GB2254830A - Manufacturing thin plates having surface structures of different depths or heights - Google Patents

Abstract

A silicon wafer (2-8) having surface structures of different depths or heights on one or both sides, e.g. for use as an integral part of a micropump is produced by a particular process. In order to reduce the number of photolithographic steps in a production process requiring several etching or growing steps, the starting wafer (3) comprises a central layer (8) which is covered on both sides with oxide layers (9a, 10) of different thicknesses. In a first process phase both oxide layers (9a, 10) receive surface cavities (12, 12a) which, on the side of the thinner layer (9b) completely traverse the latter, whereas they do not reach the central layer (8) on the side of the thicker oxide layer (10a). While a first etching or growing operation on the central layer (8) is carried out on the wafer (5) which only affects the side of the central layer (8) which carries the thinner layer (9a) while the other side is still entirely covered by the thicker layer (10a, 10c), the second etching or growing step on the central layer (8) is carried out after the thicker layer (10a) has been reduced such as to expose the central layer (8) at the initially formed cavities (12a). Thus, the depressions (14) formed at the side of the initially thicker oxide layer during this second etching step on the central layer are less deep, or in the case of a growing process the first deposits are less high than the depressions (13) or deposits formed on the other side during both etching or growing steps.

Description

METHOD OF MANUFACTURING THIN PLATES HAVING SURFACE STRUCTURES OF DIFFERENT DEPTHS OR HIGHTS The present invention relates to a method of manufacturing thin plates having surface structures of different depths or hights on one or both sides of the plate.

BACKGROUND OF THE INVENTION In the art of treating surfaces for the production of surface structures it is well known to use photolithographic techniques wherein a thin layer of a radiation-sensitive material is applied onto the surface to be treated and masked with a negative pattern of the structure to be obtained. Hereafter, the surface is exposed to radiation through the free portions of the mask and the exposed or non-exposed parts of the radiation-sensitive layer (depending on whether a positive or negative resist is used) are then developped away with a developing agent which is highly selective between exposed and non-exposed portions of the radiation-sensitive layer.

Surface structures having portions of different thicknesses or depths may be obtained hereby in repetitive process steps wherein the masks are changed between the different steps in order to subsequently etch portions of the surface such as to produce the desired pattern, whereby repetitive etching of the same portion may result in deeper cavities than one time etching.

In the production of complicated structures which would require a relatively large number of steps, the application of a photoresist on the surface which comprises already previously produced cavities, becomes more difficult with each etching step and, simultaneously it becomes more difficult to maintain the requisite accuracy.

It is therefore the object of the present invention to find a method for manufacturing thin plates with surface structures of different depth on one or both sides of this plate, which method permits to reduce the complexity andlor the number of repetitive photolithographic process steps in order to facilitate production.

SUMMARY OF INVENTION This and other objects are obtained with a method according to the present invention which is characterized in that it comprises the following steps: - providing a starting wafer having a central layer of a first material which is covered on at least one side with outer layers of a second material, at least portions of said outer layers having different thicknesses;; - producing surface cavities of essentially uniform depths in at least two portions of different thickness of said outer layers such that the depth of said cavities corresponds essentially to the thickness of the thinner portions of said outer layers in order to produce holes therein through which the central layer is exposed to the environment, whereas the surface cavities in the thicker portions of said outer layers leave the central layer fully covered by the thicker portions of said outer layers, such that the thickness of a residual layer of said thicker portions, within the cavities, corresponds essentially to the difference between the original thicknesses of the two portions of the outer layers;; - carrying out a first etching or growing operation at which first depressions or deposits are produced in or on the central layer at locations corresponding to the holes in said thinner portions of the outer layers, said first depressions or deposits having an essentially uniform intermediary depth or hight; - removing a layer of uniform thickness of said thicker and thinner portions of said outer layers, said uniform thickness essentially corresponding to the difference between the initial thicknesses of the portions of different thicknesses of the outer layers in order to remove the residual layer of the thicker portions of said outer layers within the cavities previously produced in the thicker portions of said outer layers such as to expose the central layer at locations corresponding to said cavities in the thicker portions of said outer layers;; - carrying out a second etching or growing operation at which said first depressions or deposits are deepened or hightened respectively to a uniform final depth or hight, while second depressions or deposits are produced at locations corresponding to said cavities in the initially thicker portions of said outer layers, whereby the depth or hight of said second depressions or deposits corresponds to the difference between said intermediary and said final depths of said first depressions or deposits.

The method as described above may be applied for the manufacture of wafers which carry surface structures of different depths on both sides or on one side only. In the first case the starting wafer comprises two outer layers on opposite sides of the central layer, the thickness of one outer layer or of a portion thereof being smaller than the thickness of the other outer layer or a portion thereof. In the other case, the starting wafer comprises an outer layer on one side of the central layer only, whereby this one outer layer comprises portions of different thicknesses.

According to a preferred application of the method of the pesent invention a starting wafer is produced, whereby said wafer comprises a central layer of silicon and outer layers of silicon oxide.

The method according to the present invention uses a silicon wafer who's outer oxide layers have different initial thicknesses which may be produced by first oxidizing both sides of a silicon plate to essentially the same thickness to form said outer oxide layers and subsequently Yeducing the thickness of one of said outer layers.

Advantageously, the thickness of this one oxide layer is reduced by an etching operation.

In a method according to the present invention a silicon wafer with surface structures of different depths on one or both sides of the wafer may be produced with as few as one single lithographic process step by which initial cavities in both outer layers are produced in a process where this one or more photolithographic steps may be performed on essentially planar surfaces, i.e. before any etching of depressions in the silicon is performed.

Advantageously, the removing of said essentially uniform thickness of said thicker outer layer is carried out by an etching operation which guarantees that the same thickness of the oxide layer is etched away at surface areas which were not etched before as well as in surface areas where first cavities had already been produced, such that the central layer will be exposed if the thickness which is etched away in this second etching operation corresponds to at least the initial difference of thicknesses of the thinner and thicker outer layers minus the small amount of oxide material which is undesirably but unavoidably etched away during the etching step on the material of the central layer.

Obviously, in order to obtain the best results, etching of the cavities in the outer layers and the etching of the depressions in the central layer are carried out with different etching agents, whereof at least the agent for the etching of the central layer is essentially selective such as to produce depressions only in the central layer while remaining inoffensive for the outer layers.

One of the possible applications for the method according to the present invention is for the production of wafers such as the ones carrying membranes for micropumps which are comprised of a portion of such a wafer, the membranes being constituted of surface structures of the wafer which form the pump membrane or valve components for cooperation with valve seats which are arranged on other parts of the micropump, which other parts are independent from the wafer but with which said valve components may come into releasable contact.

BRIEF DESCRIPTION OF This DRAWINGS Fig. 1 shows an untreated silicon plate; Fig. 2 shows a silicon wafer having a central layer and two oxide layers of same thickness; Fig. 3 is the silicon wafer of Fig. 2, whereby one oxide layer has been entirely reduced in thickness; Fig. 4 shows the silicon wafer of Fig. 3 wherein the two oxide layers have received a pattern of etched cavities; Fig. 5 is the wafer of Fig. 4 after a first etching operation on the central layer; Fig. 6 is the wafer of Fig. 5 after a uniform thickness of the oxide layers has been etched away; Fig. 7 is the wafer of Fig. 6 after a second etching operation on the central layer, and Fig. 8 is a simplified schematic illustration of a wafer carrying membranes as used in a micropump.

DETAILED DESCRIPTION OF THE INVENTION Figs. 1 and 2 do not require particular attention as they only illustrate the starting silicon plate 1 and the resulting wafer 2 therefrom after the silicon plate 1 has been oxidized e.g. in a suitable furnace under oxidizing atmosphere. The resulting wafer 2 comprises thus a central layer 8 and outer oxide layers 9 and 10 of more or less the same thickness.

Between the stages illustrated in Figs. 2 and 3, one of the two oxide layers has been reduced in thickness for example by half. The wafer 3 thus comprises a central layer 8 a thinner oxide layer 9a having a thickness "a" and the thicker oxide layer 10, having a thickness "b". The reduction of the thickness of layer 9a has been carried out by a one-sided etching operation or by any other kind of method known in the art. Fig. 3 also shows masks 16 and 16a which are used to define the surface structures produced lateron during the photolithographic/etching process step. In the following description only positive photoresists will be considered for the sake of simplicity, other resists may be understood to be usable in analogous ways.

Figs. 2 and 3 show the simple case where the central layer is covered by two oxide layers of different thicknesses, each of which is uniform over the entire surface. Alternatively, one or both oxide layers may include areas of different thicknesses, whereby the initial reduction of the thickness of the portion of the oxide layers which correspond to areas in which deeper final depressions or higher deposits are to be produced later, are obtained by local etching.

Fig. 4 illustrates the result of this lithographic/etching process during which both oxide layers were coated with a photosensitive layer (reference number 11 in Fig. 3) and the masks were applied to both sides of the wafer on top of the photosensitive layers 11, which masks exposed all those parts of the surfaces of the wafer which were to be kept inert during the following etching operation.

The assembly was exposed to UV light and the portions of the surfaces which were left free by the mask were thus activated by UV light to a state in which they became resistant to the used etching agent and thus inhibited access of the etching agent to the oxide layer. In the covered areas the photosensitive layer remained soluble in the etching agent and permitted thus access for the agent to the oxide layer, resulting in the formation of cavities 12 and 12a in the two oxide layers at such covered portions of the surfaces, the depth of said cavities being a function of the etching time and other parameters.The duration of the etching step was selected such as to permit complete dissolution of the thinner oxide layer 9a at the desired locations whereas the thicker oxide layer 10 only received cavities which did not traverse the entire thickness of the thicker oxide layer 10a but left a residual layer 10c of a thickness x within the cavities 12a.

If the thickness of the thinner oxide layer was selected initially as two thirds of the thickness of the thicker oxide layer, and since the cavities in both oxide layers are of the same depth, it is trivial that the thickness x of the residual oxide layer 10c within the cavities 12a is one third of the thickness of the originally thicker oxide layer 10.

Fig. 5 illustrates the wafer 5 as obtained from wafer 4 of Fig.

4 after a first etching step on the central layer 8. Immersion of the wafer into KOH for example does not significantly attack the oxide layers, but etches first depressions 13 of an intermediary depth y into the parts of the central layer 8 which are exposed to the solution through the holes 12 in the thinner oxide layer 9b, whereas the entire side of the thicker oxide layer is only insignificantly but uniformly affected by this etching step.

Hereafter, a double-sided etching step is carried out on the side of the thicker oxide layer 10a in order to take away a uniform thickness x of both oxide layersl0a and 9b all over the surface, i.e.

within and outside the cavities 12a. The thickness x of the eliminated portion of the thicker oxide layer is identical to the thickness x of the residual layer within the previously formed cavities 12a. The results of this step are shown in Fig. 6.

The duration of this double-sided etching step is calculated such as to eliminate the residual oxide layer 10c within the cavities 12a in the thicker oxide layer 10b while the portions of the thicker oxide layer which were not etched previously are reduced in thickness to e.g. a value essentially corresponding to the thickness of the remainders of the initially thinner oxide layer 9a. This step results in the exposure of the central layer 8 at locations corresponding to the previously produced cavities l2a.

Fig. 7 then shows the wafer 7 obtained from wafer 6 of Fig. 6 after a second etching step on the central layer 8, whereby the latter receives second depressions 14 on the side of the initially thicker oxide layer 10a, which second depressions have a depth z' while the first depressions 13 in the central layer 8 on the side of the initially thinner oxide layer 9a are deepened from their initial intermediary depth y to their final depth z which represents in principle the sum of y plus z'.

In fact, the speed of the deepening of the depressions in the central layer is somewhat dependent on the cross section of the cavities in the oxide layers, so as to produce slightly deeper depressions where the cavities are wider because of the easier access of fresh etching agent to and easier removal of dissolved material from the etching locations. It may be envisaged to take advantage of this effect in order to produce differences in the depths of the produced depressions in the central layer.

Figs. 7 and 8 show finished wafers 7 and 8 which have surface structures of different depths on both sides. Whereas wafers 7 carries any kind of a desired surface structure configuration , the wafer of Fig. 8 forms membranes 15 such as currently used e.g. in the art of manufacturing micropumps, where such wafers are used as integral elements, carrying membranes usefull for pumping movements or as valve elements. It is understood that the wafer of Fig. 8 has been produced by a process in which the upper side of the starting wafer included areas of different oxide thicknesses. Consequently, depressions 13b are shallower than depressions 13a and the two membranes 15a and 15 are therefore of different thicknesses.

According to examples which were carried out in order to put the present invention into practice, the thinner oxide layer had a thickness of 1.0 micrometers, the thicker layer of 1.5 micrometers and first cavities of 1.0 micrometers were formed in both layers such that the central layer was exposed through the cavities in the thinner layer whereas a residual thickness of 0.5 micrometers remained in the cavities of the thicker layer.

In a first KOH etching step the areas outside the cavities of the thinner oxide layer were insignificantly reduced from 1.0 to 0.9 micrometer thickness whereas depressions of desired depths were produced in the central layer. During this step the thickness of the thicker oxide layer was reduced outside the cavities from 1.5 to 1.4 micrometers and the residual oxide layer within the cavities of the thicker layer was reduced from 0.5 to 0.4 micrometers.

Subsequently the wafer was submitted to an etching process which reduced the thickness of all oxide surfaces by 0.4 micrometers, so that the remaining oxide layer of the initially thinner outer layer became 0.5 micrometer and the initially thicker oxide layer came down to 1.0 micrometers at locations outside the cavities, whereas the residual oxide layer within the cavities on the side of the initially thicker oxide layer completely disappeared.

A following second KOH etching step increased the depth of the previously produced first depressions on the side on the initially thinner outer layer, whereas at the same time second depressions were produced on the side of the initially thicker outer layer at locations corresponding to the previously formed cavities therein.

The method of the present invention may also be used in applications where portions of the central layer have to be etched through their entire thickness in order to produce through holes therein. For such purpose, first depressions of any desired dimensions are produced in one side of the wafer (as in the process step preceding Fig. 5) such as to almost traverse the central layer whereafter the second etching step (preceding Fig. 7) only finishes off the remaining part from the opposite side. In wafers used as integral parts of micropumps, it is commonly known to lead the liquid inlet and outlet through the wafer, thus necessitating the formation of through holes therein.

The invention has been described above by way of example only, it being understood that various variations of the process may be carried out which seem obvious to the expert without departing from the spirit of the invention. Thus the starting material is not at all limited to silicon and the outer layers need not be oxides. Other materials are equally susceptible to be processed according to the described method. Further, as already mentioned above, the differences in the initial thickness of the two outer layer need not be confined to layers on opposite sides of the central layer but may also be applied to specific areas on the same side of the wafer. It is also possible to provide wafers having areas of a number of different thicknesses on one or both sides. The invention has also been described with respect to etching processes only, it being understood that the reverse processes, i.e. growing processes wherein deposition layers are produced on areas of the exposed central layer, may also be carried out in a way analogous to the described process steps for the etching processes.

Claims (10)

1. Method of manufacturing thin plates having surface structures of different depths on one or both sides, characterized in that it comprises the following steps: - providing a starting wafer (3) having a central layer (8) of a first material which is covered on at least one side with outer layers (9a,10) of a second material, at least portions of said outer layers having different thicknesses (a, b);; - producing surface cavities (12,12a) of essentially uniform depths in at least two portions of different thickness of said outer layers such that the depth of said cavities corresponds essentially to the thickness (a) of the thinner portions of said outer layers (9a) in order to produce holes therein through which the central layer (8) is exposed to the environment, whereas the surface cavities (12a) in the thicker portions of said outer layers (10a) leave the central layer fully covered by the thicker portions of said outer layers, such that the thickness of a residual layer (10c) of said thicker portions, within the cavities (12a), corresponds essentially to the difference between the original thicknesses (a, b) of the two portions of the outer layers;; - carrying out a first etching operation at which first depressions (13) or deposits are produced in or on the central layer (8) at locations corresponding to the holes (12) in said thinner portions of the outer layers (9a), said first depressions or deposits having an essentially uniform intermediary depth (y) or hight;; - removing a layer of uniform thickness (x) of said thicker and thinner portions of said outer layers (10a, 9b), said uniform thickness corresponding to the difference between the initial thicknesses (a, b) of the portions of different thicknesses of the outer layers in order to remove the residual layer (10c) of the thicker portions of said outer layers within the cavities (12a) previously produced in the thicker portions of said outer layers such as to expose the central layer (8) at locations corresponding to said cavities (12a) in the thicker portions of said outer layers;; - carrying out a second etching or growing operation at which said first depressions (13) or deposits are deepened or hightened to a uniform final depth (z) or hight respectively, while second depressions (14) or deposits are produced at locations corresponding to said cavities (12a) in the initially thicker portions of said outer layers (10b), whereby the depth (z') or hight of said second depressions (14) or deposits essentially corresponds to the difference between said intermediary (y) and said final (z) depths or hights of said first depressions (13) or deposits.

2. The method of claim 1, characterized in that the wafer (3) comprises two outer layers (9a, 10) on opposite sides of the central layer (8), the thickness (a) of one outer layer (9a) being smaller than the thickness (b) of the other outer layer (10).

3. The method of claim 1, characterized in that the starting wafer (3) comprises an outer layer on at least one side of the central layer, at least said one outer layer comprising portions of different thicknesses.

4. The method according to any one of claims 1-3, characterized in that said starting wafer (3) comprises a central layer (8) of silicon and outer layers (9, 10) of silicon oxide.

5. The method of claim 4, characterized in that said starting wafer (3) is produced by the following steps: - providing a silicon plate (1); - oxidizing both sides of said silicon plate to essentially the same thickness (b) to form said outer oxide layers (9, 10); and - reducing the thickness of one of said outer layers (9a).

6. The method of claim 5, characterized in that the thickness of said one oxide layer (9a) is reduced by an etching operation.

7. The method of any one of the preceding claims, characterized in that said cavities (12, 12a) in both outer layers (9b, 10a) are produced by a photolithographic/etching process.

8. The method of any one of the preceding claims, characterized in that the removing of said uniform (x) thickness of said thicker portions of said outer layers (10a) is carried out by an etching operation.

9. The method of any one of the preceding claims, characterized in that the etching of the cavities (12, 12a) in the outer layers (9b, 10a) and the etching of the depressions (13, 13a, 14) in the central layer (8) are carried out with different etching agents, whereof at least the agent for the etching of the central layer (8) is essentially selective such as to produce depressions only in the central layer while remaining essentially inoffensive for the outer layers (9b, 10a, 10b).

10. Membrane for micropumps constituted by a portion of a wafer (8) carrying surface structures which form a pump membrane or a valve component, characterized in that said wafer is produced by the method according to any one of the preceding claims.