Substrate holder and method of use

The etching of a thin substrate (23) is performed using a holder (10). The holder (10) has a base (11) that has a cavity (20). The cavity (20) is pressurized to compensate for the pressure and stress that is applied to the substrate (23) by an etchant solution. The holder (10) also has a sequence of o-rings (22,24,26) that are used to hold the substrate (23) in place and to prevent etchant from leaking into the cavity (20) and attacking a bottom surface (32) of the substrate (23). The pressure necessary to hold the substrate (23) in place is applied by a cover ring (28) that is screwed onto the base (11).

BACKGROUND OF THE INVENTION 
This invention relates, in general, to semiconductor devices, and more 
particularly, to handling equipment used in the manufacture of 
semiconductor devices. 
A sensor device is one form of semiconductor device that generally uses a 
thin diaphragm over a cavity to measure a pressure. The thin diaphragm is 
commonly a layer of crystalline silicon that is on the order of 50 microns 
thick and is thin enough to flex in response to slight changes in 
pressure. The change in pressure can be determined by quantifying the 
amount of flex in the thin diaphragm over the cavity. To fabricate a 
typical sensor device, two layers of semiconductor material are bonded 
together and one of the layers is thinned to the desired thickness to 
provide the diaphragm. A specially designed selective etch is then used to 
remove a portion of the second layer of semiconductor material to form a 
cavity below the thinned first layer. 
One of the difficulties associated with manufacturing a sensor device is 
the handling of such delicate layers while forming the cavity. Again, the 
diaphragm is only 50 microns thick and is easily destroyed by stress 
applied to the sensor device. Another complication of the manufacturing 
process, is that great care must also be taken to ensure that only the 
proper portions of the semiconductor layers are exposed to the etchant 
that is used to form the cavity. Commonly used etchants will attack most 
exposed surfaces of semiconductor material, so the top of the diaphragm 
layer must be protected as the cavity is formed. 
In large scale manufacturing operations, the throughput of each processing 
step involved with the formation of a sensor structure must be as 
efficient as possible. Most of the processing equipment used to handle 
semiconductor substrates during the formation of sensor devices is 
cumbersome and requires a considerable amount of time and effort to use 
the equipment. This handling time significantly affects the throughput of 
the manufacturing operations. 
By now it should be appreciated that it would be advantageous to provide an 
apparatus for handling semiconductor substrates that meets the physical 
requirements of handling delicate layers during the formation of sensor 
structures. It would also be advantageous if the apparatus could ensure 
that only the desired portions of the semiconductor substrates are exposed 
to chemical etchants during processing. It would be of a further advantage 
if the apparatus reduced the total processing time of the manufacturing 
operation.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a holder that can be used in a manufacturing 
flow to hold a semiconductor substrate. The holder is configured so that 
it is suitable for handling a semiconductor substrate while the substrate 
is placed into a chemical bath, such as during the formation of cavities 
in the fabrication of sensor devices. For such a process, the holder must 
be able to protect one side of the substrate from harsh etchants and be 
able to hold the delicate substrate so that it is not damaged during 
processing. The holder of the present invention has many features which 
make it applicable not only for the formation of sensor devices, but the 
holder has advantages that make it suitable for other processing steps as 
well. 
As will be described in more detail below, the holder of the present 
invention comprises a sequence of o-rings or seals that provide many 
advantages over previously known holders. For example, the o-rings are 
configured such that the force used to keep the semiconductor substrate in 
place is evenly distributed around the semiconductor substrate. This is 
necessary to ensure that delicate substrates are not damaged during 
handling or processing. An additional feature of the o-ring configuration 
is that it protects one side of the substrate from etchant while the other 
side is being etched. 
During processing, the semiconductor substrate, along with the holder, is 
typically immersed into a chemical bath. The fluid in the chemical bath 
will apply pressure to the semiconductor substrate, which is proportional 
to the depth of the semiconductor substrate in the chemical bath. Another 
advantage of the holder of the present invention is that the holder has an 
opening that is used to pressurize a cavity in the holder. The pressure of 
the cavity is applied against the bottom surface of the semiconductor 
substrate to equalize the pressure on both sides of the semiconductor 
substrate during processing. This allows for thinner substrates to be used 
with the holder because the risk of breaking the substrate due to a 
pressure differential is addressed. The pressurized cavity also serves as 
a leak test to determine if the o-rings are properly sealing the holder to 
prevent etchant from attacking the wrong side of the substrate. 
An even further advantage of the holder of the present invention is that it 
provides an efficient method to load and unload the semiconductor 
substrate from the holder. This is a significant advantage in large scale 
manufacturing operations since it reduces the amount of time it takes to 
prepare a substrate for processing and unloading the substrate once the 
processing is complete. 
Referring now to the figures, a more detailed description of the present 
invention will be provided. FIG. 1 is a sectional view of a holder 10 to 
be used in the processing of a semiconductor substrate or substrate 23. An 
exploded isometric view of holder 10 is provided in FIG. 2 which can be 
used to further understand how the portions of holder 10 connect together. 
First a structural description of holder 10 will be discussed to identify 
the components of holder 10. Following thereafter, a description of an 
example of how to use holder 10 will be provided. 
Again referring primarily to FIG. 1, holder 10 comprises a base 11 that has 
a cavity 20 and an opening 12 through base 11. Opening 12 is used to 
provide an ambient to pressurize cavity 20 as will be described shortly. 
Base 11 also has a recessed area 15 which is used to provide physical 
support to a base plate 13. Base plate 13 is used to provide support for 
an electrical contact 17 and it should be understood that base plate 13 
can also be formed as part of base 11 rather than as a separate component 
of holder 10. Electrical contact 17 is preferably a spring loaded 
electrical contact, but can be any mechanism for providing electrical 
connection to the bottom surface 32 of semiconductor substrate 23. In some 
processing operations, substrate 23 can be biased to accelerate or control 
the chemical reactions that are taking place. Therefore, electrical 
contact 17 can be used to provide a voltage potential to a bottom surface 
32 of substrate 23 or be used to measure a voltage potential present at 
the bottom surface 32. It should be understood that the use of electrical 
contact 17 is optional if a processing step is not an electro-chemical 
process, or otherwise has no need to bias substrate 23. 
Base plate 13 also comprises holes or openings 14 that allow an ambient in 
cavity 20 to pass through base plate 13. This allows cavity 20 to be 
pressurized to apply pressure to bottom surface 32 of substrate 23. The 
pressure in cavity 20 can then be used to compensate for the pressure 
applied to substrate 23 during processing. Base plate 13 further includes 
a notch around its outer edge which is used to contain first o-ring 22. 
Other mechanisms can be used in base plate 13 to hold first o-ring 22 such 
as a groove or the like. First o-ring 22 is used to not only support 
substrate 23, but to provide some separation between base plate 13 and 
substrate 23. With substrate 23 resting on first o-ring 22, a second 
o-ring 24 is placed along the outer edge of a top surface 33 of substrate 
23. Second o-ring 24 is used in conjunction with first o-ring 22 to keep 
substrate 23 in place and to prevent a fluid from seeping from the top 
surface 33 and reaching the bottom surface 32 when holder 10 is placed 
into a chemical bath (not shown). 
Substrate 23 is firmly held in place during processing by a top ring 25, a 
third o-ring 26, and cover ring 28. Third o-ring 26 is used to prevent 
chemicals that might leak through top ring 25 and cover ring 28 from 
reaching bottom surface 32. Top ring 25 is placed in contact with both 
second o-ring 22 and third o-ring 26. Preferably, cover ring 28 screws 
onto base 11, which in turn, applies pressure to top ring 25 and second 
o-ring 24. It is this pressure that keeps substrate 23 in place and 
prevents etchant from attacking bottom surface 32 of substrate 23. It 
should also be understood that in other embodiments cover ring 28 can 
apply pressure to top ring 25 by other means such as by snapping onto base 
11 or the like. 
A method for using holder 10 will now be provided by way of an example 
where substrate 23 is dipped into an etch solution of potassium hydroxide 
(KOH). In this example, KOH is used to etch cavities (not shown) into top 
surface 33 of substrate 23 as might be done to form sensor devices. When 
forming sensor structures, at least a portion of substrate 23 is extremely 
thin and therefore substrate 23 must be handled with great care. In 
addition, the KOH solution must not leak into holder 10 such that KOH can 
attack the bottom surface 32 of substrate 23. To control the etching of 
substrate 23, an electrical bias is placed on substrate 23 using 
electrical contact 17 so that it is at a different voltage potential than 
the KOH etch solution. The electrical bias is used to monitor the progress 
of the KOH solution as the cavities are formed. 
Before placing holder 10 into the solution of KOH, substrate 23 is placed 
into holder 10 has shown in FIGS. 1 and 2. Then contact is made to 
electrical contact 17 with a wire 18 that passes through opening 19. Base 
plate 13 is then placed into base 11 and first o-ring 22 is placed on base 
plate 13. Substrate 23 is then placed onto first o-ring 22 using 
conventional delicate handling equipment (not shown) and second o-ring 24 
is place onto substrate 23 so that it conforms to the shape provided by 
base 11. When in place, substrate 23 will make contact with electrical 
contact 17 so a bias can be placed on substrate 23 during etching. 
One advantage to the configuration of holder 10 is that first o-ring 22 and 
second o-ring 24 will apply pressure to substrate 23 evenly as cover ring 
28 is screwed onto base 11. When placed onto substrate 23, second o-ring 
24 conforms to the configuration of base 11 so as to minimize the distance 
second o-ring overlaps substrate 23, typically this distance is only 1 mm 
to 2 mm so as to minimize the amount surface area that is not exposed to 
the KOH etchant. It should also be understood the that shape of base 11 
and base plate 13 can be altered to conform to the shape of substrate 23 
so as to accommodate the use of minor or major flats (not shown) on 
substrate 23, yet still form an etchant-tight seal. 
Third o-ring 26 is then place onto base 11 and is held in place by top ring 
25. Cover ring 28 is then placed over top ring 25 and tightened by 
screwing cover ring 28 onto base 11 using a groove pattern. Preferably, 
cover ring 28 is hand tightened to simplify the processing step for an 
operator performing the KOH etch operation. As cover ring 28 is tightened, 
pressure is applied to top ring 25, which in turn applies pressure to 
first o-ring 22 and second o-ring 24. Because the leverage on substrate 23 
is minimized by the sequence of o-rings 22, 24, and 26, it is nearly 
impossible to damage substrate 23 by over-tightening cover ring 28. This 
is due in part because first and second o-rings 22 and 24 distribute the 
pressure to substrate 23 evenly. 
Now that substrate 23 is in place in holder 10, a leak check is performed 
to test if o-rings 22, 24, and 26 will prevent etchant from attacking the 
bottom surface 32 of substrate 23. To perform a leak test, a coupling 19 
of base 11 is connected to an external ambient source (not shown) such as 
nitrogen. A leak check is then performed by placing holder 10 into a inert 
water solution and pressurizing cavity 20 with gas entering through 
opening 12. If bubbles are present along the edge of second o-ring 24, 
then o-rings 22,24, and 26 of holder 10 are rechecked and holder 10 is 
tightened. This operation is repeated until no bubbles are detected. This 
is of course an optional step, but the leak test helps insure that when 
holder 10 is placed into a KOH solution that there are no leaks that would 
allow etchant to attack the bottom surface 32 of substrate 23. 
Once the leak test is completed, holder 10 is placed into a KOH solution 
and a bias is applied to substrate 23 relative to the KOH solution. Cavity 
20 is then pressurized to the proper pressure using the ambient through 
opening 12. The proper pressure is determined by calculating the pressure 
that will be applied to substrate 23 from the top surface 33 from the KOH 
solution. This pressure will depend on the depth of holder 10 in the KOH 
solution. As the depth of holder 10 in the KOH solution is increased, the 
pressure applied to substrate 23 by the KOH solution will also increase. A 
corresponding pressure is then placed in cavity 20, which in turn applies 
pressure to the bottom surface 32 of substrate 23 (shown as arrows 31) to 
compensate for the pressure applied by the KOH solution. It should also be 
understood that in most applications the net pressure across substrate 23 
is preferably zero, however, some applications may prefer a positive or 
negative net pressure differential. 
In addition to reducing the amount of stress applied to the thin portions 
of substrate 23, the pressure in cavity 20 will also retard etchant from 
attacking the bottom surface 32 of substrate 23 should a leak develop in 
holder 10 during the etch process. Because the potentially damaging 
pressure applied by the KOH solution is offset, the present invention can 
be used to process wafers that are thinner than those that can be used in 
previously known apparatuses. For example, holder 10 can be used to 
process wafers that have portions that are only 1 micron to 10 microns 
thick. 
The materials used to form the various elements of holder 10 will depend on 
the process conditions holder 10 will be exposed to. For example, if 
holder 10 is used in an electro-chemical KOH etch process, then o-rings 
22, 24, and 26 can be "Buna" o-rings or "Kalrez" o-rings and base 11, base 
plate 13, top ring 25 and cover ring 28 can be made from "Teflon" 
material. "Buna" is a registered trademark of Bayer-Aktiengesellschaft of 
Bayerwerk, Germany. "Teflon" and "Kalrez" are polymer compounds used 
regularly in the semiconductor industry and are registered trademarks of 
E. I. Du Pont De Nemours and Company of Wilmington, Del. It should also be 
understood that holder 10 can be used in a variety of processing 
applications including, but not limited to, the etching of silicon, 
nitride, polysilicon, and photoresist and holder 10 can be made from the 
corresponding materials that are necessary for that particular process. 
A significant advantage of holder 10 of the present invention is that it is 
well suited for use in a high volume manufacturing operation. Because 
cover ring 28 is simply hand tightened onto base 11, it is readily and 
consistently used by operators performing the etching operation. Compared 
to some previously known holders used to process semiconductor wafers, 
holder 10 of the present invention has a much more efficient and 
simplified manner in which substrate 23 is placed in and taken out of 
holder 10. This improves the throughput of the etch operation and reduces 
the overall manufacturing cost of semiconductor devices that require this 
type of etch processing. 
By now it should be appreciated that the present invention provides a 
holder and a method for its use. The holder is designed so that it can be 
used to handle delicate substrates by compensating for the pressure 
applied by the etch solution. The holder also provides for a leak check 
mechanism that allows the integrity of the holder to be verified to 
prevent the wrong portions of the substrate from being exposed to a 
chemical bath. The design of the holder of the present invention is 
optimal for high volume manufacturing operations where throughput is a key 
contributor to final manufacturing cost.