Bonding of MEMs materials is carried out using microwave. High microwave absorbing films are placed within a microwave cavity, and excited to cause selective heating in the skin of the material. This causes heating in one place more than another. Thereby minimizing the effects of the bonding microwave energy

Barmatz, Martin B.

Budraa, Nasser K.

Jackson, Henry W.

Mai, John D.

Pike, William T.

NASA Technical Reports Server

(12) United States Patent 
Barmatz et al. 
(io) Patent No.: 
(45) Date of Patent: 
US 6,905,945 B1 
Jun. 14,2005 
(54) MICROWAVE BONDING OF MEMS 
COMPONENT 
Inventors: Martin B. Barmatz, La Cresenta, CA 
(US); John D. Mai, Pasadena, CA 
(US); Henry W. Jackson, La Verne, 
CA (US); Nasser K. Budraa, 
Riverside, CA (US); William T. Pike, 
London (GB) 
(75) 
(73) Assignee: California Institute of Technology, 
Pasadena, CA (US) 
Subject to any disclaimer, the term of this 
patent is extended or adjusted under 35 
U.S.C. 154(b) by 0 days. 
( * ) Notice: 
(21) Appl. No.: 10/198,656 
(22) Filed: Apr. 20, 2000 
Related U.S. Application Data 
(60) Provisional application No. 601130,842, filed on Apr. 22, 
1999. 
(51) Int. Cl? ................................................ HOlL 21/30 
(52) U.S. C1. ................... 438/455; 4381456; 2281180.22 
(58) Field of Search ................................. 4381106-109, 
180.1, 180.21, 180.22; 2571777, 778 
4381118, 119, 455-459, 612; 2281179.1, 
(56) References Cited 
U.S. PATENT DOCUMENTS 
5,172,852 A * 1211992 Bernardoni et al. ... 2281180.21 
5,346,857 A * 911994 Scharr et al. .......... 2281180.22 
5,603,795 A * 211997 Paulauskas et al. ...... 1561272.4 
5,846,854 A 1211998 Giraud et al. 
5,985,693 A * 1111999 Leedy ........................ 4381107 
6,054,693 A * 412000 Barmatz et al. ............ 2191678 
6,312,548 B1 * 1112001 Fathi et al. .............. 1561275.1 
FOREIGN PATENT DOCUMENTS 
JP 11121531 A * 411999 ........... HOlL121160 
* cited by examiner 
Primary Examine rxa r l  Whitehead, Jr. 
Assistant Examiner-Jennifer M Dolan 
(74) Attorney, Agent, or F i r m C i s h  & Richardson P.C. 
(57) ABSTRACT 
Bonding of MEMs materials is carried out using microwave. 
High microwave absorbing films are placed within a micro- 
wave cavity, and excited to cause selective heating in the 
skin of the material. This causes heating in one place more 
than another. Thereby minimizing the effects of the bonding 
microwave energy. 
10 Claims, 4 Drawing Sheets 
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U S .  Patent Jun. 14,2005 Sheet 3 of 4 US 6,905,945 B1 
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US 6,905,945 B1 
1 
MICROWAVE BONDING OF MEMS 
COMPONENT 
CROSS-REFERENCE TO RELATED 
APPLICATIONS 5 
This application claims benefit of U.S. Provisional Appli- 
cation No. 601130,842, filed Apr. 22, 1999. 
STATEMENT AS TO FEDERALLY-SPONSORED 
RESEARCH 10 
The U.S. Government may have certain rights in this 
invention pursuant to Grant No. 7-1407 awarded by NASA. 
BACKGROUND 
1s 
Microelectrical mechanical or “MEMS” systems allow 
formation of physical features using semiconductor materi- 
als and processing techniques. The techniques enable the 
physical features to have relatively small sizes. A MEMS 
structure often requires two separated parts to become 20 
bonded. This can be difficult since too much heat can 
overheat and destroy delicate components. 
SUMMARY 
The present application teaches bonding MEMS stmc- 2s 
tures using selective heating feature of microwave energy. A 
low temperature, low pressure wafer bonding, can be 
effected e.g. in a MEMS environment. 
BRIEF DESCRIPTION OF THE DRAWINGS 30 
These and other aspects will now be described in detail 
FIG. 1 shows a view of silicon substrates in a chamber; 
FIG. 3 shows a system for correcting for non-uniform 
FIG. 4 shows a heating protection element for a semi- 
with respect to the accompanying drawings, wherein: 
FIG. 2 shows a view of a silicon wafer; 3s 
heating; 
conductor wafer; 
FIG. 5 shows a high speed bonding system; and 40 
FIG. 6 shows a system for processing a large sized wafer. 
DETAILED DESCRIPTION 
Bonding of MEMS structures has been carried out in the 4s 
past using anodic bonding, thermal compression, or 
adhesives, such as polymer adhesives, between the layers. 
Other techniques have also been used. Each of these tech- 
niques has certain advantages and also its own host of 
limitations. 
The present application discloses a way of bonding sub- 
strate using films such as a metal with a large imaginary 
dielectric constant E”. Microwave energy causes heating 
effects predominately within the skin depth of such films. 
This selective heating causes the skin depth in the metal 
film to be heated more than the parts of the metal film that 
are not within the skin depth. This can be very useful when 
bonding together materials in which the metal films are thin, 
e.g., of comparable thickness to the skin depth. The films can 60 
be less than 10 pm, and excellent effects are obtained when 
the films are less 1 pm. The metal is typically attached to a 
substrate, e.g., a silicon substrate. The silicon substrate may 
include semiconductor materials, e.g. materials which can 
An embodiment is shown in FIG. 1. This embodiment 
discloses bonding of two silicon substrates, each with two 
so 
The skin depth can be, for example, about 1 pm. 5s 
be sensitive to heat. 65 
2 
metal films, to each other. The metal is a high E” material 
while the silicon substrate lower E” material. The MEMS 
device is placed in a single mode cavity 110. Microwave 
radiation 120 is introduced into the cavity 110. The micro- 
wave radiation 120 selectively heats the materials in the 
cavity. Most of the heating effect from the microwave is 
deposited in the skin depth 101 of the metal 102. Note that 
the skin depth can be smaller or larger than the thickness of 
the metal film. This effectively concentrates the deposition 
energy in that skin depth causing the thin metal film to 
rapidly heat and melt. Bonding occurs relatively quickly, 
with minimal heating of the substrate 104. Of course, the 
substrate 104 is heated in the area of the gold 102 when the 
heat escapes from the heated gold. However, heating in the 
area 108 will generally be minimal due to the large heat 
capacity of the substrate 104. 
Moreover, the bonding process time can be short, allow- 
ing for reduced diffusion of the metallization 102 into the 
silicon 104. 
The microwave bonding can be carried out with no 
pressure or low pressure. This means that mechanically- 
induced stresses can be minimized. 
As shown in FIG. 1, micromachining techniques may 
form a small cavity 130, e.g. of 0.1 to 8 microns in size. By 
surrounding this cavity with a continuous metal film, the 
heating can hermetically seal the cavity. This technique can 
lead to obtain leak rates at equal to or better than 3x109 
atm-cc/s. The microwave cavity 110 can be evacuated or the 
substrates to be bonded can be within a vessel such as a 
quartz tube, that is evacuated to form a vacuum around the 
substrates. 
This technique allows bonding using microwave heating 
only, requiring no pressure in the bonding area beyond the 
weight of the substrate connections. Furthermore, in a 
vacuum environment, hermetic seals can be formed where 
the pressure in the hermetic sealed cavity would not return 
to atmospheric for over one year. 
The present application uses a system disclosed herein. 
Two four-inch silicon wafers are used. One of those wafers 
is shown as 200 in FIG. 2. A mask of photoresist 205 is 
provided to )thographically define a concentric square bond 
area. 150 A of chromium is dTposited as a first layer, 
followed by deposition of 1200 A (0.12 pm) of gold as a 
second layer 220. The remaining photoresist 205 is then 
lifted off. 
The wafer is etched in a solution of ethylenediamene+ 
pyrocathecol (“EDP’) for about 80 minutes. 
This produces pits of approximately 3 mmxlOO pm deep. 
The pits are surrounded by a 2 mm wide plateau of gold on 
all sides. 
If multiple parts. are formed on the wafer, the wafer can 
then be diced to form separated parts (1021104) shown in 
FIG. 1. 
Microwave bonding is carried out, as shown in FIG. 1, in 
a cylindrical cavity 110 that may be excited by an azimuth- 
ally symmetric TM,,, mode at 2.45 GHz by a microwave 
source 122. The cavity can have a 12.7 centimeter diameter. 
The loaded Q of the empty cavity may be approximately 
2500. 
The first substrate 102 is simply placed on top of the 
second substrate 104 so that the deposited film patterns 
overlay. Microwave energy is applied in order to fuse the 
matching metallic parts on the two substrates. The high 
vacuum within the cavity in many cases is desired in order 
to form a vacuum within the cavity 130. This vacuum can 
US 6,905,945 B1 
3 4 
also avoid the formation of an underscrable a plasma during Items can be loaded onto the conveyor 510 in advance. If 
the bonding process. vacuum is desired, the entire operation shown in FIG. 5 can 
The only pressure applied comes from the wafer's weight. actually be within a vacuum. 
The wafers are optimally placed at the area of the highest FIG. 6 shows a system in which two wafers to be bonded 
magnetic field intensity, and are oriented SO their surfaces are inserted into the chamber through a slit 600 in the 
are parallel to the magnetic field. chamber. The wafers are round and are rotated together, as 
Different power-time profiles can be used. Some of these shown by the arrow 610. Each portion of the wafer that 
are high power and short times, e.g. a 300 watt pulse for 2-3 enters the chamber is heated during the time it is in the 
seconds. Others use the opposite, e.g., 30 seconds at 100 chamber. This allows simultaneous bonding at multiple 
watts or less. Different time-power profiles can be used with positions larger wafers in a relatively small chamber. 
different materials and substrate sizes and position in the According to a particular embodiment, the metallization 
620 at various positions is formed of a graded material using cavity. 
The hermetic seal in the cavity is maintained for over a metals of varying melting points, The material towards the 
year is quite good. Moreover, since the cavity can be formed 1~ end 622 has a higher melting point, while the material 
within silicon, it can be small, e& less than 5 ium in towards the end 624 has a lower melting point. The micro- 
diameter, more Preferably less than 1 ium which may be wave energy may follow the curve 626 shown in FIG. 6. 
desirable for MEMS devices. Therefore, more microwave energy is presented at the area 
The above has disclosed bonding MEMS wafers together 622 and less at the area 624. 
and forming hermetically sealed enclosures using a single 20 
mode microwave cavity. The concentration of the heat on 
the metal films join the two surfaces together without 
external pressure. The substrates temperature rise only 
slightly and due mostly to heat being transferred from the 
metal films. Metal diffusion into the silicon substrates is zs 
relatively limited because of short film required for the 
bonding. 
Different combinations of substrates and metallic layers, 
such as PlatiIlUm-titUliUm, Copper, aluminum are COntem- 
plated. 
Another embodiment is shown in FIG. 3. If the sample 
300 is very large, e.g., greater than 10% of the size of the 
microwave wavelength 310, then the microwaves may actu- 
ally induce a heat gradient along the substrate. For example, 
the microwave may have a sinusoidal shape in the cavity 3s 
shown as sinusoid 310. This would mean that the heating 
effect would be greatest at the area 302, and somewhat less 
at the area 304. A heat conducting plate 320 is added to 
either the top of the silicon wafer 300. The heat plate 320 can 
be made of, for example, a sapphire material. 
This system can avoid the uneven heating effect which 
could otherwise could not be avoided no matter where the 
sample was placed in the cavity. 
some materials may actually require one or more electronic 
shown as 400 on the silicon wafer 405. The system prefer- 
entially heats the metallizations 410, 412. The microwave 
heating may also heat the circuitry 400, especially if the so 
circuitry 400 includes metal. This system places at least one first material with a first metallic portion; 
shield element 420, 422 on the substrate surface so as to 
block the microwave energy from penetrating the substrate portion; and 
of the surface. This shield element can reduce, at least 5s to the second metallic portion without heating the 
somewhat, the heating effect of the microwave energy. semiconductor portion significantly. 
An automation system is shown in FIG. 5 .  A number of 9. Amethod as in claim 8, wherein said first portion is held 
samples, 500, 502 are placed on a conveyor element 510. 
The conveyor element can be a set of non metallic support 10. A method as in claim 9, wherein the first portion 
wires or a belt for example. The conveyor element takes 60 includes an indented portion therein, and said bonding 
each of the samples into the microwave area 520, and comprises hermetically sealing around an outside of the 
irradiates them with microwave while they are in the area. indented portion. 
After the irradiation, the samples can be removed from the 
Other modifications are contemplated, 
What is 
'. A method comprising: 
placing a first semiconductor substrate with a first metal 
part against a second semiconductor substrate with a 
second metal part; 
aligning said first metal part with said second metal part; 
and 
applying microwave energy to the first and second sub- 
strates to bond the first substrate to the second sub- 
strate. 
2. A method as in claim 1, wherein the first substrate is 
placed on top of the second substrate, and is held only by 
gravity during bonding. 
3. A method as in claim 1, wherein said bonding includes 
hermetically sealing a cavity. 
4. A method as in claim 3, wherein one of said metal 
portions includes an indented part and said hermetically 
sealing comprises hermetically sealing completely said 
40 cavity, to form a hermetically sealed cavity inside the 
indented Part. 
is: 
30 
5 .  A method as in claim 2, wherein said metal is gold. 
6. A method as in claim 5,  wherein said substrate are 
'. A method as in '3 wherein said first part has an Another embodiment shown in FIG. 4 recognizes that 45 
components such as a transistor and/or electrical leads Outer surface formed Of a with a low imaginary 
constant e". 
8. A 
Providing a semiconductor substrate having a first Portion 
providing a second material, having a second metallic 
comprising; 
and heating the component 400. This should cover about z/3 using microwave energy to bond the first metallic portion 
on the second portion by gravity only. 
area by moving the conveyor element. * * * * *  


Microwave bonding of MEMS component

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