Patent ID: 12187605

DETAILED DESCRIPTION OF THE DISCLOSURE

This disclosure describes a method for manufacturing a micromechanical structure in the structural layer of a wafer by forming a first gap and a second gap in the structural layer. The first gap has a first gap width and the second gap has a second gap width. The second gap width is greater than the first gap width.

The method comprises the following step: (1) depositing and patterning a first etching mask and a second etching mask on a horizontal face of the structural layer. The first etching mask has a first opening which defines the location and dimensions of the first gap. The width of the first opening is equal to the first gap width. The first opening forms a first unprotected area. The first etching mask also has a second opening which defines the location and dimensions of the second gap. The width of the second opening is equal to the second gap width.

The second etching mask comprises a load-reducing part within the second opening in the first etching mask. The load-reducing part divides the second opening into a temporarily protected area which is covered by the load-reducing part and at least one second unprotected area which is not covered by the load-reducing part. The width of the at least one second unprotected area is substantially equal to the width of the first opening.

The method also comprises the following steps: (2) etching trenches through the wafer in the first and second unprotected areas which are not protected by the first etching mask or the second etching mask, (3) coating at least the sidewalls of the trenches with a protective layer and removing the second etching mask at least from the second opening in the first etching mask, so that the temporarily protected area is exposed, and (4) etching away the structural layer in the exposed temporarily protected area.

The wafer may be a silicon wafer. The “structural layer” of the wafer may be a silicon layer where the mobile parts of a MEMS device are manufactured. The wafer may also comprise other layers. These other layers may for example provide support for the structural layer or contain contacts which facilitate electrical measurements. The structural layer may be etched after it is fixed to a support layer.

The micromechanical parts which are separated from each other by the first gap in one region of the structural layer may be the same parts which are separated from each other by the second gap in another region of the wafer. Alternatively, the micromechanical parts which are separated from each other by the first gap may differ from the micromechanical parts which are separated from each other by the second gap. Alternatively, asFIG.1illustrates, a central part (112) may be separated from an adjacent part (111) on one side by the first gap and from another adjacent part (113) on the opposite side by the second gap. This last option will be the primary illustration in the figures of this disclosure, but the method can be equally well employed when the parts which are to be separated from each other by the first gap are distant and completely separate from the parts which are to be separated from each other by the second gap or when two parts are to be separate from each other by a narrow gap in one region of the xy-plane and by a broader gap in another region of the xy-plane.

Although only one first gap and one second gap will be discussed and illustrated in this disclosure, there could in practice be lots of gaps which have the same width as the first gap and lots of gaps which have the same width as the second gap. The expressions “a first gap” and “a second gap” could therefore alternatively be “at least one first gap” and “at least one second gap”.

Furthermore, the first and second gaps will often be illustrated in this disclosure as substantially parallel elongated gaps, but their geometry and mutual orientation in the plane determined by the structural layer (the xy-plane) could be of any kind. The first gap could for example be perpendicular to the second gap, or it could be oriented at any angle in relation to the second gap. The shape of the first and/or second gaps may be rectangular or meandering, but these gaps could alternatively have any other shape. The first gap and second gap could also be concatenated—so that they together form an extended gap which is narrow in a first section and broader in a second section. This will be illustrated in the practical examples below.

In this disclosure the “width” of a gap refers to the smallest dimension of the gap in the xy-plane—not to its dimension in any particular direction.

The word “horizontal” refers in this disclosure to the plane defined by the structural layer, which is illustrated as the xy-plane. The z-axis, and words such as “vertical”, “up” and “down”, refer to the direction which is perpendicular to the horizontal plane. These words do not imply anything about how the wafer should be oriented during manufacturing or how a manufactured device should be oriented when it is used.

FIG.2aillustrates the first stage in the deposition and patterning of the first and second etching mask.21is structural layer. A first etching mask22has been deposited and patterned on the surface of the structural layer, and a second etching mask23has been deposited after the first etching mask. An opening in the first etching mask allows the second etching mask to form a load-reducing part230which reaches down to the surface of the structural layer21. The second etching mask23is here used for completing the patterning the first etching mask22—the material of the first etching mask22will be removed in regions defined by the gaps231—233in the second etching mask23. It could alternatively be possible to complete the patterning of the first etching mask before the second etching mask is deposited and patterned.

FIG.2billustrates the first and second etching masks after the patterning of the first etching mask22has been completed by removing the areas of the first etching mask22which lay under the gaps231—233in the second etching mask23. InFIG.2bthe first etching mask22has a first opening221which defines a first unprotected area261on the surface of the structural layer21, where the first gap will be formed. In other words, the area of the first opening on the horizontal face of the structural layer corresponds to the area where the first gap will be etched. The width W1of the first opening will determine the width of the first gap.

The first etching mask22also has a second opening—here formed by the two openings222—223and the intermediate space where the load-reducing part230of the second etching mask23is located. The area of the second opening on the horizontal face of the structural layer corresponds to the area where the second gap will be etched. The width of this second opening—which has been indicated with W2inFIG.2, and which is equal to W3+W5+W4inFIG.2b—will eventually determine the width of the second gap.

The load-reducing part230is formed from the material of the second etching mask23, as described above. This part divides the surface of the structural layer within the second opening into a temporarily protected area264and two second unprotected areas262and263. A temporary part219will be formed in the structural layer beneath the temporarily protected area264.

By dimensioning the load-reducing part suitably, the DRIE load on the sidewalls of the second gap can be made as low as the DRIE load on the sidewalls of the first gap. This can be achieved by dimensioning the openings222and223so that the widths of the second unprotected areas262and263is at least approximately equal to the width of the first opening—in other words, W3≈W1and W4≈W1. The width W1may for example be in the range 1 μm-10 μm, 1 μm-5 μm or 2 μm-5 μm.

FIG.2cillustrates the next step where trenches241—243is etched through the wafer in a DRIE etching process. Due to the fact that the widths of the trenches241—243are substantially equal, none of the sidewalls in these trenches will be subjected to a high DRIE load. The trench241is a permanent trench which will form the first gap which separates part211from part212. It will remain at the width it has been given inFIG.2c. Trenches242and243, on the other hand, are temporary trenches in the sense that they will eventually be merged into the second gap which will separate part212from part213. In other words, trenches242and243will form parts of the second gap. InFIG.2cthe structural layer still contains a temporary part219between the parts212and213. This temporary part219will be removed later, as described below. The second gap will then be formed by trenches242and243and by the space between these trenches, i.e. the region which was occupied by temporary part219before it was removed.

In the next step, illustrated inFIG.2d, the second etching mask23is removed and the sidewalls of the trenches (such as2411,2421and2431) are coated with a protective layer251. The second etching mask23may be removed before the sidewalls are coated, or the sidewalls may be coated before the second etching mask23is removed. The protective layer251can be made of any material which is suitable for deposition in narrow trenches, which is sturdy enough to withstand the subsequent etching step where temporary part219is removed, and which can be easily removed after that etching step. The protective layer251may for example be a silicon dioxide layer. It may for example be deposited in a chemical vapour deposition process where tetraethylorthosilicate (TEOS) is used as a precursor.

In the structural layer21illustrated inFIG.2d, the second etching mask23has been removed and the sidewalls of the trenches have been coated with a protective layer251. After the removal of the second etching mask23, the temporarily protected area264, which forms the top surface of the temporary part219, is unprotected.

Finally, the temporary part219is removed. It may for example be removed in a DRIE etching process or a wet etching process. The sidewalls are protected by protective layer251during this step and will therefore not be damaged.FIG.2eillustrates a device where the temporary part219has been completely removed, so that part211is separated from part212by a first gap28and part212is separated from part213by a second gap29. The second gap29is wider than the first gap28, but the sidewalls are intact in both gaps. The protective layer251has also been removed. The first mask22may be removed in a subsequent step.

The step of coating at least the sidewalls of the trenches with a protective layer may comprise filling the trenches with the material of the protective layer. In other words, the protective layer251may be so thick that it fills the permanent trench241and the temporary trenches242and243. This option is illustrated inFIG.2f, which illustrates an alternative step which would follow the step illustrated inFIG.2c(instead of the step swown inFIG.2d). Here the protective layer251fills the trenches. The temporary part219is then removed with any of the methods mentioned above. After that, and after the protective layer251has been removed, the structure will be the same as inFIG.2e.

The width W5of the load-reducing part230—and the width of the underlying temporary part219of the structural layer—does not have to be close to equal to the width W1of the first opening221in the first etching mask22. The load-reducing part230could alternatively be much wider than the first opening221, as described below.

The load-reducing part may comprise a rectangular section on the horizontal face of the structural layer. The load-reducing part may comprise of one rectangle on the horizontal face of the structural layer. This rectangle may extend from a first edge of the second opening to an opposing second edge of the second opening.

FIG.3aillustrates a part of the horizontal face of the structural layer in the xy-plane, with the first and second mask on top. Reference numbers32,33,321-323and330correspond to reference numbers22,23,221-223and230, respectively, inFIGS.2a-2b. Reference number32/33indicates the area where at least the first etching mask32is present. The second etching mask33may lie on top of the first etching mask32in these areas, asFIG.2billustrates.

InFIG.3athe second opening in the first etching mask32has a first edge371and an opposing second edge372. The load-reducing part330extends from the first edge371to the second edge372. In the illustrated case, the load-reducing part has been placed substantially in the middle of the second opening, so that the at least one second unprotected area here comprises two rectangular unprotected areas on opposing sides of the load-reducing part, illustrated by the openings322and323between the load-reducing part330and the first etching mask32.

The load-reducing part could alternatively extend from the first edge371to the second edge372along one side of the second opening, instead of in the middle. This is illustrated inFIG.3b. The at least one second unprotected area is in this case one rectangular area which lies next to the load-reducing part330, illustrated by gap323. The difference between the arrangement illustrated inFIG.3aand the one illustrated inFIG.3bis that inFIG.3aboth the left and right sidewalls of the second gap (that is, the sidewalls which lie opposed to each other in the x-direction) will be etched with a DRIE load which is as low as the load on the sidewalls in the first gap. In contrast, inFIG.3bonly the sidewall on the right will be etched with this low DRIE load. This may be acceptable if the quality of the left sidewall is not important.

As illustrated inFIG.3b, the load-reducing part330may be much wider than the first gap.FIG.3cillustrates an option where a wide load-reducing part330is placed in the middle of the second opening. AsFIG.3cillustrates, the load-reducing part330may be wider than the sum of the widths of the adjacent openings322and323(W5>W3+W4). On the other hand, the load reducing part could alternatively be narrower than either of the openings322and323(W5<W3and W5<W4). These width options apply to all embodiments presented in this disclosure.

The load-reducing part may alternatively comprise of one rectangle on the horizontal face of the structural layer, wherein the rectangle extends from a first edge of the second opening to a first point inside the second opening. The distance from the first point to a second edge of the second opening, which is opposite to the first edge, may be substantially equal to the width of the first opening.

FIG.3dillustrates a device where the load-reducing part330is a rectangle which extends from the first edge371toward the second edge372, not to all the way to the second edge. Instead it ends at a first point. If the distance W6is substantially equal to the width W1, the sidewall which will be formed under the second edge372of the second opening will also be subjected only to a low DRIE load, comparable to the load on the sidewalls of the first gap.

This method can also be extended to the sidewall formed under the first edge371, asFIG.3eillustrates. The load-reducing part330here comprises one rectangle on the horizontal face of the structural layer, wherein the rectangle extends from a first point381inside the second opening to a second point382inside the second opening. Both the distance (W6) from the first point381to a first edge371of the second opening and the distance (W7) from the second point382to a second edge372of the second opening, which is opposite to the first edge, may be substantially equal to the width (W1) of the first opening.

The load-reducing part may alternatively or complementarily comprise a convex section and/or a concave section on the horizontal face of the structural layer. In other words, one or both sides of the load-reducing part could in some places have a convex or concave shape.FIG.3fillustrates a load-reducing part330which has a straight edge on the left side and a concave shape on the right side. Here the right-hand edge of the second opening has a convex shape, and the concave shape of the load-reducing part330allows the width W4to be equal to the width W1of the first opening along the edge.

The load-reducing part can alternatively have any other geometry which is determined by the features of the second opening in the xy-plane.FIG.3gillustrates a load-reducing part330which comprises two straight sections and a bent section which accommodates the convex and concave edges of the second opening so that the widths W3and W4remain substantially equal to the width W1.

FIG.4illustrates a first practical example. The micromechanical structures in the structural layer here comprise a mobile rotor and a fixed stator. Interdigitated comb structures prepared in the structural layer to measure the movement of the rotor in relation to the stator. Only a small part of the device is illustrated inFIG.4. Reference numbers42,43,421,422,423and430correspond to reference numbers32,33,321,322,323and330, respectively, inFIGS.3a-3g. The first etching mask42comprises areas which define the stator45, first and second rotor electrodes441—442and a first stator electrode451which is paired with the first rotor electrode441. The second rotor electrode442is in turn paired with a second stator electrode, which is not illustrated. The rotor electrodes extend from the rotor toward the stator45. The rotor is not illustrated.

The first opening421will define a narrow gap between the first rotor electrode441and the first stator electrode451. The second opening, on the other hand, which will be located in the area422+423+430, will define a broader gap between the first stator electrode451and the second rotor electrode442. By implementing a second etching mask43with a load-reducing part430, and by dimensioning the widths W3and W4of openings422and423substantially equal to the width W1of the first opening421, the sidewalls4221and4231of the electrodes can be protected from structural damage.

FIG.5illustrates a second practical example. The micromechanical structures in the structural layer here again comprise a mobile rotor and a fixed stator. Interdigitated comb structures prepared in the structural layer to measure the movement of the rotor in relation to the stator. Only a small part of the device is illustrated inFIG.5. Reference numbers52,53,521,522,523and530correspond to reference numbers32,33,321,322,323and330, respectively, inFIGS.3a-3g. The first etching mask52comprises areas which define the rotor54and the stator55. The rotor also comprises a motion limiter bump541. If the rotor moves toward the stator, the motion limiter bump541will make contact with the stator55before any other part of the rotor54touches the stator55. Such motion limiter bumps are used to prevent short-circuits and structural damage for example in situations where the MEMS device is subjected to a sudden external shock.

In this example the first gap and the second gap are concatenated. Together they separate the rotor54from the stator55. The first opening521defines the first gap which will be the motion limiter gap where the rotor54is designed to first come into contact with the stator55. The second opening, which, as before, will be formed by openings522and523and the region covered by the load-reducing part530after the temporary structure has been removed, defines the second gap which will be broader than the first. The widths W3and W4of openings522and523is substantially equal to the width W1of the first opening521, so the sidewalls of the rotor and stator will not be damaged.