High temperature micro-machined valve

Details High temperature micro-machined valve High temperature micro-machined valve

2001-08-08

2004-04-06

Mancene, Gene (Department: 3754)

Valves and valve actuation

Reciprocating valve

Diaphragm

C251S129010, C251S129060, C137S859000

Reexamination Certificate

active

06715733

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to micro-machined valves capable of regulating flow in micro electromechanical systems (MEMS) devices and also relates to methods of making such valves.
2. Description of the Related Art
U.S. Pat. No. 6,056,269 to Johnson et al. (the Johnson '269 patent), incorporated herein in its entirety by reference, discloses the micro-miniature valve
5
having a silicon diaphragm illustrated in FIG.
1
. The valve
5
includes a diaphragm
10
, a valve body
12
(made up of a seat substrate
15
and a base
17
), a valve seat
20
, a well or recess
30
, an orifice
40
, an inlet port
50
, an inlet channel
60
, an outlet port
70
and an outlet channel
80
. The seat substrate
15
and the diaphragm
10
are made of silicon. The base
17
is made with glass.
A recess or well
30
is formed in the seat substrate
15
by a first etch step. Inside the recess or well
30
is a valve seat
20
, formed by a second etch step. Further, a third etch step is sometimes used to align the features on the front of the seat substrate
15
to the features on the back of the seat substrate
15
. The inlet port
50
, inlet channel
60
and outlet channel
80
are formed in the seat substrate
15
via a fourth etch step. The orifice
40
and outlet port
70
are chemically etched in the interior of the valve seat
20
, by a fifth etch step, such that it extends through the seat substrate
15
and connects to the inlet port
50
and the inlet channel
60
. If a double-sided aligner is used, the third etch step can be eliminated. Therefore, depending upon whether a double-sided aligner is used or not, the same piece of silicon that makes up the valve
5
is etched either four or five times.
Since each photolithography, handling and etching step inherently has associated yield problems, a few wafers are lost at each step. Assuming that each step has an associated loss of 10% of the wafers, the total yield of valves
5
according to the method discussed above is 90% raised to the power of 4 or 5. Hence, only between 59 and 66% of the valves
5
manufactured by the process described above will be operational.
In operation, the valve
5
is opened and shut by the diaphragm
10
. Whether the diaphragm
10
is in the open or closed position is dependant on a control pressure applied on the top surface of diaphragm
10
. When the control pressure is high, the diaphragm
10
deflects onto and forms a seal with the valve seat
20
, thereby closing the valve
5
. However, when the pressure is reduced, the diaphragm
10
relaxes away from the valve seat
20
and opens the valve
5
.
When the diaphragm
10
is relaxed away from the valve seat
20
, gas or liquid can pass into the inlet port
50
, through the inlet channel
60
and out of the orifice
40
. Then, the gas or liquid can flow into the recess
30
and can drain through the outlet port
70
, the outlet channel
80
and out of the valve
5
. When the diaphragm
10
is positioned directly atop the valve seat
20
, the seal created prevents gas or liquid from flowing out of the orifice
40
. Hence, neither gas nor liquid can escape via the outlet channel
80
and the valve
5
is in a closed position. In some instances, the direction of flow can be reversed, making the inlet an outlet and vice versa
The diaphragm
10
can be made from a relatively thick piece of silicon bonded to the body
12
and then chemically etched from one side, leaving somewhere on the order of a 5-to 80-micron-thick diaphragm
10
. However, because semiconductor-processing equipment is designed to handle wafers of certain thickness ranges, it is generally preferred to etch the diaphragm
10
before performing the bonding process. Further, a pre-etched diaphragm
10
is preferable to bonding wafers and then etching them because of wafer-to-wafer thickness variation and thickness variations at different regions on the same wafer, as discussed below.
Typically, thickness variation from one wafer to another is approximately 25 microns. This means that, in a batch of wafers specified as being 500 microns thick, some wafers may be only 487 microns thick while others may have a thickness of 512 microns. If a 500-micron etch were to be performed on all of the wafers in a batch after they were attached to a set of bodies
12
, the 487-microns-thick wafers would be etched completely through while the 512-micron-thick wafers would leave 12 microns of thickness that could be used as a diaphragm
10
. Hence, diaphragm
10
thickness could not be controlled precisely by standard batch manufacturing processes and the cost of manufacturing valves
5
would increase substantially.
Thickness variations at different regions on the same wafer would increase processing complexities and cost even more. Under such conditions, the diaphragm
10
could be completely etched away in some regions while too thick of a diaphragm
10
could be left in other regions. Therefore, as stated above, pre-etched diaphragms
10
are preferred.
Once a diaphragm
10
has been obtained, the fusion bonding process is used to affix the diaphragm
10
to the seat substrate
15
and to affix the seat substrate
15
to the base
17
. This process requires that two very clean and flat silicon wafer surfaces be in contact with each other. Once the surfaces are in contact, the bonding process starts and a strong bond can be formed after annealing, typically in a high-temperature environment of greater than 1100° C. The end product of the fusion bonding process can be a silicon structure that is almost monolithic. However, according to certain types of fusion bonding, one wafer can be oxidized and placed in contact with a bare silicon wafer.
Although the fusion bonding process can theoretically produce strong bonding, certain requirements and specifications have to be met. For example, many studies on wafer specifications have been performed and the need for an approximately 5 nanometer rms surface roughness is generally accepted as being required for proper bonding.
Also, extremely clean surfaces are required in order to carry out the fusion bonding process. Generally, wafer surfaces are first treated according to the well-known RCA etch/cleaning process (developed by the RCA Corp.) and immediately thereafter are bonded together. Further, the cleanliness required for fusion bonding typically necessitates the use of a class
10
or, preferably, a class
1
clean-room environment. Because such environments are expensive to maintain, the fusion bonding process is not conducive to commercial production.
The wafer surfaces must also be free of chipping. When a wafer is chipped, the chips themselves can become bare silicon surfaces. Should the chips (or particles from the chips) fall back onto either of the wafer surfaces, a gap would inevitably remain as the surfaces are placed in contact with each other. Such a gap would render wafer-to-wafer bonding impossible. Hence, having to avoid gap formation renders the manufacturing process of the valves
5
discussed above even more problematic.
If all smoothness and cleanliness conditions discussed above are not met, the silicon-to-silicon bonds holding the diaphragm
10
to the seat substrate
15
in the valve
5
either never form or are highly susceptible to delamination. Under non-ideal conditions, even when bonds form, the bonds are weak and simply inserting one's fingernail between the two wafers causes the wafers to peel away from each other.
Assuming that ideal bonding conditions have been met, the valve
5
still has inherent design flaws that limit its use. For example, the diaphragm
10
stands a high risk of cracking during use when pressure from the top or back side of the valve
5
(opposite the body
12
) is too great. Under such conditions, the diaphragm
10
is pushed against the valve seat
20
with such force that the diaphragm
10
attempts to conform to the shape of the valve seat
20
. Therefore, especially at the edges of the valve seat
20
, the diaphragm
10
experiences tremend

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