Patent Description:
Microelectromechanical (MEMS) systems often comprise micromechanical structures formed in the structural layer of a device wafer by etching. These micromechanical parts can for example include mobile structures that are suspended with flexible suspenders from adjacent fixed parts of the structural layer. MEMS systems which comprise mobile micromechanical structures include, for example, acceleration sensors, gyroscopes, micromirrors, optical switches and scanners.

Micromechanical structures are created by separating some parts of the wafer from other parts, for example by etching a trench all the way through the structural layer so that a gap is formed between two structures. <FIG> illustrates schematically a structural layer <NUM> where structures <NUM> - <NUM> are separated from each other in the illustrated region. The micromechanical structures often include some (such as <NUM> and <NUM>) which have to be separated from each other by a narrow gap <NUM>, and others (such as <NUM> and <NUM>) which have to be separated from each other by a significantly broader gap <NUM>.

Deep-reactive ion etching (DRIE) is a common method for etching gaps and trenches in silicon wafers. When narrow gaps such as <NUM> and broad gaps such as <NUM> are etched in the same DRIE process, the sidewalls <NUM> of the broad gap <NUM> are often burdened by DRIE load effects such as striation. The structural damage caused by load effects can introduce measurement errors or cause short-circuits in the MEMS device. It would therefore be beneficial to perform a DRIE etch without producing load effects on the any sidewalls.

An object of the present disclosure is to provide a method for alleviating problems that result from DRIE load effects when narrow and wide gaps are etched simultaneously.

The object of the disclosure is achieved by a method which is characterized by what is stated in the independent claim. The preferred embodiments of the disclosure are disclosed in the dependent claims.

The disclosure is based on the idea of leaving a temporary structure in the middle of the broader gap, so that two temporary gaps are formed on opposing sides of the broader gap. These temporary gaps have substantially the same width as the narrow gap and they are etched in the same step as the narrow gap. An additional etching step is then performed to remove the temporary structure. An advantage of this method is that the sidewalls of the finished broad gap will not suffer from DRIE load effects. <CIT> discloses a method of reactive ion etching a substrate to form at least a first and a second etched feature, wherein the first etched feature has a greater aspect ratio than the second etched feature. The method comprises the steps of in a first etching stage, etching said substrate so as to etch only the first feature to a predetermined depth; and thereafter in a second etching stage etching the substrate so as to etch both the first and said second features to a respective depth.

The invention is set out in claim <NUM>. The invention relates to 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: (<NUM>) 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: (<NUM>) etching trenches in a DRIE process through the structural layer in the first and second unprotected areas which are not protected by the first etching mask or the second etching mask, (<NUM>) 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 (<NUM>) etching away the structural layer in the exposed temporarily protected area, said sidewalls of the trenches being protected by the protective layer during said etching.

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, as <FIG> illustrates, a central part (<NUM>) may be separated from an adjacent part (<NUM>) on one side by the first gap and from another adjacent part (<NUM>) 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> illustrates the first stage in the deposition and patterning of the first and second etching mask. <NUM> is structural layer. A first etching mask <NUM> has been deposited and patterned on the surface of the structural layer, and a second etching mask <NUM> has been deposited after the first etching mask. An opening in the first etching mask allows the second etching mask to form a load-reducing part <NUM> which reaches down to the surface of the structural layer <NUM>. The second etching mask <NUM> is here used for completing the patterning the first etching mask <NUM> - the material of the first etching mask <NUM> will be removed in regions defined by the gaps <NUM> - <NUM> in the second etching mask <NUM>. It could alternatively be possible to complete the patterning of the first etching mask before the second etching mask is deposited and patterned.

<FIG> illustrates the first and second etching masks after the patterning of the first etching mask <NUM> has been completed by removing the areas of the first etching mask <NUM> which lay under the gaps <NUM> - <NUM> in the second etching mask <NUM>. In <FIG> the first etching mask <NUM> has a first opening <NUM> which defines a first unprotected area <NUM> on the surface of the structural layer <NUM>, 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 W<NUM> of the first opening will determine the width of the first gap.

The first etching mask <NUM> also has a second opening - here formed by the two openings <NUM> - <NUM> and the intermediate space where the load-reducing part <NUM> of the second etching mask <NUM> is 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 W<NUM> in <FIG>, and which is equal to W<NUM> + W<NUM> + W<NUM> in <FIG> - will eventually determine the width of the second gap.

The load-reducing part <NUM> is formed from the material of the second etching mask <NUM>, as described above. This part divides the surface of the structural layer within the second opening into a temporarily protected area <NUM> and two second unprotected areas <NUM> and <NUM>. A temporary part <NUM> will be formed in the structural layer beneath the temporarily protected area <NUM>.

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 openings <NUM> and <NUM> so that the widths of the second unprotected areas <NUM> and <NUM> is at least approximately equal to the width of the first opening - in other words, W<NUM> ≈ W<NUM> and W<NUM> ≈ W<NUM>. The width W<NUM> may for example be in the range <NUM> - <NUM>, <NUM> - <NUM> or <NUM> - <NUM>.

<FIG> illustrates the next step where trenches <NUM> - <NUM> is etched through the wafer in a DRIE etching process. Due to the fact that the widths of the trenches <NUM> - <NUM> are substantially equal, none of the sidewalls in these trenches will be subjected to a high DRIE load. The trench <NUM> is a permanent trench which will form the first gap which separates part <NUM> from part <NUM>. It will remain at the width it has been given in <FIG>. Trenches <NUM> and <NUM>, on the other hand, are temporary trenches in the sense that they will eventually be merged into the second gap which will separate part <NUM> from part <NUM>.

In other words, trenches <NUM> and <NUM> will form parts of the second gap. In <FIG> the structural layer still contains a temporary part <NUM> between the parts <NUM> and <NUM>. This temporary part <NUM> will be removed later, as described below. The second gap will then be formed by trenches <NUM> and <NUM> and by the space between these trenches, i.e. the region which was occupied by temporary part <NUM> before it was removed.

In the next step, illustrated in <FIG>, the second etching mask <NUM> is removed and the sidewalls of the trenches (such as <NUM>, <NUM> and <NUM>) are coated with a protective layer <NUM>. The second etching mask <NUM> may be removed before the sidewalls are coated, or the sidewalls may be coated before the second etching mask <NUM> is removed. The protective layer <NUM> can 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 part <NUM> is removed, and which can be easily removed after that etching step. The protective layer <NUM> may 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 layer <NUM> illustrated in <FIG>, the second etching mask <NUM> has been removed and the sidewalls of the trenches have been coated with a protective layer <NUM>. After the removal of the second etching mask <NUM>, the temporarily protected area <NUM>, which forms the top surface of the temporary part <NUM>, is unprotected.

Finally, the temporary part <NUM> is removed. It may for example be removed in a DRIE etching process or a wet etching process. The sidewalls are protected by protective layer <NUM> during this step and will therefore not be damaged. <FIG> illustrates a device where the temporary part <NUM> has been completely removed, so that part <NUM> is separated from part <NUM> by a first gap <NUM> and part <NUM> is separated from part <NUM> by a second gap <NUM>. The second gap <NUM> is wider than the first gap <NUM>, but the sidewalls are intact in both gaps. The protective layer <NUM> has also been removed. The first mask <NUM> may 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 layer <NUM> may be so thick that it fills the permanent trench <NUM> and the temporary trenches <NUM> and <NUM>. This option is illustrated in <FIG>, which illustrates an alternative step which would follow the step illustrated in <FIG> (instead of the step swown in <FIG>). Here the protective layer <NUM> fills the trenches. The temporary part <NUM> is then removed with any of the methods mentioned above. After that, and after the protective layer <NUM> has been removed, the structure will be the same as in <FIG>.

The width W<NUM> of the load-reducing part <NUM> - and the width of the underlying temporary part <NUM> of the structural layer - does not have to be close to equal to the width W<NUM> of the first opening <NUM> in the first etching mask <NUM>. The load-reducing part <NUM> could alternatively be much wider than the first opening <NUM>, 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> illustrates a part of the horizontal face of the structural layer in the xy-plane, with the first and second mask on top. Reference numbers <NUM>, <NUM>, <NUM> - <NUM> and <NUM> correspond to reference numbers <NUM>, <NUM>, <NUM> - <NUM> and <NUM>, respectively, in <FIG>. Reference number <NUM> / <NUM> indicates the area where at least the first etching mask <NUM> is present. The second etching mask <NUM> may lie on top of the first etching mask <NUM> in these areas, as <FIG> illustrates.

In <FIG> the second opening in the first etching mask <NUM> has a first edge <NUM> and an opposing second edge <NUM>. The load-reducing part <NUM> extends from the first edge <NUM> to the second edge <NUM>. 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 openings <NUM> and <NUM> between the load-reducing part <NUM> and the first etching mask <NUM>.

The load-reducing part could alternatively extend from the first edge <NUM> to the second edge <NUM> along one side of the second opening, instead of in the middle. This is illustrated in <FIG>. The at least one second unprotected area is in this case one rectangular area which lies next to the load-reducing part <NUM>, illustrated by gap <NUM>. The difference between the arrangement illustrated in <FIG> and the one illustrated in <FIG> is that in <FIG> both 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, in <FIG> only 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 in <FIG>, the load-reducing part <NUM> may be much wider than the first gap. <FIG> illustrates an option where a wide load-reducing part <NUM> is placed in the middle of the second opening. As <FIG> illustrates, the load-reducing part <NUM> may be wider than the sum of the widths of the adjacent openings <NUM> and <NUM> (W<NUM> > W<NUM> + W<NUM>). On the other hand, the load reducing part could alternatively be narrower than either of the openings <NUM> and <NUM> (W<NUM> < W<NUM> and W<NUM> < W<NUM>). 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> illustrates a device where the load-reducing part <NUM> is a rectangle which extends from the first edge <NUM> toward the second edge <NUM>, not to all the way to the second edge. Instead it ends at a first point. If the distance W<NUM> is substantially equal to the width W<NUM>, the sidewall which will be formed under the second edge <NUM> of 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 edge <NUM>, as <FIG> illustrates. The load-reducing part <NUM> here comprises one rectangle on the horizontal face of the structural layer, wherein the rectangle extends from a first point <NUM> inside the second opening to a second point <NUM> inside the second opening. Both the distance (W<NUM>) from the first point <NUM> to a first edge <NUM> of the second opening and the distance (W<NUM>) from the second point <NUM> to a second edge <NUM> of the second opening, which is opposite to the first edge, may be substantially equal to the width (W<NUM>) 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> illustrates a load-reducing part <NUM> which 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 part <NUM> allows the width W<NUM> to be equal to the width W<NUM> of 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> illustrates a load-reducing part <NUM> which comprises two straight sections and a bent section which accommodates the convex and concave edges of the second opening so that the widths W<NUM> and W<NUM> remain substantially equal to the width W<NUM>.

<FIG> illustrates 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 in <FIG>. Reference numbers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> correspond to reference numbers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, respectively, in <FIG>. The first etching mask <NUM> comprises areas which define the stator <NUM>, first and second rotor electrodes <NUM> - <NUM> and a first stator electrode <NUM> which is paired with the first rotor electrode <NUM>. The second rotor electrode <NUM> is in turn paired with a second stator electrode, which is not illustrated. The rotor electrodes extend from the rotor toward the stator <NUM>. The rotor is not illustrated.

The first opening <NUM> will define a narrow gap between the first rotor electrode <NUM> and the first stator electrode <NUM>. The second opening, on the other hand, which will be located in the area <NUM> + <NUM> + <NUM>, will define a broader gap between the first stator electrode <NUM> and the second rotor electrode <NUM>. By implementing a second etching mask <NUM> with a load-reducing part <NUM>, and by dimensioning the widths W<NUM> and W<NUM> of openings <NUM> and <NUM> substantially equal to the width W<NUM> of the first opening <NUM>, the sidewalls <NUM> and <NUM> of the electrodes can be protected from structural damage.

<FIG> illustrates 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 in <FIG>. Reference numbers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> correspond to reference numbers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, respectively, in <FIG>. The first etching mask <NUM> comprises areas which define the rotor <NUM> and the stator <NUM>. The rotor also comprises a motion limiter bump <NUM>. If the rotor moves toward the stator, the motion limiter bump <NUM> will make contact with the stator <NUM> before any other part of the rotor <NUM> touches the stator <NUM>. 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.

Claim 1:
A method for manufacturing a micromechanical structure in the structural layer (<NUM>) of a wafer by forming a first gap (<NUM>) and a second gap (<NUM>) in the structural layer so that the first gap has a first gap width (W<NUM>) and the second gap has a second gap width (W<NUM>), wherein the second gap width (W<NUM>) is greater than the first gap width (W<NUM>), and the method comprising the following steps:
depositing and patterning a first etching mask (<NUM>, <NUM>) and a second etching mask (<NUM>, <NUM>) on a face of the structural layer, so that
- the first etching mask has a first opening (<NUM>, <NUM>) which defines the location and dimensions of the first gap (<NUM>), so that the width of the first opening is equal to the first gap width (W<NUM>) and the first opening forms a first unprotected area (<NUM>).
- the first etching mask (<NUM>,<NUM>) has a second opening which defines the location anc dimensions of the second gap (<NUM>), so that the width of the second opening is equal to the second gap width (W<NUM>), and
- the second etching mask comprises a first part (<NUM>, <NUM>) within the second opening in the first etching mask (<NUM>,<NUM>), so that the first part divides the second opening into a temporarily protected area (<NUM>) which is covered by the first part (<NUM>, <NUM>) and at least one second unprotected area (<NUM>, <NUM>) which is not covered by the first part (<NUM>, <NUM>), wherein the width (W<NUM>, W<NUM>) of the at least one second unprotected area is substantially equal to the width of the first opening,
etching trenches (<NUM>, <NUM>, <NUM>) in a DRIE process through the structural layer (<NUM>) in the first and second unprotected areas which are not protected by the first etching mask (<NUM>,<NUM>) or the second etching mask (<NUM>,<NUM>),
coating at least the sidewalls (<NUM>, <NUM>, <NUM>) of the trenches (<NUM>, <NUM>, <NUM>) with a protective layer (<NUM>) and removing the second etching mask (<NUM>,<NUM>) at least from the second opening in the first etching mask (<NUM>,<NUM>), so that the temporarily protected area (<NUM>) is exposed,
etching away the structural layer (<NUM>) in the exposed temporarily protected area, said sidewalls (<NUM>, <NUM>, <NUM>) of the trenches (<NUM>, <NUM>, <NUM>) being protected by the protective layer (<NUM>) during said etching.