predictable temperatures and deposition rates. The geometry appropriate to this problem is shown in Fig. 2 . Cylindrical coordinates will be used with height x measured above the heat sink which is held at temperature To. Variable r is the radius from the center of the circular hot spot. The substrate is assumed to extend to infinity in the r direction. Zero heat flux out of the top surface (x = L, r > a) will also be assumed. Because there are no sources of heat within the domain of the solution, the appropriate heat equation is =
0.
(3)
This equation was solved using separation of variables with boundary conditions as stated in Fig. 2 . The solutions for Regions I and I1 were found to be =
C an
2aG Kl(a,p)Io(a,,r)sin ( a n x ) + Gx LCY,
(- 1)""
+ To (4)
and
300
Fig. 3. Temperature versus distance from center of hot spot (theoretical).
A. General Solution [17]
T,(r, x)
200
100
DISTANCE FROM CENTER OF HOT SPOT (pm)
where
111. THEORY
V2T(r, x)
(r,x)
a, =
~
+ L
,
The remaining relationship required for a specific temperature distribution involves that of the constant G in (4) to the heat flux into the circular area. Using the following boundary condition: (7)
G can be related to the thermal flux Q using
where k is the thermal conductivity of the substrate. The thermal flux Q is related to the input power to the resistor P by (1). Using this theory and the fact that a temperature of 80°C is needed at the edge of the hot spot to prevent Parylene-C deposition, it was found that an input power of 0.8 W was required. A theoretical plot of temperature versus distance near the edge of a thin film resistor disspating 0.8 W is shown in Fig. 3. IV. EXPERIMENTAL The three layers which make up the coating are deposited in succession, without breaking vacuum in the reactor, which is shown in Fig. 4. Before deposition, oxygen gas is fed slowly into the vacuum chamber surrounded by an RF coil which sustains a plasma within the chamber. This results in a short etch of the chamber and sample and produces a cleaner environment for the depositions which follow. Methane gas is then fed into the chamber and is plasma polymerized on the sample. This methane primer layer has
204
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992
Fig. 4. Polymer deposition reactor.
been shown to adhere tenaciously to common substrate materials 1181. During methane polymerization, a known amount of Parylene-C dimer is heated to 60°C in the sublimation portion of the reactor. The resulting dimer vapor passes through the pyrolysis tube where it is heated to 650°C. Pyrolysis of the dimer yields monomeric Parylene-C radicals. These reactive groups polymerize on all available surfaces in the deposition chamber, which is typically at room temperature during conventional Parylene deposition procedures. For the first 5 min of the Parylene-C deposition, the RF plasma is maintained, resulting in an intermediate plasma deposited layer of Parylene between the plasma polymerized methane primer layer and the bulk Parylene protective coating. A . Results To test the feasibility of the implementation suggested in the theory section, a sample was fabricated and the deposition was carried out as has been described. Fig. 5 shows a scanning electron micrograph of the resulting opening in the coating. The metal conductor which had the opening on it was contacted by a probe at a point some distance away from the opening to create one terminal for resistance measurements. A small amount of saline solution was poured onto the coating at the opening, making sure that it did not touch the conductor at any point which was not protected by the coating film. A graphite rod was placed in the saline solution to be the other terminal for the measurements. Even a thin film, 1-2 pm, of Parylene-C at the opening produced a capacitive reading, typically in the megohm range with a phase angle of - 88". This was the same as was measured on areas where the coating was 10 pm thick. When the opening was free of Parylene, resistance measurements were on the order of tens of thousands of ohms with only 3-5" phase angles. To predict the thickness gradient at the edge of the resistor from the theoretical temperature distribution, data on deposition rate versus substrate temperature is required. Fig. 6 shows this data for our system in the temperature range from -60°C to 100°C on oxidized silicon substrates. Using the results of temperature versus distance calculations shown in Fig. 3, and assuming To to be
Fig. 5. Scanning electron micrograph of a selectively deposited opening in a 40 pm thick Parylene-C film.
Z
0.800-
0.700v
w
0.600-
2
0.500-
t
.
0.100
0.000 -60
-40
-20
0
20
40
60
80
SUBSTRATE TEMPERATURE ("C)
Fig. 6. Experimentally determined deposition rate versus substrate temperature on an oxidized silicon substrate.
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992
75 0-0
A
-A
THEORY EXPERlMENl
/ LINE SPACING
’.IA,
0 4 =-•
0
100
200
300
4
I
DISTANCE FROM CENTER OF HOT SPOT ( p m )
Fig. 7. Theoretical and experimental plot of thickness versus distance from center of hot spot.
- I O T , curves of thickness versus distance were calculated for the sample whose scanning electron micrograph is shown in Fig. 5. These data are shown in Fig. 7 along with measured values taken using a Tencor Alpha Step 200 profilometer. These coatings were tested for adhesion using a modified version of the ASTM standard adhesion test D3359-83 (“Measuring adhesion by tape test,”--1984 ASTM Standards). This test involves scribing a pattern of regularly spaced lines in the coating on the sample prior to boiling the sample in simulated extracellular fluid (ECF), which is a 0.9% saline solution. The adhesion of the coating is evaluated according to the number of squares which detach from the substrate when adhesive tape is applied and subsequently removed. The tape test is performed and the rating recorded after regular one hour intervals of boiling the sample in ECF, which is a form of accelerated life testing. A typical set of satisfactory coatings was found to have adhesion ratings in the two highest ranges after one hour of boiling in ECF. Coatings were also deposited on a set of interdigitated comb patterns, shown in Fig. 8. Since the comb pattems have 130 pm width and 130 pm spacings, the line width to line spacing ratio is 1. The leakage current between combs was monitored while the samples were under soak in ECF at room temperature under continuous 3 V bias. Fig. 9 shows a comparison between the leakage currents of three typical comb pattern samples. One sample was coated with an opening created at a distance of 130 pm from one of the comb fingers. Another sample was coated with no opening created in the coating, and a third sample was coated with sufficient power supplied to the resistor to permit deposition of a thin film of Parylene-C on the heated area. Transistors have also been coated and tested over time while soaking in ECF under the same conditions as the comb patterns. A continuous 3 V bias was applied between the source and drain of the transistors. Preliminary results indicate that typical coatings, with openings created in the coating using the selective deposition technique, can protect the transistors for periods similar to the comb pattern samples, and the change in the leakage current and threshold voltage of the devices is within 15% of the values before soaking was started.
V. CONCLUSION
A method has been presented for selectively depositing Parylene-C using a thin film structure fabricated as part of the normal processing for implantable monolithic integrated probes and sen-
MEASUREMENT IN CENTIMETERS
Fig. 8. Abbreviated drawing of interdigitated comb test pattern (not to scale).
&--I
0
WITHOUT OPENING
10
20
30
40
DAYS
Fig. 9. Leakage current ( A ) versus time under soak (days) for typical comb patterns.
sors. Data from a planar resistor implementation of the method on an oxidized silicon substrate show a sharp gradient of thickness versus distance and ability to withstand soak under bias in simulated extracellular fluid with low leakage current. The theory presented should allow prediction of first order results of applying thermal selective deposition to a variety of integrated circuit technologies. Modified versions of this technique could be used in various applications. External heat sources, such as lasers, could be used instead of the thin film resistors provided substrates are clamped to heat sinks, such as the Peltier coolers we use, so that temperature rise is confined to the area of the beam. Also, a gradual decrease in the thickness of the Parylene-C coating over the length of a neural probe could be achieved if the tip of the probe is heated and the other end is on a heat sink. This protective coating would fulfill the requirements proposed by some workers [19].
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 39, NO. 2, FEBRUARY 1992 ACKNOWLEDGMENT
The authors are grateful t o R. Faup o f t h e UMC College o f Agriculture Electron Microscope Facility for taking t h e scanning electron micrograph. REFERENCES [ l ] J. T. Mortimer, “Electrical excitability: the basis for applied neural control,” Eng. Med. Biol., vol. 2, pp. 12-13, Sept. 1983. [2] G. A. May, S. A. Shamma, and R. L. White. “A tantalum-on-saDphire microelectrode array,” IEEE Trans. Electron. D e v . , vol. ED26, pp. 1932-1939, Dec. 1979. S. L. BeMent, K. D. Wise, D. J. Anderson, K. Najafi, andK. Drake, “Solid-state electrodes for multichannel multiplexed intracortical neuronal recording,” IEEE Trans. Biomed. Eng., vol. BME-33, pp. 230-241, Feb., 1986. P. E. Crago, H. J. Chizeck, M. R. Neuman, and F. T. Hambrecht, “Sensors for use with functional neuromuscular stimulation,” IEEE Trans. Biomed. Eng., vol. BME-3, pp. 256-268, Feb. 1986. W. J. Heetderks and F. T. Hambrecht, “Applied neural control in the 1990s,” Proc. IEEE, vol. 76, no. 7, pp. 1115-1121, Sept. 1988. B. Smith, P. Peckham, M. Keith, and D. Roscoe, “An externally powered multichannel, implantable stimulator for versatile control of paralyzed muscle,” IEEE Trans. Biomed. Eng., vol. BME-34, pp. 499-508, July 1987. E. M. Schmidt, J. S. McIntosh, and M. J. Bak, “Long-term implants of Parylene-C coated microelectrodes.” Med. B i d . Eng. Comput., vol. 26, pp. 96-101, 1988. E. J. Charlson, E. M. Charlson, A. K. Sharma, and H. K. Yasuda, “Electrical properties of glow-discharge polymers, Parylenes, and composite films,” J. Appl. Polym. Sci: Appl. Polym. Symp., vol. 38, pp. 137-148, 1984. M. F. Nichols, A. W. Hahn, W. J. James, A. K. Sharma, a n d H . K. Yasuda, “Evaluating the adhesion characteristics of glow-discharge plasma-polymerized films by a novel voltage cycling technique,” J. Appl. Polym. Sci: Appl. Polym. Symp., vol. 38, pp. 21-33, 1984. J. T. C. Yeh and K. R. Grebe, “Patterning of poly-para-xylylenes by reactive ion etching,” J. Vac. Sei. Technol. A, vol. 1, pp. 604608, Apr.-June, 1983. B. P. Levy, S. L. Campbell, and T. L. Rose, “Definition of the geometric area of a microelectrode tip by plasma etching of Parylene,” IEEE Trans. Biomed. Eng., vol. BME-33, pp. 1046-1049, Nov. 1986.
1121 Union Carbide, Bound Brook, NJ. [I31 A. Dilks and A. Van Laken, Physiochemical Aspects of Polymer Surfaces, vol. 2, New York: Plenum, 1983, p. 749. [14] Y.-Y. J. Yang, S.-J. Lee, and S. D. Allen, “Carbon dioxide and excimer laser ablation of parylene,” Summ. Pap. Presented Conf. Laser Electro O p t . , Apr. 1989, pp. 264-265. 1151 P. Kramer, A. K. Sharma, E. E. Henneke, and H. Yasuda, “Polymerization of Para-Xylylene Derivatives (Parylene Polymerization). I. Deposition Kinetics for Parylene N and Parylene C,” J. Polym. Sci., vol. 22, pp. 475-491, 1984. [16] M. Gazicki, G. Surendran, W. James, and H. Yasuda, “Polymerization of Para-Xylylene Derivatives (Parylene polymerization). 11. Heat effects during deposition of Parylene C at different temperatures,” J. Polym. Sei., vol. 23, pp. 2255-2277, 1985. [I71 Nat. Inst. Health Contract Nol-NS-4-2374, Quarterly Progr. Rep. 9 , 1986. [IS] A. H. Hahn, H. K. Yasuda, W. J. James, M. F. Nichols, R. K. Sadhir, A. K. Sharma, 0. A. Pringle, D.H. York, and E. J. Charlson, “Glow discharge polymers as coatings for implanted devices,” in Proc. 18th Rocky Mountain Bioeng. Symp. Laramie, WY, April 20-22, 1981, pp. 109-113. [19] K. Najafi and K. D. Wise, “An implantable multielectrode array with on-chip signal processing,” IEEEJ. Solid Stare Circuits, vol. SC-21, pp. 1035-1044, Dec. 1986.
Correction to “Laser Doppler Velocimetry Stabilized in One Dimension” M. T. Milbocker, G. T. F e k e , a n d D.G. Goger T h e byline o f t h e a b o v e paper was published incorrectly. Please note t h e c h a n g e regarding t h e authorship o f t h e communication. Manuscript received November 15, 1991. The authors are with the Eye Research Institute, Boston, MA 021 14. IEEE Log Number 9 105635. M. T. Milbocker et a l . , IEEE Trans. Biomed. Eng., vol. 38, pp. 928930, 1991.
