U.S. patent application number 09/527095 was filed with the patent office on 2003-05-01 for method of preparing silicon-on-insulator substrates particularly suited for microwave applications.
Invention is credited to De Los Santos, Hector J., Lin, Yu-Hua Kao.
Application Number | 20030082886 09/527095 |
Document ID | / |
Family ID | 24100073 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030082886 |
Kind Code |
A1 |
De Los Santos, Hector J. ;
et al. |
May 1, 2003 |
METHOD OF PREPARING SILICON-ON-INSULATOR SUBSTRATES PARTICULARLY
SUITED FOR MICROWAVE APPLICATIONS
Abstract
A method of directly and indirectly bonding a microwave
substrate 14 and a silicon substrate 12 is described. The method
for directly bonding a silicon substrate includes the steps of
cleaning the microwave substrate and cleaning the silicon
substrate. Then, the microwave substrate and the silicon substrate
are stacked together. The stack is placed in a furnace. The
temperature of the furnace is increased to a predetermined
temperature at a predetermined rate. The temperature of the furnace
is reduced at a second predetermined rate. The method of indirectly
bonding includes sputtering a silicon dioxide layer on the
microwave substrate and silicon substrate prior to placing them
together.
Inventors: |
De Los Santos, Hector J.;
(Del Aire, CA) ; Lin, Yu-Hua Kao; (Oak Park,
CA) |
Correspondence
Address: |
JOAN A. ARTZ
ARTZ & ARTZ
28333 TELEGRAPH ROAD, SUITE 250
SOUTHFIELD
MI
48034
US
|
Family ID: |
24100073 |
Appl. No.: |
09/527095 |
Filed: |
March 16, 2000 |
Current U.S.
Class: |
438/404 ;
257/E21.122; 257/E21.279; 257/E21.567; 438/107; 438/406;
438/455 |
Current CPC
Class: |
H01L 21/76251 20130101;
H01L 21/2007 20130101; H01L 21/31612 20130101 |
Class at
Publication: |
438/404 ;
438/455; 438/107; 438/406 |
International
Class: |
H01L 021/44; H01L
021/76 |
Claims
What is claimed is:
1. A method of bonding a microwave substrate to a silicon substrate
comprises the steps of: cleaning the microwave substrate; cleaning
the silicon substrate; stacking the microwave substrate and the
silicon substrate together to form a stack; placing the stack in a
furnace; increasing the temperature of the furnace to a
predetermined temperature at a predetermined rate; and decreasing
the temperature of the furnace to at a second predetermined
rate.
2. A method as recited in claim 1 wherein the step of cleaning the
microwave substrate comprises the steps of: immersing the microwave
substrate in a substrate cleaner; and rinsing the microwave
substrate in water.
3. A method as recited in claim 2 wherein said substrate cleaner
comprises Burmar 922.
4. A method as recited in claim 1 further comprising the steps of
drying the substrate with nitrogen gas.
5. A method as recited in claim 1 wherein the step of cleaning the
microwave substrate comprises the steps of: immersing the microwave
substrate in acetone; and rinsing the microwave substrate.
6. A method as recited in claim 1 wherein the step of cleaning the
microwave substrate comprises the steps of: immersing the microwave
substrate in isopropyl alcohol; and rinsing the microwave
substrate.
7. A method as recited in claim 1 further comprising the step of
purging the furnace with nitrogen.
8. A method as recited in claim 1 wherein the step of cleaning the
silicon substrate comprises the steps of: removing organic
contamination from the silicon substrate; and removing inorganic
contamination from the silicon substrate.
9. A method as recited in claim 8 wherein the step of removing
organic contamination form the substrate comprises the steps of:
cleaning the silicon substrate in acetone; rinsing the substrate in
water; cleaning the silicon substrate in methanol; and rinsing the
substrate in water.
10. A method as recited in claim 8 wherein the step of removing
inorganic contamination from the silicon substrate comprises:
cleaning the silicon substrate in a hydochloric acid/hydrogen
peroxide mixture; rinsing the silicon substrate in water; cleaning
the silicon substrate in hydrofluoric acid; rinsing the silicon
substrate in water; cleaning the silicon substrate in a hydrogen
peroxide/ammonium hydroxide mixture; and rinsing the silicon
substrate in water.
11. A microwave and silicon substrate assembly formed according to
the method of claim 1.
12. A method as recited in claim 1 further comprising the steps of
sputtering a SiO.sub.2 coating on said microwave substrate prior to
the step of stacking.
13. A method as recited in claim 1 further comprising the steps of
sputtering a SiO.sub.2 coating on said silicon substrate prior to
the step of stacking.
14. A method as recited in claim 1 wherein the first predetermined
rate and the second predetermined rate are about 1 degree C. per
minute.
15. A method as recited in claim 1 wherein the microwave substrate
comprises alumina.
16. A method of forming a microwave circuit assembly comprising:
bonding a silicon substrate to a microwave substrate to form an
assembly; and micromachining the assembly.
17. A method as recited in claim 16 wherein the step of bonding
comprises bonding the silicon substrate to a microwave substrate
comprises the step of heat-treating the silicon substrate and the
microwave substrate.
18. A method as recited in claim 17 wherein the step of heat
treating comprises stacking the microwave substrate and the silicon
substrate together to form a stack; placing the stack in a furnace;
increasing the temperature of the furnace to a predetermined
temperature at a predetermined rate; and decreasing the temperature
of the furnace to at a second predetermined rate.
19. A method as recited in claim 17 wherein prior to the step of
heat treating, performing the steps of: cleaning the microwave
substrate; and cleaning the silicon substrate.
20. A method of bonding an alumina substrate to a silicon substrate
comprises steps of: cleaning the microwave substrate; cleaning the
silicon substrate; stacking the microwave substrate and the silicon
substrate together to form a stack; placing the stack in a furnace;
increasing the temperature of the furnace to a predetermined
temperature at a predetermined rate; and decreasing the temperature
of the furnace to a second predetermined rate.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to microwave
substrates, and more particularly, to a method for bonding
microwave substrates and silicon substrates that leverages silicon
integrated circuit fabrication and silicon micromachining
technologies.
BACKGROUND OF THE INVENTION
[0002] Microwave communication devices such as satellites employ
hybrid microwave circuits such as switching matrices and phased
array antennas.
[0003] The microwave circuits can include miniature
electromechanical (MEM) switches. Commonly, silicon-on-insulator
techniques are used in integrated circuits. One method for forming
silicon-on-insulator substrates is by the implantation of oxygen
ions into a silicon wafer. In this process, oxygen ions are
injected deep into the silicon wafer and the wafer is annealed
under high temperature to form the buried SiO.sub.2 layer. Another
method for forming silicon-on-insulator devices uses a first
silicon wafer onto which the desired structures are etched. Then,
in a post processing step, a second silicon wafer is bonded to the
first wafer to enclose the structures. This post processing
technique adds to the cost of the devices.
[0004] Another method for silicon-on-insulator bonding for two
silicon wafers is to use a silicon dioxide bond between two silicon
layers. Silicon dioxide is used to bond the two silicon wafers
together.
[0005] One drawback to the above mentioned silicon-on-insulator
methods is that they are intended for the high volume integrated
circuit industry. The silicon-on-insulator wafers are extremely
pure and thus very expensive. However, because MEMS technology has
been highly developed, the cost of processing circuits using this
technology is reduced.
[0006] It is commonly thought, however, that silicon-based MEMS
fabrication processes are not amenable to the fabrication of
microwave devices. Silicon has poor microwave properties and thus
it was previously thought that the silicon substrate must be
removed. Such devices are flip chip bonded to an insulating device
where the silicon substrate can then be etched away. This adds to
processing steps and increases the cost of the device.
SUMMARY OF THE INVENTION
[0007] The present invention therefore provides a methods for
bonding a silicon substrate to a microwave substrate that will
enable the use of mature silicon MEMS fabrication technology.
[0008] The present invention provides two alternative techniques to
bonding silicon with a microwave substrate. The first method is a
direct fusion bonding method wherein silicon is directly bonded
with a microwave substrate. In the second method, an SiO.sub.2
layer is deposited on the silicon and microwave substrate and the
bond is formed between the two SiO.sub.2 layers.
[0009] In one aspect of the invention, the direct fusion technique
includes the steps of:
[0010] cleaning the microwave substrate;
[0011] cleaning the silicon substrate;
[0012] stacking the microwave substrate and the silicon substrate
together to form a stack;
[0013] placing the stack in a furnace;
[0014] increasing the temperature of the furnace to a predetermined
temperature at a predetermined rate; and
[0015] decreasing the temperature of the furnace to at a second
predetermined rate.
[0016] In the indirect method, prior to the steps of stacking the
microwave substrate and the silicon substrate, an SiO.sub.2 layer
is sputtered onto the silicon layer and the microwave
substrate.
[0017] One advantage of the invention is that the present invention
enables the fabrication of low insertion loss microwave circuits
and MEMS devices on silicon-based technology. The present invention
also alleviates the need for post processing of the silicon wafers
after MEMS processing.
[0018] Other objects and features of the present invention will
become apparent when viewed in light of the detailed description of
the preferred embodiment when taken in conjunction with the
attached drawings and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a perspective view of a silicon substrate directly
bonded to a microwave substrate.
[0020] FIG. 2 is a flow chart of a process used to form the silicon
and microwave substrate assembly of FIG. 1.
[0021] FIG. 3 is a silicon substrate indirectly bonded to a
microwave substrate through layers of SiO.sub.2.
[0022] FIG. 4 is a flow chart of a process for forming the silicon
and microwave substrate assembly of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] Referring now to FIG. 1, an assembly 10 having a silicon
substrate 12 directly bonded to a microwave substrate 14 as
illustrated. Silicon substrate 12 may be less pure than that
commonly used in the integrated circuit industry since the present
application is intended for microwave circuits. Microwave substrate
14 is preferably an alumina (Al.sub.2O.sub.3). However, other
microwave substrates such as quartz may also be used.
[0024] Referring now to FIG. 2, a method for direct fusion bonding
of silicon substrate 12 to microwave substrate 14 is illustrated.
In step 16, the silicon substrate 12 and microwave substrate 14 are
cleaned. The process is described with respect to an alumina
microwave substrate. The preferred method for cleaning the alumina
substrate is hereinafter described. The substrate is preferably
immersed in a cleaner such as Burmar 922 cleaner at 60 to 95 C for
three to five minutes. The alumina substrate is then removed and
rinsed in hot tap water for five minutes. The substrate is then
rinsed in cold tap water for five minutes. Thereafter, the alumina
substrate is rinsed in deionized water for five minutes. Then, the
alumina substrate is immersed in clean acetone for one minute and
rinsed in deionized water again for five minutes. The alumina is
soaked in isopropyl alcohol for one minute and then rinsed in
deionized water again for five minutes. Finally, the alumina
substrate may be dried by blow drying the substrate with nitrogen
gas. Although the above process is specific with respect to times
and temperatures, these times and temperatures are not meant to be
limiting since those of ordinary skill in the art would recognize
that variations in times and temperatures and materials may
exist.
[0025] The silicon substrate 12 should be cleaned for both
inorganic contamination and organic contamination. It is important
that the instruments and beakers for cleaning the silicon are clean
as well. For cleaning a beaker, preferably a mixture of nitric and
sulfuric acid are used for about five minutes. The beaker is then
rinsed in deionized water. If tweezers or other instruments are
used it is preferred that they are cleaned in acetone in an
ultrasonic cleaner then cleaned in methanol in an ultrasonic
cleaner.
[0026] To clean the surface of the silicon substrate, a cotton swab
in acetone is wiped on the surface for five minutes. The silicon
wafer is then soaked in acetone for ten minutes and rinsed in
deionized water for four minutes. Then, the silicon substrate is
placed in methanol at 40 C for about ten minutes and placed in a
deionized water flow for four minutes. The silicon wafer is then
blow dried with nitrogen.
[0027] To remove inorganic material from the surface of the wafer,
a water/hydrochloric acid/hydrogen peroxide mixture in the ratio of
5:1:1 at 70 degrees C. is used to soak the wafer for about ten
minutes. The silicon wafer is then placed in a deionized water flow
for about four minutes. The silicon wafer is then placed in a water
and hydrofluoric acid mixture dip for about 15 seconds. The
water/hydrofluoric acid mixture ratio is about 50:1. After the
water/hydrofluoric acid mix, the silicon wafer is then placed in a
deionized water flow for about four minutes. The silicon wafer is
then placed in a water/hydrogen peroxide/ammonium hydroxide
solution at 70 degrees C. The ratio of water/hydrogen
peroxide/ammonium hydroxide solution is 5:1:1. Preferably, the
silicon wafer is placed in the bath for about ten minutes. After
the ten minutes, the silicon wafer is placed in a deionized water
flow for about four minutes. Thereafter, the silicon wafer is blow
dried with nitrogen.
[0028] In step 18, the clean silicon substrate and microwave
substrate are placed together face to face prior to bonding.
[0029] In step 20, the stack is placed into a furnace. Preferably,
the furnace is purged with nitrogen gas. In one method performed
according to the present invention, nitrogen at a rate of 10 cubic
feet per hour was introduced into the furnace for 20 minutes.
[0030] The temperature of the nitrogen atmosphere furnace is ramped
up at a predetermined rate. In one performed method, the heat
treatment was performed for 12 hours. The maximum temperature
achieved was 1200 degrees C. and was achieved at a ramp up rate of
1 centimeter per minute.
[0031] In step 24, the temperature is ramped down to ambient
temperature at the same rate of 1 centimeter per minute.
[0032] During the heat treating process, the silicon substrate
becomes bonded to the microwave substrate by direct fusion bonding.
That is, the silicon substrate and microwave substrate become one
layer due to migration of molecules.
[0033] Referring now to FIG. 3, an assembly 10 having a silicon
substrate 12 and a microwave substrate 14 is illustrated. In this
example, a layer 26 of SiO.sub.2 is sputtered upon silicon
substrate 12 and microwave substrate 14 before placing them
together. This process forms an indirect bond between silicon
substrate 12 and microwave substrate 14.
[0034] Referring now to FIG. 4, each of the steps of the process
illustrated in FIG. 4 is similar to that in FIG. 3 with the
addition of step 28. Therefore, the similar steps are designated
with an "A" thereafter.
[0035] In step 16A, the wafers are preferably cleaned with the same
process described in step 16 above.
[0036] In step 28, a layer of SiO.sub.2 is sputtered upon silicon
substrate 12 and microwave substrate 14. The SiO.sub.2 may, for
example, be about 6000 .ANG.. The wafers with the SiO.sub.2 layer
are placed together so their SiO.sub.2 layers are in contact in
step 118A. In step 20A, 22A and 24A, the stack is thus heated in a
furnace as in steps 22 and 24. Preferably, the same ramping rate
temperature and time are used.
[0037] After the assembly is formed according to the processes
shown in FIGS. 2 and 4, the assembly is subjected to
microelectromechanical fabrication processes. In such processes,
the silicon does not have to be removed. The devices formed may be
used for a variety of microwave applications such as switching
devices.
[0038] While particular embodiments of the invention have been
shown and described, numerous variations and alternate embodiments
will occur to those skilled in the art. Accordingly, it is intended
that the invention be limited only in terms of the appended
claims.
* * * * *