Vacuum-sealed silicon incandescent light

A microlamp including a polysilicon filament coated with a protective layer and enclosed by a window in a vacuum-sealed cavity.

BACKGROUND OF THE INVENTION 
The present invention relates generally to miniaturized incandescent lamps, 
and more particularly to a silicon-filament, vacuum-sealed incandescent 
light source. 
Miniaturized incandescent lamps were reported for use in displays in the 
1970's. See P.M. Alt, "Performance and design considerations of the 
thin-film tungsten matrix display," IEEE Trans. Electron Devices, vol 
ED-20, pp 1006-1015, Nov. 1973; and F. Hochberg, H.K. Seitz, and A.V. 
Brown, "A thin-film integrated incandescent display" , IEEE Trans. 
Electron Devices, vol ED-20, pp. 1002-1005, Nov. 1973. These devices, 
which consisted of thin-film tungsten filaments suspended from a glass 
substrate, were fabricated using hybrid circuit technology. More recently, 
miniature incandescent light sources have been fabricated utilizing 
silicon integrated circuit (IC) technology. See H. Guckel and D.W. 
Burns,"Integrated transducers based on blackbody radiation from heated 
polysilicon films", Transducers' 85, pp. 364-366, Jun. 11-14, 1985; and G. 
Lamb, M. Jhabvala, and A. Burgess, "Integrated-circuit broadband infrared 
source", NASA Tech. Briefs, p. 32, Mar. 1989. In this later work, the 
incandescent element was an electrically-heated polycrystalline-silicon 
microbridge resistor elevated a few micrometers above a silicon substrate 
and exposed to air. Guckel et al. demonstrated the use of this device to 
make an all-silicon optical coupler. 
Even though the polysilicon filaments described in Guckel et al. and Lamb 
et al are capable of high temperature operation, they are not isolated 
from the environment, and therefore are susceptible to particle 
contamination or possibly to damage caused by physical contact. In 
addition, the useful device life-time is limited by oxidation of the 
exposed silicon filament when operated in free air. Guckel et al. 
suggested that the lifetime could be increased if the filaments were 
coated with silicon nitride to reduce their rate of oxidation. 
In view of the foregoing, an object of the present invention is to provide 
a microlamp in which oxidation and contamination problems are 
substantially eliminated. 
More specifically, an object of the present invention is to provide a 
vacuum-sealed microlamp. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
claims. 
SUMMARY OF THE INVENTION 
The present invention is directed to a microlamp. The microlamp may 
comprise a substrate having a cavity formed therein. A polysilicon 
filament may be disposed in the cavity. A window means is provided to 
vacuum seal the cavity. Radiation emitted by the filament is visible 
through the window means. 
This silicon-filament, vacuum-sealed incandescent light source may be 
fabricated using IC technology. The incandescent light may include a 
heavily doped p.sup.+ polysilicon filament coated with silicon-nitride and 
enclosed in a vacuum-sealed (.perspectiveto. 80 mT) cavity. The filament 
may be electrically heated to reach a maximum temperature in the range of 
1500-1600K corresponding to a peak wavelength of approximately 2 .mu.m. 
The power required to achieve this temperature for a filament 
350.times.3.times.1 .mu.m.sup.3 is 3-4 mW. The cavity may be sealed with a 
silicon-nitride window through which the radiation emitted by the 
incandescent filament is visible. The turn-off time for the 350 .mu.m 
device is 2-4 ms.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring in detail to the drawings, wherein like reference numerals 
designate like parts in several figures, and initially to FIG. 1, a 
vacuum-sealed micro miniature incandescent light source or microlamp 10 in 
accordance with the present invention is illustrated. As shown, microlamp 
10 includes an incandescent filament 12 built inside a vacuum-sealed 
cavity 14 that is sealed by a window 16. The cavity is formed in a 
substrate 18. The window 16 transmit the filament black-body radiation 
with very low loss. 
In lamp 10, both oxidation and contamination problems are eliminated. 
Because the incandescent filament 12 is enclosed in a vacuum environment, 
it cannot oxidize. Furthermore, window 16 protects the fragile 
incandescent filament 12 from external contamination or physical 
disturbance. The lamp can even operate when submerged in liquids. 
With silicon nitride used as a window material, it is possible to build 
active devices such as MOSFETS or bipolar transistors on the same 
substrate after the process to build the microlamps has been completed. 
Thus, microlamps can easily be integrated with active circuits. 
Refractory materials might be used as filaments in the microlamp or in 
variations such as the micro-vacuum-amplifying tube. These materials 
include the refractory metals tungsten, tantalum, platinum, palladium, 
molybdium, zirconium, titanium, nickel, chromium, nickel-chromium plus, 
possibly, conducting compounds like the silicides of titanium, zirconium, 
hafnium, niobium, tantalum, chromium, molybdium, tungsten, iron, cobalt, 
nickel, platinum, and palladium. Other similar materials are also 
possible. 
In addition to display and optical-coupling applications, the microlamp 10 
is suitable for high-efficiency thermal print heads since the filament 
requires little power to achieve a temperature of 1300K. The 
silicon-nitride housing provides an excellent wear resistance for contact 
with paper. The lamp can also be the transmitter for an IC-fabricated 
optocoupler. The structure has potential biomedical applications that are 
especially attractive because of the lamp housing and the possibility of 
operation in liquid environments. It can act as a black-body source for 
infrared energy, and it can be laid out in planar-array form. If it faced 
a series of infrared (IR) detectors, this could be very useful for 
measurements of IR absorption in intervening layers of tissue or other 
materials. 
The technology to produce the microlamp can also be applied to build 
micromachined vacuum tubes. For example, a micromachined vacuum diode 
could be fabricated by building both an incandescent cathode filament and 
a anode inside a single vacuum-sealed chamber. The cathode material should 
have a low work function and a high carrier density to obtain significant 
thermionic emission of electrons. 
FIG. 2 shows a cross section of the actual device. In this structure, the 
incandescent filament 12 is placed between an anisotropically etched 
silicon V-groove 20 in substrate 18 and a low stress silicon-nitride 
window 16 that is transparent to the filament black-body radiation. The 
window hermetically seals the cavity 14 at the time of the deposition of 
silicon nitride. In this structure, the V-groove silicon walls 21 and 22 
are partial reflectors for the filament radiation. The maximum depth of 
the V-groove is approximately 20 to 25 .mu.m. 
Silicon is transparent for wavelengths longer than 1.1 .mu.m reflecting for 
shorter wavelengths; therefore it is not a good mirror for infrared 
radiation. The infrared reflectance of the silicon walls can be improved 
if they are heavily doped. We expect that the wall reflectance can also be 
improved by the deposition or growth of a thin SiO.sub.2 film. 
The thermal radiator may be a p.sup.+ polysilicon filament. The filament 12 
may be coated with low-stress silicon nitride. The conductive polysilicon 
and insulating silicon-nitride coating may be 0.9 to 1.mu.m and 0.3 to 0.5 
.mu.m thick, respectively. Filament lengths from 110 to 510 .mu.m (in 40 
.mu.m intervals) may be made in a single wafer run. Silicon-nitride coated 
filaments can operate at higher temperatures than uncoated filaments, 
since the melting point of silicon nitride is 220K (compared to 1900K for 
silicon). Thus, even if the silicon filament melts, it is held in place by 
the silicon-nitride "skin" or protective layer that encloses it. 
In operation, the filament is electrically heated until it glows. The 
maximum achievable temperature is determined by the decomposition rate of 
silicon nitride and the expected lifetime of the microlamp. Studies 
conducted on bulk silicon-nitride samples show that the vaporization rate 
of silicon nitride starts to be significant at 1900K, corresponding to a 
peak in the spectrum of the emitted radiation of approximately 2.mu.m. 
The window material, silicon nitride, is transparent to wavelengths between 
0.28 and 8 .mu.m. Thus, the window transmits most of the radiation emitted 
by the incandescent filament. The window must be thick enough to undergo 
negligible deflection due to the pressure difference between the chamber 
and the outside environment. A window thickness of 2.5 to 2.8 .mu.m is 
adequate for this purpose. 
The cavity seal is achieved by filling lateral etch channels 24 (see also 
FIG. 3) with additional silicon nitride after the filament has been 
released and the V-groove etched. A similar sealing technique for an 
absolute pressure sensor is discussed in S. Sugiyama, T. Suzuki, K. 
Kawahata, K. Shimaoka, M. Takigawa, and et al., "Micro-diaphragm pressure 
sensor" Tech. Digest, IEEE International Electron Devices Meeting, pp. 
184-187, 1986. 
Generally, the channels are completely filled with silicon nitride. The 
surface of the silicon nitride near the etch-channel seal may be very 
smooth, and may show no evidence of cracks. 
Once the vacuum over the lamp has been sealed, it is straightforward to 
make MOSFETS or bipolar devices on the same wafer. Therefore, the 
microlamp can easily be integrated with an active drive circuit. 
The microlamp fabrication process is shown in FIGS. 4(a) through 4(k). The 
process may start with a &lt;100&gt; silicon wafer 30. A thin layer 31 of 
low-stress silicon-nitride 0.5 .mu.m thick may be deposited on the wafer, 
and it is etched to define the edge of the silicon V-groove. (FIG. 4(a)). 
The etching is followed by a subsequent deposition of 0.7 .mu.m of 
phosphosilicate glass (PSG) 33 to provide a spacer layer between the 
filaments and the substrate; thus the silicon etchant can flow underneath 
the filament once the spacer has been etched. The PSG layer is patterned 
to conform to the silicon V groove to provide as much planarization as 
possible, thus precise alignment is needed. (FIG. 4(b). This first 
silicon-nitride film is not required, and precision alignment is not 
needed if skipped. 
The PSG etch is followed by the deposition of 0.3 .mu.m of low-stress 
silicon-nitride 34 which will form the base of the filament. After the 
deposition, the residual oxide of the silicon-nitride is removed by an HF 
dip, and 1 .mu.m of undoped polysilicon 35 is grown. The wafer is then ion 
implanted with boron to make the polysilicon conductive. The ion implant 
cam be omitted if an in-situ boron-doped polysilicon deposition is 
performed. (See FIGS. 4(c)). 
The polysilicon 35 is then plasma etched, and the residual oxide on the 
bottom silicon-nitride layer is carefully removed. A 0.3 .mu.m layer of 
low-stress silicon-nitride 36 is subsequently deposited to form the upper 
part of the filament seal. (FIG. 4(d)). Notice that it is essential to 
remove the residual oxide of the bottom nitride layer. If the residual 
oxide is removed, both silicon-nitride layers (top and bottom) will bond. 
The bond is believed enhanced by the presence of boron from the 
polysilicon filament. The top and bottom silicon-nitride layers are next 
patterned to form a silicon-nitride coating around the filament 37 so it 
is completely sealed. (FIG. 4(e)). 
A thick layer 38 of PSG 3 .mu.m thick is then deposited to form a glass 
mesa on top of the filament 37 (FIG. 4(f)). The wafers are heated at 
1050.degree. C. for 30 minutes to activate the filament dopants and to 
reflow the thick PSG glass. After the reflow, the PSG mesa is etched in 
buffered HF. The reflow step is necessary to achieve a satisfactory etch 
of the PSG mesa. If the reflow step is not performed, the wet etchant 
penetrates the oxide at the interface between the filament and the PSG, 
thus creating long channels inside the mesa. 
After the mesa is formed, a subsequent deposition of 0.8 .mu.m of PSG is 
performed to form the lateral etching channel. (FIG. 4 (g)) . The height 
of this channel can be reduced. The etching channels may be 0.15 .mu.m 
high. The etching channel glass is also removed in some regions to provide 
an anchor for the silicon-nitride window. 
After the PSG is etched, any residual oxide on the nitride regions is 
carefully stripped, and a layer 40 of 1 .mu.m of low-stress 
silicon-nitride is deposited. (FIG. 4(h)). This layer represents 
approximately one half of the thickness of the silicon-nitride window. The 
nitride layer is patterned and etched on the periphery of the V groove, 
down to the PSG of the etching channel 41. These openings on the nitride 
are the etching holes leading to the etching channels through which the 
sacrificial PSG and silicon substrate will be etched. 
After the etch holes have been made, the wafer is immersed in concentrated 
HF for 2.5 minutes to remove the PSG under the silicon-nitride window and 
underneath the filament. (FIG. 4(i) The wafers are then immersed in a hot, 
concentration of KOH for 90 minutes to etch the V groove in the silicon 
substrate. 
The wafers or samples are subsequently cleaned, and any residual oxide is 
removed from the silicon-nitride using HF. An additional layer 42 of 
silicon-nitride is then deposited, filling the etching holes and 
hermetically sealing the cavity containing the filament. (FIG. 4(j)). 
After this step, the contact holes are opened, and the wafers are 
metalized and sintered. (FIG. 4(k)). 
A more detailed description of the microlamp fabrication process is 
described in the attached Appendix I: 
The vacuum-sealed micromachined-silicon infrared source 10 can easily be 
integrated with MOS or bipolar circuits. The technology demonstrated can 
be extended to also build a micromachined vacuum tube. 
Stoichiometric silicon nitride is transparent to radiation with wavelengths 
between 0.28 and 8 .mu.m. The low-stress nitride window is not 
stoichiometric having a composition of Si.sub.1.0 N.sub.1.1 and a 
refractive index of 2.4. FIG. 5 shows the optical transmittance of a 1.3 
.mu.m-thick low-stress silicon-nitride membrane measured using a FTIR 
spectrophotometer. The oscillatory nature of the transmittance is caused 
by interference in the thin membrane. The nitride is transparent between 
0.5 to 8 .mu.m; hence it transmits most of the radiation emitted by the 
incandescent filament. The increase of the lower wavelength absorption 
edge in the low-stress nitride compared to that of the stoichiometric 
nitride is expected because of the excess silicon in the film. 
The quality of the nitride seal was tested as follows. First, the low bias 
I-V curves of a sealed device were measured inside a vacuum system at both 
atmospheric pressure and at 5 .mu.T. Then the silicon-nitride window was 
punctured using a fine probe, and the measurements were repeated. FIG. 6 
shows the I-V curves for both the sealed and punctured devices. For the 
sealed microlamps (data points (a) and (b)), there is no dependence in the 
I-V curves on vacuum system pressure. For the punctured devices, however, 
the characteristics (data points (c) and (d)) are strongly dependent on 
vacuum-system pressure. The similarity in the dependence of data points 
(a), (b), and (d) indicates that the background pressure in the sealed 
devices is a good vacuum. 
FIG. 7 shows the electrical characteristics of a microlamp device with a 
polysilicon filament 350 .mu.m long and 5 .mu.m wide. Initially, the 
device resistance increased since polysilicon has a positive TCR. At 
higher bias, there is a kink point P where the resistance actually 
decreases. At this point, the polysilicon filament may have been heated 
sufficiently to cause thermal breakdown. For voltages higher than those at 
point P in FIG. 7, the electrical characteristics are irreversible, and 
the device is typically not operated in this region. 
FIG. 8 shows the optical power of a microlamp as a function of the applied 
bias measured with an optical pyroelectric detector. The point P 
corresponds to the kink in the I-V characteristic. The radiated power 
emitted by the incandescent filaments is in the order of .mu.W and clearly 
visible to the naked eye. The power needed to reach visible incandescence 
is approximately 5 mW for a 510.times.5.times.1 .mu.m.sup.3 device. 
The time for the filament to cool down from its temperature at 
incandescence to room temperature has been measured. This time, easily 
observed by monitoring the near-zero-bias resistance after power is 
removed from the lamp, depends on the filament length and is typically 
several ms (for the full transient) for microlamps exceeding 300 .mu.m in 
length. 
Microlamps with polycrystalline-silicon filaments have been built which 
generate wideband visible and IR light. Typical microlamps operate at 5V 
and 3mW of power. The measured radiant power is in the order of 
microwatts. 
The use of polycrystalline silicon for the microlamp filament permits the 
structure to be incorporated in an IC process that is nearly conventional 
(the low-stress silicon nitride being exceptional). With the incorporation 
of other metalizations, such as tungsten or tantalum, filaments capable of 
higher-temperature operation are potentially able to be produced by this 
technique. Microlamps with these filaments can be expected to operate with 
much higher optical efficiencies. The IC-processed microlamps, which can 
be operated in liquids, have many potential applications as wideband 
infrared and visible radiation sources. 
Although demonstrated for the purpose of building microlamp, the 
fabrication sequence described here can also be used to provide a 
microvacuum housing for other structures on a silicon substrate. The 
microlamp of the present invention has a number of applications. For 
example, in chemical analysis, the wideband spectrum of the lamp is useful 
to analyze the light absorption of samples. Other potential uses are in 
displays, infrared scene generation, and the calibration of photosensors. 
Although certain embodiments of the invention have been described herein in 
detail, the invention is not to be limited only to such embodiments, but 
rather only by the appended claims. 
APPENDIX I 
1. PREATION 
1.1 Wafer Selection : p-type wafer, 18-22 .OMEGA.-cm, &lt;100&gt; 
2. INITIAL SILICON NITRIDE DEPOSITION 
2.1 Standard clean wafers 
1. Piranha cleaning (H.sub.2 SO.sub.4 :H.sub.2 O.sub.2, 5:1for 10 min. 
-sink8 
2. 3 DI water rinses, 1 min. each--sink8 
3. Wafer drying 
4. Steps 1, 2 on sink6, rinse to 8 M.OMEGA.-cm 
5. Surface oxide removal (mandatory here) (H.sub.2 O.sub.2 :HF, 25:1) until 
hydrophobic, rinse to 12 M.OMEGA.-cm - sink6 
6. Wafer drying 
2.2 Deposit 0.5 .mu.m of low-stress silicon nitride - program SNITC.V, 
SiCl.sub.2 H.sub.2 =75 sccm, NH.sub.3 =15 sccm at a pressure of 300 mT and 
a temperature of 835.degree. C. The deposition rate is 3.7 nm/min. 
T.sub.dep =136 min. 
3. INITIAL SILICON NITRIDE DEFINITION (INE) 
3.1 Standard clean wafers 
3.2 Standard lithography--layer INE 
1. Dehydration bake--VWR oven, 20 min. @ 120.degree. C. 
2. HMDS--2 minutes sink5 
3. Photoresist spinning and pre-bake. - Eaton program 10, Kodak 820, 4800 
rpm. 30 sec., 120.degree. C. 60 sec. 
4. Exposure--GCA wafer stepper, T.sub.exp =0.153s 
5. Develop.--MTI program 1, Kodak 932:H.sub.2 O=1:1, 60 sec. 
6. inspection 
7. Photoresist descum--Technics-c, 1 min. O.sub.2 plasma, 300 mT @ 50 W. 
8. Hard bake--VWR oven, 120.degree. C. for 20 min. 
4. INITIAL SILICON NITRIDE ETCH 
4.1 Plasma etch - LAM2, P=700W, R=3.7 nm/sec., T.sub.etch =136 sec. 
4.2 Standard resist stripping: plasma ash in Technics-c : O.sub.2, 300W, 
300 mT, 10 min. 
5. SER 1 PSG DEPOSITION 
5.1 Standard clean wafers--sink8, sink6 
5.2 PSG deposition, T.sub.ex =0.7 .mu.m, program SDOLTOD, tylan12, 
T.sub.dep =36 minutes. 
6. SER 1 DEFINITION (LTO1) 
6.1 Standard lithography layer LTO1 
7. SER I PSG ETCH 
7.1 PSG etch in fresh 5:1 BHF for 45 sec. 
7.2 Photoresist removal and piranha clean. 
8. SILICON NITRIDE DEPOSITION 
8.1 Standard wafer clean--sink8, sink6 : 1:25 HF dip, 10 sec. 
8.2 Low-stress silicon nitride deposition : tylan9, program SNITC.V, 
SiCl.sub.2 H.sub.2 =70 sccm, NH.sub.3 =15 sccm, at 835.degree. C., P=300 
mT. The target thickness is 300 nm. T.sub.dep =1 hr. 
9. POLYSILICON DEPOSITION 
9.1 Clean wafers--sink8. 1:10 HF dip 30 sec (mandatory) 
9.2 after nitride deposition, deposit 1 .mu.m of undoped polysilicon. 
tylan11, program SUNPOLYA, T.sub.dep =86 min. 
10. POLYSILICON ION IMPLANTATION 
10.1 Blanket implant, B.sup.11, 50 KeV. dose=1.1.times.10.sup.16 cm-2. 
11. POLYSILICON DEFINITION (NP) 
11.1 Standard clean. 1:25 HF dip, 30 sec. (or until dewets) 
11.2 Standard lithography--layer NP 
12. POLYSILICON ETCH 
12.1 Plasma etch poly in LAM1. T.sub.etch .apprxeq. 150 sec. Do 50% 
overetch to remove any polysilicon rings. Test the ring removal using the 
iv station 
12.2 Photoresist removal. 
13. SILICON NITRIDE DEPOSITION 
13.1 Standard wafer clean--sink8, sink6 : 1:10 HF dip, 30 sec. (mandatory) 
13.2 Low-stress nitride deposition : tylan9, program SNITC,V, SiCl.sub.2 
H.sub.2 =70 sccm, NH.sub.3 =15 sccm, at 835.degree. C., P=300 mT. The 
target thickness is 0.3 .mu.m. T.sub.dep .apprxeq. 1 hr. 
14. SILICON NITRIDE SANDWICH DEFINITION (NIT2) 
14.1 Standard lithography--layer NIT2, 2 photoresist layers 
15. SILICON NITRIDE SANDWICH ETCH 
15.1 Silicon nitride etch, technics-c, P=35W, SF.sub.4 =13 sccm, 
He.sub.2 =21 SCCM. The etching rate is approximately 25-30 nm/min. 
T.sub.etch .apprxeq. 25 min. 
15.2 Photoresist removal. 
16. SER II PSG DEPOSITION 
16.1 Standard clean 
16.2 Deposit 3.0 .mu.m of phosphosilicate glass. Program SDOLTOD, T.sub.dep 
=3.5 hrs. Use a blank control wafer to determine the oxide thickness. 
17. SER II PSG REFLOW AND IMPLANT ACTIVATION 
17.1 Transfer wafers directly to tylan7. Program N2ANNEAL, 30 min @ 
1050.degree. C. 
18. SER II PSG DEFINITION 
18.1 Standard lithography--2 layers of photoresist 
19. SER II PSG ETCH 
19.1 Oxide etch--5:1 BHF, sink8, T.sub.etch 10-15 min. 
19.2 Photoresist removal 
20. OXIDE PEDESTAL PSG DEPOSITION 
20.1 Standard clean wafers - sink8, sink6 
20.2 PSG deposition, T.sub.ex =0.8 .mu.m. Program SDOLOTD, T.sub.dep =58 
min. Expected thickness is 0.8 .mu.m. 
20. OXIDE PEDESTAL DEFINITION (NPED) 
21.1 Standard lithography.--Layer NPED, 2 layers (mandatory), overexpose 
200% 
22. OXIDE PEDESTAL ETCH 
22.1 Oxide etch, 5:1 BHF, T.sub.etch .apprxeq. 90 sec. 
23. SILICON NITRIDE CAP DEPOSITION 
23.1 Standard clean wafers--30 sec 10:1 HF dip (mandatory). 
23.2 Low-stress silicon nitride deposition, SiCl.sub.2 H.sub.2 =70 sccm, 
NH.sub.3 =15 sccm @835.degree. C., P=300 mT. The target thickness is 1 
.mu.m. T.sub.dep .apprxeq. 3.5 hrs. 
24. SILICON NITRIDE CAP DEFINITION 
24.1 Standard lithography--2 photoresist layers, overexpose 200%, 
overdevelop 10 sec. using hand development 
25. SILICON NITRIDE CAP ETCHING 
25.1 Silicon nitride etch LAM2, P=85OW, T.sub.etch =2.5 min, R =0.6 
.mu.m/min 
25.2 Photoresist removal 
26. OXIDE SER ETCHING 
26.1 Standard wafer cleaning 
26.2 Oxide etching, concentrated (49%) HF for 2.5 min 
27. SILICON ETCH 
27.1 10:1 BHF dip, 1 min 
27.2 KOH anisotropic etch @ 80.degree. C., T.sub.etch .apprxeq. 90 min 
28. SILICON NUTRIDE VACUUM SEALING DEPOSITION 
28.1 Standard wafer cleaning - 10:1 HF dip, 30 sec. 
28.2 Low-stress silicon nitride deposition, SiCl.sub.2 H.sub.2 =70 sccm, 
NH.sub.3 =15 sccm @ 835.degree. C., P=300 MT. The target thickness is 1 
.mu.m. T.sub.dep .apprxeq. 3.5 hrs. 
29. CONTACT HOLE DEFINITION 
29.1 Standard lithography--3 layers of photoresist 
30. CONTACT HOLE ETCH 
30.1 Silicon nitride etch, LAM2, P=85OW, R=0.6 .mu.m/min. Measure the 
conductivity of the contact opening in the iv probe to detect the 
endpoint, T.sub.etch .apprxeq. 4.5 min 
31. METALIZATION 
31.1 Standard wafer cleaning--10:1 HF dip, 30 sec. 
31.2 Dehydrate water--120.degree. C. oven 
31.3 Residual oxide removal, LAM2, P=700W, T.sub.etch =15 sec. 
31.4 Metal sputtering--cpa, Al-Si target, P=4.5 kW, Ar=120 sccm, P=6mT, 
track speed=8 .mu.cm/min. Expected thickness is 0.9 .mu.m. 
32. METAL DEFINITION (NM) 
32.1 Standard lithography--Layer NM, 2 photoresist layers, Texp .apprxeq. 
25% less. 
33. METAL ETCH 
33.1 Al etch--Sink8, Al etchant type A. (H.sub.3 PO.sub.4 :HC.sub.3 O.sub.2 
H) .apprxeq. 
45.degree. C., T.sub.etch .apprxeq. 90 sec. 
33.2 Silicon etch dip--15 sec. 
34. SINTERING 
34.1 Program SINT460, tylan 13, 450.degree. C. for 20 min. 
35. END PROCESS