DESCRIPTION

Method for manufacturing MEMS devices with moveable structure

FIELD OF THE INVENTION

The current invention is related to a method for manufacturing a MEMS device with a moveable structure.

BACKGROUND OF THE INVENTION

The manufacturing process of MEMS devices with moveable structures is always confronted with the problem of stiction of the moveable structure. There is a danger that the moveable structure sticks to an adjacent layer or substrate without a possibility to release it. Stiction of the moveable structure can happen during the release of the moveable structure or during the subsequent processing.

In US 6,666,979 a method of fabricating a surface within a MEM is described, which surface is free moving in response to stimulation. The free moving surface is fabricated in a series of steps, which includes a release method, where release is accomplished by a plasmaless etching of a sacrificial layer material. An etch step is followed by a cleaning step in which by-products from the etch step are removed along with other contaminants which may lead to stiction. There are a series of etch and then clean steps so that a number of "cycles" of these steps are performed. Between each etch step and each clean step the process chamber pressure is typically abruptly lowered, to create turbulence and aid in the removal of particulates, which are evacuated from the structure surface and the process chamber by the pumping action during lowering of the chamber pressure. The final etch/clean cycle may be followed by a surface passivation step in which cleaned surfaces are passivated and/or coated. The release of the free moving surface is time consuming and expensive.

SUMMARY OF THE INVENTION

It's an objective of the current invention to provide a cost effective method for manufacturing MEMS device with moveable structure minimizing the danger of stiction.

The objective is achieved by means of a method for manufacturing MEMS device with a moveable structure comprising the steps of: providing a substrate; providing at least one sacrificial layer directly or indirectly attached to the substrate; providing at least one functional layer; providing the moveable structure with a pattern of etch holes in the functional layer; releasing the moveable structure by etching the sacrificial layer via the pattern of etch holes and forming at least one anti-stiction structure by means of residues of the sacrificial layer.

The anti-stiction structure or anti-stiction structures if more than one is needed are formed by means of controlled etching of the sacrificial layer or layers. The etchant used to etch the sacrificial layer preferably doesn't etch the substrate alternatively an etch stop layer can be provided in order to prevent etching of the substrate. The etching of the sacrificial layer or layers starts at every etch hole provided in the functional layer with the consequence that the sacrificial layer or layers are subsequently etched. The notation "etch hole" is a general description of all structured areas in the functional layer in order to release the moveable structure. The etching of the sacrificial layer is stopped e.g. by means of controlling the etching time when the moveable structure is released but before all residues of the sacrificial layer are etched away. The residues of the sacrificial layer form the anti-stiction structures. The small surface of the anti-stiction structures in comparison to the area of the moveable structure limits the adhesion between the moveable structure and an adjacent layer e.g. the etch stop layer limiting the etching to the sacrificial layer. The anti-stiction structures remain between the moveable structure and the adjacent layer during the further processing of the MEMS device preventing stiction of the moveable structure by means of this mechanic measure simplifying the processing of the MEMS device in comparison to prior art.

In a further embodiment of the current invention the moveable structure is released and the anti-stiction structure is formed by means of wet etching. Preferred methods in order to prevent stiction are the usage of dry or vapor etching avoiding a continuous liquid phase between the moveable structure and the adjacent layer in order to prevent stiction but being rather expensive due to special equipment and slower etch rates than wet etching methods. Furthermore, plasma etching or vapor etching chemistry might not be compatible with used materials for the sacrificial and structural layers. Alternatively cheaper wet etching methods are used but in combination with the expensive Critical Point Drying (CPD) avoiding the phase transition between liquid and gas in order to prevent stiction caused by the adhesion of the liquid between the moveable structure and the adjacent layer and the surface tension of the liquid. Furthermore, CPD step adds several extra steps to the process flow thereby increasing process complexity. The anti-stiction structures enable the use of wet etching methods without CPD by stopping the wet etching procedure after releasing the moveable structure but before the sacrificial layer is completely etched away. The moveable structure doesn't touch the adjacent layer during conventional drying of the released and cleaned MEMS device by means of heating or spin-drying. The moveable structure is forced to move to the adjacent layer during removing of the liquid (cleaning liquid) between the moveable structure and the adjacent layer by means of the adhesion between the liquid and the surfaces of the moveable structure and the adjacent layer and the surface tension of the liquid. The movement stops as soon as the moveable structure touches the anti-stiction structures whereby the liquid is further removed. The small parts of the surface of the moveable structure or the adjacent layer touching the anti- stiction structures result in limit adhesion not sufficient to cause stiction between the moveable structure and the adjacent layer. The anti-stiction structures form a kind of spacer between the moveable structure and adjacent layer. The anti-stiction structures remain functional during the further processing protecting the moveable structure with respect to stiction and improving the yield of the process.

In another embodiment of the current invention the sacrificial layer or sacrificial layers etch isotropically. The combination of material used for the sacrificial

layer and the appropriate etching method enables an isotropic etching of the sacrificial layer. The advantage is the etching of the sacrificial layer is more predictable and the allocation of the anti-stiction structures can be determined easily. Regular patterns of etch holes in the functional layer result in regular patterns of anti-stiction structures essentially having the same size. Examples of regular pattern are square pattern, hexagonal pattern and the like. One method to minimize the number of anti-stiction structures is to use a regular pattern of etch holes where at certain positions no etch hole is provided. One example is a square pattern where every e.g. third position in a row and every third position in a line one etch hole is left out. The distance between the anti- stiction structures can be designed according to other functional parameters of the moveable structure as e.g. the elasticity of the used material or materials and the layer thickness. The etching time needed in order to release the moveable structure is not reduced since the distance between the etch hole can be adapted to the etching process. In another embodiment of the current invention the sacrificial layer comprises a first sacrificial layer and a second sacrificial layer, whereby the first sacrificial layer is characterized by a lower etching rate in comparison to the etching rate of the second sacrificial layer, and the first sacrificial layer is directly attached to the functional layer. The etching rate of a sacrificial layer depends on the material where the sacrificial layer consists of and the etchant where the sacrificial layer is etched with. Providing two sacrificial layers with different etching rate with respect to a specific etchant enables the preparation of anti-stiction structures attached to the functional layer of the MEMS device if the etching rate of the first sacrificial layer attached to the functional layer is slower as the etching rate of the second sacrificial layer separated from the functional layer by means of the first sacrificial layer. The first sacrificial layer is always etched first since it is firstly exposed to the etchant through the etch holes. The first sacrificial layer is etched and due to the relation between the layer thickness of the first sacrificial layer being in the order of some micrometer and the distance between the etch holes provided in the functional layer being in the order of some ten micrometer the etchant reaches the boundary between the first sacrificial layer and the second sacrificial layer before the first sacrificial layer is totally removed at the boundary between the

functional layer and the first sacrificial layer. This process can be easily controlled if the first sacrificial layer etches isotropically. The second sacrificial layer is then etched away again before the first sacrificial layer is totally removed at the boundary between the functional layer and the first sacrificial layer due to the faster etching rate. This can be achieved by adapting the relation between the etching rates of the first and the second sacrificial layer (depending on materials and etchant) and again the relation between the layer thickness of the first sacrificial layer and the distance between the etch holes provided in the functional layer. The second sacrificial layer etches preferably isotropic simplifying the control of the process. After the second sacrificial layer is removed the functional layer is released. Depending on the etch time, etching rates and layer thickness individual anti-stiction structures of about bulb shape attached to the functional layer are left.

In one embodiment of the current invention only one sacrificial layer is provided. The sacrificial layer is etched first at the boundary between the sacrificial layer and the functional layer. The functional layer is released at the boundary between the sacrificial layer and the functional layer and no residues of the sacrificial layer are attached to the functional layer. The residues of the sacrificial layer forming the anti- stiction structures are attached to an adjacent layer preferably a layer essentially not etched by the etchant used to release the moveable structure. Since the anti-stiction structures are not attached to the functional layer the mass of the moveable structure is not influenced. A precise definition of the mass of the moveable structure is essential if the moveable structure is part of a resonator. In this case the mass of the resonator strongly influences the resonance frequency of the resonator.

In another embodiment of the current invention silicon on insulator (SOI) wafer is used for manufacturing a MEMS device with a moveable structure. A silicon substrate is provided with a buried oxide sacrificial layer sandwiched between a mono- crystalline or poly-crystalline silicon functional layer and the silicon substrate.

The silicon functional layer is oxidized and the oxide layer is structured. The structuring of the oxide layer can be used to remove the silicon functional layer up to the buried oxide sacrificial layer providing the etch holes and the contour of the

moveable structure. Alternatively a resist mask can be used. Finally the buried oxide sacrificial layer is isotropically etched in a controlled way by means of wet etching (HF solution) until the moveable structure is released but residues of the buried oxide sacrificial layer are still present. The residues of the buried oxide sacrificial layer form the anti-stiction structures. The oxide removal is determined by controlling the etching time of the buried oxide whereby the etching time is determined by the etchant (e.g. concentration of HF solution), the thickness of the buried oxide sacrificial layer, the thickness of the silicon functional layer, the diameter of the etch holes and the distance between adjacent etch holes and the pattern of the etch holes. In one embodiment of the current invention essentially pyramid shaped anti-stiction structures are formed. Providing one sacrificial layer the shape of the anti- stiction structures attached to the adjacent layer facing the moveable structure is determined by means of the pattern of the etch holes in the functional layer and the etching of the sacrificial layer. Especially if a regular pattern of etch holes is provided and the sacrificial layer etches isotropically the shape of the anti-stiction structure is pyramid like. The number of etch holes circumventing one point and this etch holes having the same distance to that point determines the number of sides of the pyramid. If e.g. a square pattern is provided four etch holes circumvent the point having the same distance to the etch holes (intersection of the diagonals of the square). The etching of the sacrificial layer starts at the etch holes and etches the sacrificial layer spherically around the etch holes. Depending on the etch time the spheres coalesce finally resulting in a four- sided pyramid residue of the sacrificial layer forming one of the anti-stiction structures.

It's further an objective of the current invention to provide a low cost, reliable MEMS device with moveable structure.

The objective is achieved by means of a MEMS device with a moveable structure comprising anti-stiction structures, and the anti-stiction structures are residues of at least one sacrificial layer. The anti-stiction structures improve the yield during the production process since the stiction of the moveable structure is prevented. Cost effective wet etching without critical point drying can be used to release the moveable

structure. The reliability of the MEMS device is enhanced since the anti-stiction structures also prevent the sticking of the moveable structure during operation. Examples for MEMS devices with moveable structures where the anti-stiction structures can be used are e.g. MEMS resonators and MEMS switches.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in greater detail with reference to the figures, in which the same reference signs indicate similar parts, and in which:

Fig. Ia and Ib show schematic drawings of cross sections of one embodiment of the current invention based on a SOI wafer.

Fig. 2 shows a top view of a structured silicon functional layer after removing the plate resonator with a regular pattern of etch holes. Fig. 3 shows a top view of one anti-stiction structure

Fig. 4 shows a top view of a structured silicon functional layer after removing the plate resonator with a not completely regular pattern of etch holes.

Fig. 5 shows a schematic of a cross section of a SOI wafer if no anti- stiction structure is left between adjacent etch holes. Fig. 6 shows a photograph of a silicon wafer with a regular pattern of anti-stiction structures.

Fig. 7 shows a photograph of an anti-stiction structure with a pyramid shape.

Fig. 8 shows a schematic of an etch monitor to control the etching procedure of the sacrificial layer.

Fig. 9a - 9b show schematic drawings of cross sections of an embodiment of the current invention with upside down anti-stiction structures.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. Ia shows a cross section of a SOI wafer where already trenches are etched in the silicon functional layer 20 building the etch holes 30. The buried oxide (SiC^) sacrificial layer 10 sandwiched between the silicon functional layer 20 and the silicon substrate 5 (not shown) can be etched via the etch holes 30. In Fig. Ib the buried oxide sacrificial layer 10 is partly removed by means of wet etching with HF solution. The moveable structure formed by parts of the silicon functional layer 20 is removed and anti-stiction structures 40 attached to the silicon substrate 5 are formed by means of residues of the buried oxide sacrificial layer 10. The anti-stiction structures 40 prevent a direct contact between the structured silicon functional layer 10 and the silicon substrate 5. During the e.g. drying procedure the moveable structure 25 moves towards the silicon substrate 5 and stops when the tip of the anti-stiction structures 40 are touched. The small contact area between the anti-stiction structure 40 and the moveable structure 25 made of the silicon functional layer 20 reduces adhesion and prevents sticking of the moveable structure 25. The boundary condition for forming anti-stiction structures 40 by means of residues of the buried oxide is given by

with Dl the dis ftanc-e between adjafcent etch-holefs 30 ci ⁺ rc ¹ umventing a point where an anti-stiction structure 40 should be left. In a square pattern of etch holes 30 Dl is the length of the diagonals between the etch holes 20 at the corners of the square pattern. R is the etch radius of the sphere istropically etched in the buried oxide sacrificial layer 10 around the etch holes 30 (It's no perfect sphere since the etch holes 30 do have a finite diameter or size). T is the thickness of the buried oxide sacrificial layer 10. The left part of the unequation determines the boundary condition to release the moveable structure 25 whereby the right side of the unequation determines that a residue of the buried oxide sacrificial layer 10 is left. The radius R is determined by the etchant, the quality of the buried oxide and the etching time.

Fig. 2 shows a top view of the moveable structure 25 of a plate resonator 100. A regular square pattern of etch holes 30 is provided on the moveable structure 25

in order to release the moveable structure. The essentially square shaped moveable structure 25 is suspended at the edges of the moveable structure 25 with suspension structures 110. Fig. 3 shows a top view of an anti-stiction structure if the moveable structure 25 is transparent. The etch holes 30 are arranged in a square pattern around the anti-stiction structure 40. The tip of the anti-stiction structure 40 is essentially at the point of intersection between the diagonals of the square formed by the etch holes 30. Fig. 4 shows a top view of the moveable structure 25 of a further plate resonator 100. A square pattern of etch holes 30 is provided on the moveable structure 25 in order to release the moveable structure. The essentially square shaped moveable structure 25 is suspended at the edges of the moveable structure 25 with suspension structures 110. At some anti-stiction points 50 no etch hole 30 is provided. For example at the crossing of row 2 and line 2 is no etch hole 30 taking the regular square pattern of Fig. 2 and the upper left corner of the essentially square like moveable structure 25 as reference. The anti-stiction points 50 are again circumvented by a square pattern of etch holes 30 but this square pattern is tilted 45 degrees with respect to the regular square pattern. The length of the diagonal between the etch holes 30 building the tilted pattern is longer as the length of the diagonal between the etch holes of the regular square pattern. The etching of the buried oxide sacrificial layer 10 can now be controlled in a way that the buried oxide sacrificial layer 10 is totally removed at all positions where the regular square pattern is provided. This situation is shown in Fig. 5 where the length of the diagonal D2 between the etch holes 30 of the regular square pattern for a given etch radius R is to small. The right side of the unequation shown above is not fulfilled replacing Dl by D2 in the equation. No anti-stiction structure 40 is left. At the anti- stiction positions 50 the length Dl of the diagonals between the etch holes 30 of the tilted square pattern circumventing the anti-stiction positions 50 is longer than D2. Dl fulfills the unequation shown above for the given etch radius R. An anti-stiction structure 40 is left on the silicon substrate 5 underneath the anti-stiction positions 50. The pattern of the anti-stiction structures 40 can be designed according the requirements of the application e.g. one anti-stiction structure 40 in the middle of a moveable structure 25.

Fig. 6 shows a photograph of a MEMS resonator where the moveable structure 25 has been removed after the etching of the buried oxide sacrificial layer 10. A regular square pattern of etch holes 30 had been provided in the silicon functional layer 20 resulting in a regular square pattern of anti-stiction structures 40 attached to a silicon substrate 5.

Fig. 7 shows a photograph of one single anti-stiction structure 40 attached to a silicon substrate 5 after the moveable structure 25 has been removed. The anti-stiction structure resulted from etching the buried oxide sacrificial layer 10 via four etch holes 30 provided in the silicon functional layer 20. The anti-stiction structure 40 essentially has the shape of a four-sided pyramid.

Fig. 8 shows an example of an etch monitor 200 to control the etching procedure of the buried oxide sacrificial layer 10. The time of the buried oxide sacrificial layer 10 etch is critical. The proper etch time is obtained by means of an etch monitor 200. The etch monitor 200 consists of a series of small plates 210. The plates 210 may be essentially square and the silicon functional layer 20 is removed around the plates 210. Additionally etch holes 30 are provided in the plates 210. The pattern of the etch holes 30 of each plate is identical to the pattern of etch holes 30 provided on the silicon functional layer 20. In the series of small plates 210 the pitch P between the etch holes 30 increases from the left plate 210 (plate no. 1) to the right plate 210 (plate no. 10). During the buried oxide sacrificial layer 10 etch the plates 210 are released from left to right. In this way the progress of the etching is recorded by the release of the plates 210. A potential problem is contamination of the wafer (and the etch bath) by the released plates 210. To prevent contamination the plates 210 could be suspended by a single poly silicon spring connecting the plates 210 with the residues of the poly silicon functional layer 20.

Fig. 9a shows a cross section of a SOI wafer where already trenches are etched in the silicon functional layer 20 are etched building the etch holes 30. A first sacrificial layer 11 and a second sacrificial layer 12 are sandwiched between the silicon functional layer 20 and the silicon substrate 5 (not shown). The sacrificial layers 11 and 12 have different etch rates with respect to an etchant that can be provided via the etch

holes 30 whereby the etch rate with respect to said etchant of the first sacrificial layer 11 attached to the structured silicon functional layer 20 is slower than the etch rate with respect to said etchant of the second sacrificial layer 12 attached to the silicon substrate 5. In Fig. 9b an optional step of plasma etching is shown deepen the etch holes 30 up to the second sacrificial layer 12 in order to enable the etching of the second sacrificial layer 12 as soon as the etchant is provided. Fig. 9c shows an intermediate step after providing the etchant but before releasing the moveable structure 25. The second sacrificial layer 12 is etched faster resulting in broad residues of the first sacrificial layer 11 connected by means of narrow pillar like residues of the second sacrificial layer 12 connecting the first sacrificial layer 11 with the silicon substrate 5. In Fig. 9d the moveable structure 25 is finally released by removing the second sacrificial layer 12 underneath the moveable structure but leaving a residue of the first sacrificial layer 11 building an anti-stiction structure 40 attached to the moveable structure 25.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. Any reference signs in the claims shall not be construed as limiting the scope. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, first, second and the like in the description and the claims are used for descriptive purposes and not necessarily for describing

relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.