Patent Description:
Traditionally, two methods have been employed to manufacture MEMS. On the one hand, dry etch techniques using chlorine based chemistries and reactive ion etching are the most prevalent. However, these typically require the use of very reactive and toxic gases and the use of specialized equipment, which necessitate implementation of complex systems and capital intensive investments in order to meet desired capabilities while also meeting stringent safety requirements. As such, dry etch suffers from not only high initial investment costs, but also relatively low throughput capabilities. On the other hand, wet etch techniques suffer from inconsistent etch rates for some materials. As such, previous techniques have not provided a way to reliably pattern those materials.

Methods of depositing aluminum nitride layers and methods of etching aluminum nitride layers are known from:.

A method of manufacturing a patterned aluminum nitride layer includes growing an amorphous patterned layer on a seed layer. The amorphous patterned layer promotes growth of a first type aluminum nitride layer that has a disordered crystallographic structure. The seed layer promotes growth of a second type aluminum nitride layer with a vertically oriented columnar crystal structure. The method also includes depositing an aluminum nitride layer over the amorphous patterned layer and the seed layer to form the first type aluminum nitride layer with the disordered crystallographic structure over the amorphous patterned layer and the second type aluminum nitride layer with the vertically oriented columnar crystal structure over the seed layer. The method also includes depositing a masking layer over the second type aluminum nitride layer and etching away the first type aluminum nitride layer.

<FIG> shows a process of patterning an AlN layer in one embodiment of the disclosure.

The present disclosure relates generally to the manufacturing of MEMS. More specifically, this disclosure relates to the patterning of an aluminum nitride (AlN) layer within a MEMS using a wet etch technique.

AlN is a material of interest in the MEMS manufacturing industry because it has desirable dielectric and piezoelectric properties and is compatible with current CMOS manufacturing processes. AlN also has a high thermal conductivity. As such, AlN can withstand relatively harsh working environments, such as elevated temperature and pressure, which may be present in a working aircraft engine. For example, a MEMS sensor having an AlN layer may be desirable to use within a working aircraft engine because of AlN's tolerance to elevated temperatures and pressures.

However, the ability to selectively etch away AlN to form a patterned layer has been difficult to achieve. As such, AlN layers with a non-uniform or unknown crystal quality leads to devices of poor quality because the etching rate of the AlN is highly dependent upon the crystal quality of the AlN layer. This leads to inconsistent AlN pattern formation. High quality AlN films can have a vertically oriented columnar crystal structure. For consistent AlN pattern formation, previous patterning methods have required the use of dry etch techniques, which is slow and expensive. Described herein is a method for depositing layers of AlN with a locally controlled and modified crystal quality. For example, an AlN layer with a desired pattern can be deposited having a high crystal quality, such as a vertically oriented columnar crystal structure, whereas, on other areas (the negative pattern) an AlN layer having a poor crystal quality, such as a disordered or random crystal structure, can be deposited. The AlN having a relatively poor crystal quality can then be selectively etched away using wet etch techniques.

<FIG> shows a process of patterning an AlN layer in one embodiment of the disclosure. In a first step, an amorphous layer <NUM>, having a negative pattern relative to the desired final AlN patterned layer, is deposited on top of a base layer <NUM>. <FIG> is a cross-sectional view of base layer <NUM> with amorphous layer <NUM> (having a desired pattern that is the negative image of the desired pattern of the AlN layer) deposited on top of base layer <NUM>. Base layer <NUM> can be any crystal substrate which promotes growth of an AlN layer having a highly ordered crystal structure. Base layer <NUM> can be, for example, a silicon crystal surface or a molybdenum layer.

Amorphous layer <NUM> can be any formed of any material which promotes growth of an AlN layer having a poorly ordered crystal structure relative to the AlN layer grown on base layer <NUM>. Amorphous layer <NUM> can be formed of, for example, silicon dioxide (SiO<NUM>) or silicon nitride (Si<NUM>N<NUM>). The thickness of amorphous layer <NUM> is typically from <NUM> to <NUM> measured in a perpendicular direction from the top of base layer <NUM>. Amorphous layer <NUM> can be deposited by any technique known in the art. For example, amorphous layer <NUM> formed of SiO<NUM> or Si<NUM>N<NUM> having an appropriate thickness can be deposited using plasma enhanced chemical vapor deposition in about <NUM> minutes at <NUM>° C. Alternatively, SiO<NUM> can also be thermally grown at temperatures around <NUM>° C and above. Alternatively, a thermal oxide layer may already be present from a previous processing step. The thermal oxide layer may already be deposited having a desired pattern or the thermal oxide layer may need further processing to achieve the desired pattern.

In a next step, AlN is deposited simultaneously on top of both amorphous layer <NUM>, which has the negative image of the desired pattern of the AlN layer, and base layer <NUM>, which has the desired pattern of the final AlN layer. <FIG> is a cross-sectional view of base layer <NUM> with amorphous layer <NUM> deposited on top of base layer <NUM>, first type AlN layer <NUM> deposited on top of amorphous layer <NUM>, and second type AlN layer <NUM> deposited directly on top of base layer <NUM>. The simultaneous deposition of AlN on both amorphous layer <NUM> and base layer <NUM> results in two distinct AlN layers being formed, differentiated by the resulting crystal quality of first type AlN layer <NUM> compared to second type AlN layer <NUM>. First type AlN layer <NUM> has a poorly ordered crystal structure whose disordered growth is promoted by underlying amorphous layer <NUM>. Second type AlN layer <NUM> has a more ordered crystal structure relative to first type AlN layer <NUM>. The more ordered crystal structure is a vertically oriented columnar crystal structure whose growth is promoted by underlying base layer <NUM>. First type AlN layer <NUM> and second type AlN layer <NUM> may be deposited by any known method in the art. For example, AlN is usually deposited by sputtering techniques at or below <NUM>° C. The thickness of the AlN layer can be from <NUM> to <NUM> measured in a perpendicular direction from the top of the adjacent underlying layer. The thickness of the AlN layer can also be from <NUM> to <NUM>. The thickness of the AlN layer can also be from <NUM> to <NUM>.

In a next step, masking layer <NUM> is deposited on top of second type AlN layer <NUM>. <FIG> is a cross-sectional view of base layer <NUM> with amorphous layer <NUM> deposited on top of base layer <NUM>, first type AlN layer <NUM> deposited on top of amorphous layer <NUM>, second type AlN layer <NUM> deposited directly on top of base layer <NUM>, and masking layer <NUM> on top of second type AlN layer <NUM>. Masking layer <NUM> can be formed from any material which inhibits the etching rate of second type AlN layer <NUM> relative to the etching rate of first type AlN layer <NUM>. For example, masking layer <NUM> can be a photoresist, which is typically a light-sensitive organic material. A photoresist layer can be applied and then a patterned mask is used to selectively block the light. A developing agent is then applied, which removes the masking material from unwanted areas. The desired pattern of masking layer <NUM>, which can have the same pattern as second type AlN layer <NUM>, is left behind as shown in <FIG>. Alternatively, masking layer <NUM> can be formed from a metal oxide such as SiO<NUM>.

In a next step, first type AlN layer <NUM> is removed. <FIG> is a cross-sectional view of base layer <NUM> with amorphous layer <NUM> deposited on top of base layer <NUM>, second type AlN layer <NUM> deposited directly on top of base layer <NUM>, and masking layer <NUM> on top of second type AlN layer <NUM>. First type AlN layer <NUM> is preferably removed by a wet etching process, such as potassium hydroxide or phosphoric acid based chemistries. For example, the commercial composition AZ® <NUM>, which contains NaOH and KOH, can etch AlN at the rate of hundreds of nm/min. The exact rate depends primarily upon the concentration of KOH and the wet etch temperature used.

The wet etch rates for disordered structures of AlN are much higher than that for more ordered crystal structures of AlN, such as vertically oriented columnar crystal structures. As such, first type AlN layer <NUM> is selectively removed at a much higher rate compared to second type AlN layer <NUM> due to the relative disordered crystal structure of first type AlN layer <NUM> compared to the ordered crystal structure of second type AlN layer <NUM>. Masking layer <NUM> protects second type AlN layer <NUM> from wet etching. Although the etching selectivity for first type AlN layer <NUM> is higher compared to second type AlN layer <NUM>, some undercutting of second type AlN layer <NUM> is likely to occur, especially near the masking layer transitions.

In a next step, masking layer <NUM> is removed. <FIG> is a cross-sectional view of base layer <NUM> with amorphous layer <NUM> deposited on top of base layer <NUM> and second type AlN layer <NUM> deposited directly on top of base layer <NUM>. Masking layer <NUM> can be removed by any method known in the art.

In a next step, amorphous layer <NUM> is removed. <FIG> is a cross-sectional view of base layer <NUM> and second type AlN layer <NUM> deposited directly on top of base layer <NUM>. Amorphous layer <NUM> can be removed by any method known in the art. For example, hydrogen fluoride can be used to remove SiO<NUM> at <NUM>° C in less than <NUM> minutes. Alternatively, phosphoric acid can be used to remove Si<NUM>N<NUM> at <NUM>° C in about <NUM>-<NUM> minutes.

Alternatively, amorphous layer <NUM> can be removed prior to masking layer <NUM>. Alternatively, masking layer <NUM> and amorphous layer <NUM> can be removed simultaneously in a single step. For example, if amorphous layer <NUM> and masking layer <NUM> are both formed of SiO<NUM>, then both layers can be removed simultaneously using hydrogen fluoride at <NUM>° C.

Additional layers of AlN or other alloys or other metals can be added using a variety of thin-film and bonding techniques as well as by etching through sacrificial layers to build a final structure. This can continue until a MEMS with a desired geometry has been built, which can be used, for example, as sensors, actuators, and complex systems. These MEMS can include, for example, motors, bearings, gears, and linkages formed by using appropriate deposition patterns, masking patterns, and etching techniques. These MEMS can be manufactured together with an integrated circuit or manufactured separately and assembled later.

Using wet etching chemistries to remove the relatively disordered AlN crystal structure obviates the need to use dry etch techniques, which require expensive equipment and materials to start up and the use of toxic chemicals. Furthermore, using wet etch chemistries to remove the relatively disordered AlN allows for batch processing, which greatly increases throughput compared to dry etch techniques. Wet etching of AlN also does not introduce additional contaminants into current CMOS manufacturing processes.

A method of manufacturing a patterned aluminum nitride layer includes growing an amorphous patterned layer on a seed layer that promotes growth of a first type aluminum nitride layer having a disordered crystallographic structure. The seed layer promotes growth of a second type aluminum nitride layer with a vertically oriented columnar crystal structure. The method also includes depositing aluminum nitride over the amorphous patterned layer and the seed layer to form the first type aluminum nitride layer with the disordered crystallographic structure over the amorphous patterned layer and the second type aluminum nitride layer with the vertically oriented columnar crystal structure over the seed layer. The method also includes depositing a masking layer over the second type aluminum nitride layer and etching away the first type aluminum nitride layer.

The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following:.

The method includes removing the masking layer and removing the amorphous patterned layer.

The seed layer is formed of silicon or molybdenum.

The amorphous patterned layer is formed of silicon dioxide or silicon nitride.

A thickness of the second type aluminum nitride layer is from <NUM> to <NUM>, inclusive.

The etching away the first type aluminum nitride layer further includes exposing the first type aluminum nitride layer to potassium hydroxide.

The potassium hydroxide is provided by the chemical composition AZ® <NUM>.

The etching away includes exposing the aluminum nitride covered amorphous patterned layer to phosphoric acid.

The method also includes using the patterned aluminum nitride layer in a sensor.

A method of manufacturing a patterned aluminum nitride layer includes growing an amorphous patterned layer on a seed layer that promotes growth of a first type aluminum nitride layer having a disordered crystallographic structure. The seed layer promotes growth of a second type aluminum nitride layer with a vertically oriented columnar crystal structure. A thickness of the first and second type aluminum nitride layer is from <NUM> to <NUM>, inclusive. The method also includes depositing aluminum nitride over the amorphous patterned layer and the seed layer to form the first type aluminum nitride layer with the disordered crystallographic structure over the amorphous patterned layer and the second type aluminum nitride layer with the vertically oriented columnar crystal structure over the seed layer. The method also includes depositing a masking layer over the second type aluminum nitride layer; etching away the first type aluminum nitride layer; removing the masking layer; and removing the amorphous patterned layer.

The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following:
The etching away the first type aluminum nitride layer further includes exposing the first type aluminum nitride layer to potassium hydroxide.

Claim 1:
A method of manufacturing a patterned aluminum nitride layer, the method comprising:
growing an amorphous patterned layer (<NUM>) on a seed layer (<NUM>), wherein the amorphous patterned layer promotes growth of a first type aluminum nitride layer having a disordered crystallographic structure and wherein the seed layer promotes growth of a second type aluminum nitride layer with a vertically oriented columnar crystal structure;
depositing aluminum nitride over the amorphous patterned layer and the seed layer to form the first type aluminum nitride layer (<NUM>) with the disordered crystallographic structure over the amorphous patterned layer and the second type aluminum nitride layer (<NUM>) with the vertically oriented columnar crystal structure over the seed layer;
depositing a masking layer (<NUM>) over the second type aluminum nitride layer; and
etching away the first type aluminum nitride layer.