Device for one-sided etching of a semiconductor wafer

A device for one-sided etching of a semiconductor wafer (silicon wafer) is proposed, which consists in the manner of an etching box of a trough-shaped basic body (3, 23) and a lid (2, 22) which matches the latter hermetically. The lid (2, 22) has on the topside an opening (5, 25) for the passage of the etching liquid. The etching box bears at least two O-rings (7, 8, 27, 28), of which one is arranged centrally in the basic body (3, 23) and the other centrally in the lid (2, 22). The wafer (1) is clamped between the O-rings (7, 8, 27, 28). A wire (10) connected to the wafer by means of a plate of a spring contact (11, 41) is guided out of the etching box through a bore (4) extending through the basic body (3, 23).

PRIOR ART 
The invention relates to a device for one-sided etching of semiconductor 
wafers (silicon wafers), which is constructed like an etching box. Such a 
device is known from the journal Vacuum Science Technology, issue of 
July/August 1985, pages 1015 ff., G. Kaminsky, "Micro-machining of silicon 
mechanical structures". Described therein is an etching mount for silicon 
wafers in the form of a Teflon plate at the edge of which there is 
arranged in a groove a O-ring on which the wafer to be etched is 
supported. The wafer is mounted on the Teflon plate by a ring made from 
Teflon, which is connected to the base plate by means of nylon screws. 
Etching solution reaches the surface of the wafer through the Teflon ring. 
The O-ring prevents etching solution from reaching the rear. However, the 
etching solution can wet the edge of the wafer on both sides up to the 
contact zone between the wafer and 0-ring. The mechanical stability is 
therefore diminished in the edge region of the wafer, and in addition 
possibilities for machining the wafer after the etching process are 
restricted. During the etching operation, the pressure of the air situated 
under the wafer rises as a consequence of the high etching temperatures. 
The risk of breakage therefore increases, particularly at the edge region 
of the wafer, where it rests on the O-ring. A device for pressure 
compensation for the air space produced under the wafer is not provided. 
Again, the possibility of electrical contacting of the wafer in order to 
be able to carry out an electrochemical etching stop is not provided. 
Furthermore, as basic material, Teflon has a dimensional stability which is 
too low at high etching temperatures for the envisaged application. In 
solutions, Teflon varies its volume by swelling, and thus its shape. An 
etching mount made from Teflon therefore cannot be multiply used. 
It is the object of the invention to specify a device for the one-sided 
etching of silicon wafers which imposes a low mechanical stress on the 
wafer to be etched, ensures complete protection of the wafer edges, and 
permits an electrochemical etching stop. 
The object is achieved according to the invention by the characterising 
features of claim 1. In this case, use is made of a mount like an etching 
box having an open upper part which is provided with two O-rings between 
which the wafer is clamped and which are arranged such that the etching 
solution can reach only the inner region, situated inside the O-ring, 
which is on the lid side, of the side of the wafer to be etched. The 
cavity produced under the wafer backside is connected to the external 
environment via a bore which adjoins a tube or a hose. The production of 
an overpressure under the wafer is thereby excluded. Guided simultaneously 
through the bore is the lead for the electrical contacting of the wafer 
which is required for the electrochemical etching stop. 
ADVANTAGES OF THE INVENTION 
The device according to the invention has the advantage that the wafer edge 
is protected absolutely securely against contact with the etching 
solution. The O-rings holding the wafer are arranged precisely one above 
the other, so that both sides of the wafer are subjected to symmetrical 
holding forces. Due to the fact that it is mounted only via O-rings, the 
wafer is elastically supported. It is contacted electrically from the 
backside in a manner protected against the etching solution. Even at high 
temperatures of the etching solution, no overpressure is produced in the 
cavity under the wafer backside because of the connection to the external 
environment. The device therefore permits maximum safety against breakage. 
The good dimensional stability of the materials proposed permits multiple 
use of the etching box. The electrical contacting of the wafer is 
advantageously performed by spring contacts. 
Further advantageous developments of the device follow from the measures 
specified in the subclaims. 
When producing the etching box from a polyvinylidene fluoride (PVDF) 
plastic, it is expedient respectively to provide in the lid and base part 
grooves which fix the O-rings. Since the O-rings swell due to the contact 
with the etching solution, the volume of the grooves is dimensioned such 
that the pressure on the wafer remains constant. In the contact region 
between the lid and base part, it is expedient to provide a third O-ring 
in order to achieve additional security that no etching solution can 
penetrate into the edge region. It is advantageous to use a spring-loaded 
pin for the electrical contacting of the wafer backside. 
In another advantageous embodiment, the etching box is derived from a 
polypropylene (PP) transport box available commercially. An etching box 
fabricated in this way is of milky transparency and permits any leaks of 
the etching box to be detected immediately from outside. Furthermore, such 
an etching box consisting of PP is exceptionally cost-effective. It is 
expedient to introduce silicone paste as an additional seal into the 
contact region between the lid and base part. A spring part in the basic 
body permits reliable electrical contacting of the wafer backside and 
quick insertion and removal of the wafer. 
Two exemplary embodiments of the invention are explained in more detail 
with the aid of drawings and the following description.

DESCRIPTION 
The first exemplary embodiment, shown in FIG. 1, consists of the basic body 
3 and lid 2 as main parts. The two parts are fabricated from the plastic 
PVDF. The material is dimensionally stable even at high temperatures, and 
permits multiple use of the etching box. The entire etching box, formed 
from the lid 2 and basic body 3, preferably has a round shape, although 
other shapes are also possible. The lid 2 and basic body 3 are configured 
with a shape such that they match one another. For this purpose, the edge 
of the lid 2 is lowered in the shape of a step with respect to the inner 
region 12, while the edge of the basic body 3 is raised in the shape of a 
step. The steps are configured such that when the etching box is closed 
with the wafer situated therein the edge regions of the lid 2 and of the 
basic body 3 lie flush against one another, the O-rings 7, 8, between 
which the wafer 1 is clamped, being squeezed by approximately 20% of its 
cord thickness. The diameter of the depression on the inner region 13, or 
of the elevation on the inner region 12 are somewhat larger than the 
diameter of the wafer 1, for which the etching box is designed. The basic 
body 3 advantageously has two concentric grooves 18 and 19, of which one 
is worked into the edge region elevated in the shape of a step, while the 
other is worked in on the edge of the inner region 13. An O-ring 8, 9 is 
situated respectively in the grooves 18, 19. The basic body surface 14 
situated inside the inner O-ring 8 is connected to the outside of the 
basic body via a bore 4 whose diameter is expediently 0.1-5 mm. 
Immediately adjoining the outlet of the bore 4 on the outside of the basic 
body is a small tube 6 or a hose, which continues the bore 4. On the one 
hand, the pressure compensation between the environment and the space 
surrounded by the wafer 1, 0-ring 8 and basic body surface 14 is performed 
through the bore 4. On the other hand, the wire 10 required for the 
electrical contacting of the wafer 1 is guided to the outside through the 
bore 4. The bore 4 expediently extends at right angles from the inner 
surface to the narrow lateral outer surface of the basic body. Due to the 
rectangular guidance, the etching box can conveniently be introduced into 
the etching solution vertically. For this purpose, the bore 4 is continued 
at its outlet in the form of an adjoining straight small tube 6 or hose 
which projects from the etching solution. Of course, other ways of guiding 
the bore 4 are equally valid. A spring-loaded contact 11 which ensures 
reliable electrical contacting of the wafer 1 is attached to the inner 
basic body surface 14 at the end of the wire 10 guided through the bore 4. 
In the inner region 12, which is elevated with respect to the edge, the 
etching box lid 2 has a groove 17 which is constructed such that it is 
situated precisely over the inner groove 18 of the basic body when the 
etching box is closed. A further O-ring 7 is arranged in the groove. 
The outer O-ring 9 in the basic body 3 and the O-ring 7 placed in the lid 2 
are each exposed from one side to the etching solution during the etching 
process, as a result of which they swell slightly. This can lead to an 
elevated pressure on the wafer clamped between the O-rings 7 and 8. The 
form of the grooves 17 to 19 which fix the O-rings 7 to 9 is therefore 
expediently configured such that an enlargement in volume of the O-rings 7 
to 9 due to swelling is absorbed by the grooves 17 to 19, and the pressure 
on the wafer 1 thereby remains approximately constant at any time. 
The lid 2 and basic body 3 of the etching box are held together by screws 
which are inserted through bores 15 arranged for this purpose on the edges 
of the lid and basic body. Alternatively, connecting the lid and basic 
body can also be performed by quick-action fasteners. It is necessary in 
this case to take measures on the topside of the lid and the underside of 
the basic body to fix the quick-action clamps. The pressure with which the 
etching box parts are pressed together is to be selected such that the 
stepped edges of the lid 2 and basic body 3 lie fixedly against one 
another, the 0-rings 7 and 9, which are visible from outside, being 
pressed virtually completely into the grooves 17 and 19, respectively. 
For the purpose of applying the etching box, the wafer 1 is placed into the 
basic body 3 onto the inner O-ring 8 arranged therein. The side to be 
protected points towards the basic body surface 14. The lid 2 is 
subsequently put on and connected to the basic body 3 by tightening the 
screws. The sealed etching box can be exposed to the etching solution in 
any way. In particular, the duration and temperature of the etching 
process can be freely selected, since it is ensured by the 
pressure-compensating bore 4 that no overpressure, and thus no risk of 
breakage, can be produced in the cavity provided under the wafer 1. 
A second exemplary embodiment is shown by FIGS. 2a and 2b. Parts 
corresponding to the first exemplary embodiment are provided with the same 
reference numerals. 
The exemplary embodiment shown in FIG. 2a consists, as the main parts, of 
the lid 22 and basic body 23, as well as two pressure rings 44. The basic 
body 23 is the lower part of a commercial wafer transport box such as is 
available, for example, from the FLUOROWARE company. The etching box 
consists of polypropylene (PP), a material which has a good dimensional 
stability even at high etching temperatures and is very cost-effective. 
Situated in the slightly conical base trough of the basic body 23 is an 
0-ring 28, which is inserted loosely centrally and whose diameter is 
approximately 4/5 of the etching box diameter, as well as a cruciform 
spring part 41 which is arranged inside the O-ring 28 and is likewise 
available commercially. One of the four resilient tongues 42 of the spring 
part 41 is sheathed by an electrically conductive plate 43 with a 
thickness of approximately 0.05 mm. On the one hand, when the etching box 
is closed the spring part 41 stabilises the wafer 1, which is to be 
etched, in the central region, and on the other hand the tongue 42 which 
is sheathed by a plate serves the purpose of the electrical contacting of 
the wafer backside. Said spring part can be fixedly connected to the basic 
body 3 or loosely inserted. In the latter case, it is expedient to provide 
a bearing for the spring part 41, for example in the form of a ring 29. 
Soldered to the metal plate 43 is a wire 10 made from platinum, which is 
guided out of the basic body through a bore 4 placed in the base trough of 
the basic body 23. Adjoining the bore 4 in a hermetically sealed fashion 
on the outside of the base trough is a small tube 6 or a hose or the like, 
which projects from the etching solution and in which the contacting wire 
10 is guided out of the etching solution. The lid 22 is derived from the 
upper part of the transport box, which belongs in standard fashion to the 
basic body 23. Turned into the topside is a circular opening 25 through 
which the etching solution can pass. Its diameter is expediently 60 to 70% 
of the entire etching box diameter, for example 70-80 mm in the case of an 
etching box provided for four-inch wafers. 
In order to apply the etching box, the wafer 1 is placed with the side to 
be etched upwards into the basic body 23 onto the O-ring 28 arranged 
therein. The spring part 41, which is a spring cross, for example, effects 
the electrical contacting of the wafer 1 by pressing lightly against the 
wafer backside on the underside. A second O-ring 27, which is 
concentrically aligned, is placed onto the upward pointing wafer side. The 
second O-ring 27 has the same diameter as the ring in the basic body 23. 
The cross-sections of the two 0-rings 27, 28 are dimensioned such that the 
wafer 1 clamped between the O-rings 27, 28 are supported fixedly and in a 
nonslip fashion when the etching box is closed. A slight swelling of the 
O-rings 27, 28 does not change the contact pressure on the wafer 1. The 
lid 22 is subsequently put onto the basic body 23, pasty silicone being 
applied to the sealing surface 37 between the lid 22 and basic body 23 in 
order to improve the tightness. Another possibility of achieving a good 
seal consists in configuring the sealing surfaces 37 between the basic 
body 23 and lid 22 in a conical fashion, so that they lie locked against 
one another by being pressed together. As is represented in FIG. 2b, the 
lid 22 and basic body 23 of the etching box are held together externally 
by quick-action fasteners 43 in the form of clamp fasteners. They are 
expediently fixedly connected to one of the pressure rings 44, and are 
fixed to the other by suitable devices. 
Alternatively, the etching box consisting of the lid 22 and basic body 23 
is held together by two pressure rings 44 made from an acid-resisting 
material such as, for example, hard plastic, by being clamped between the 
rings 44. For their part, the pressure rings 44 are pressed against one 
another by means of screws. The pressure rings are provided in this case 
with bores through which the screws are inserted.