Patent Description:
wherein the method comprises performing a series of steps on a monocrystalline wafer substrate.

Manufacturing of MEMS (microelectromechanical systems) devices is well known in the art, for example the manufacturing of probes like AFM (atomic force microscope) probes.

For instance, <CIT> describes a method making a device with a nanometre scale opening in the form of a tip on a layer of material by forming a mould in the surface of a monocrystalline wafer, lining the surface of the mould by conformal thin film deposition and then removing at least some of the substrate. The mould has the shape of the inverse of the tip which is a pyrimidal recess having a deep thin hole extending from its apex.

In this method the sacrificial substrate is provided on the upper side with a tapering recess which has a tip portion and side walls, and the upper side of the substrate is covered at least in the region of the recess with a layer made of an etchable material. According to the known method the opening is produced from the upper side by selective opening of the layer by means of an anisotropic plasma etching method which is matched to the material of the layer, the material, the etching gases and the etching parameters being chosen such that in the region of a tip portion of the layer, which tip portion lies on the tip portion of the substrate, a greater etching rate is produced than in the region of side walls of the layer which lie on the side walls of the substrate.

A particular type of known MEMS structure comprises an oblong protrusion.

These oblong protrusions are typically vulnerable, in that they may bend in an undesirable manner (affecting the performance of the MEMS device) or even break off.

The object of the present invention is to provide a generic method of manufacturing a protrusion with reduced vulnerability.

To this end, the invention provides a method according to claim <NUM>.

Thus a protrusion of filler material is created comprising a relatively broad base provided by the upper recess section, allowing a lower (oblong) section of the protrusion as defined by the lower recess section to be shorter and hence less vulnerable.

The method may involve removal of the cover layer, for example together with the removal of as least part of the substrate but preferably before introducing the filler material.

However, the cover layer may also be used in a further step on the manufacturing of the MEMS device, and may serve as a masking layer for backside etching.

Corner lithography was introduced and used to create a nano wire pyramid (see E. Sarajlic, E. Berenschot, G. Krijnen, M. Elwenspoek, "<NPL>).

The method is very useful for the manufacture of probes, such as AFM probes. For a probe, said main body comprises a body and a cantilever, said cantilever extending from said body, and the cantilever being provided with the protrusion.

The wafer has a first main side and a second main side. With the recess being created at the first main side, the removal of at least part of the wafer substrate material so as to expose the filler material is typically performed at the second main side, but it is feasible to do this within a cavity inside the wafer.

The protrusion may be an elongated (i.e. extending in a direction parallel to the first main side) protrusion, akin to a wall, in which case the recess is a groove so as to provide a base with a triangular cross-section. For many interesting applications the recess comprises a pyramidal recess with three or four side walls all intersecting at substantially the same location (i.e. the upper recess section is a pit).

Also in the method of claim <NUM> an oblique end to the protrusion of filler material in the substrate wafer is provided. In case of a tip, an oblique end may cut more easily into a cell, if it is desired to take a sample from the cell or injecting material into the cell.

This method allows for the batch-wise production of protrusions having oblique ends.

The length of the protrusion is determined by the size and the location of the cavity (distance of the second recess to the first recess).

Typically, if a hollow needle is formed, the second recess is formed after providing the first recess with sacrificial silicon as further filler material.

For a probe comprising a hollow channel in the cantilever, the wafer is provided with a silicon nitride layer protected with a silicon oxide cover layer before the second recess is formed.

In the field of taking a sample from a cell or injecting a liquid into a cell, the oblique distal end of the needle is very beneficial.

According to a favourable embodiment, the monocrystalline substrate is silicon, and the corner lithography steps comprise.

Thus a convenient method is provided to manufacture a MEMS device with a protrusion that is less vulnerable.

The silicon is preferably <<NUM>> silicon.

According to a favourable embodiment, the upper recess section is a pit, and the mask at the nadir is a masking dot.

This allows for the manufacture of a probe as the MEMS device. More specifically, the embodiment allows fabrication of a narrow (cylindrical) protrusion at the apex of a pyramidal tip base, the tip base being defined by the upper recess section. In this way, we can not only obtain a mechanically robust tip (less chance to break off) but also a tip that is suitable for the access to a relatively deep part of the sample surface.

According to a favourable embodiment, - before the step of forming the recess.

Thus a MEMS device, such as a probe, of silicon nitride can be formed, with a relatively broad pyramidal base and the protrusion at the apex of the pyramidal base.

According to a favourable embodiment, the step of introducing filler material comprises introducing silicon nitride as the filler material as a layer.

By providing the filler material as a layer, i.e. with a circumferentially extending inner surface in the lower recess section, a hollow protrusion is provided. A hollow protrusion can be used as a needle, which allows micro-scale manipulation of a liquid and or taking of samples, for example from a cell. It also allows for injection of a liquid, for example in a cell.

Thus, this embodiment allows for the creation of a hollow tip which can be manufactured attached to a hollow cantilever to obtain a nanopipette that can be used for the injection of biological cells. A protrusion provided according to the present invention comprising a pyramidal base with a narrow (cylindrical) lower protrusion section will cause less damage to the cells during the injection compared with a standard pyramidal tip. At the same time, the mechanical strength and stiffness of the protrusion will be largely preserved. In addition, a relatively low flow resistance of the protrusion is possible, as the lower section of the protrusion will be relatively short and the broad base of the protrusion may have a relatively large lumen.

To create the hollow needle, sacrificial silicon may be introduced as further filler material after providing the layer of silicon nitride in the recess and on the silicon wafer before covering the sacrificial silicon with further silicon nitride. The sacrificial silicon may now be removed, typically simultaneously with the removal of silicon of the substrate wafer. The distal end of the needle may be removed in a conventional manner to provide an opening at the distal end.

For a probe comprising a hollow cantilever, a cantilever recess will be formed in the substrate, and the recess will be formed in said cantilever recess. To create the hollow cantilever, sacrificial silicon will be introduced as further filler material after providing the layer of silicon nitride in the recess and on the silicon wafer before covering the sacrificial silicon with further silicon nitride. The sacrificial silicon may now be removed, typically simultaneously with the removal of silicon of the substrate wafer.

According to a favourable embodiment, the step of using the wall as a barrier comprises providing the cavity with an isotropic etchant to subject the filler material to isotropic etching.

Thus the substrate material of the wall serves a plane delimiting the etching of the filler material.

According to a favourable embodiment, the step of using the wall as a barrier comprises providing the second recess with a barrier layer and subjecting the wafer substrate to etching so as to remove said wafer substrate material.

This embodiment allows for the simultaneous removal of wafer substrate material and filler material, which is advantageous as removal of wafer substrate material is typically a step that is performed when preparing MEMS devises anyway.

Optionally, the barrier layer may be removed in the end, but it may also be part of the MEMS device. A suitable barrier layer may be formed by oxidizing silicon substrate material.

According to a favourable embodiment, to form the second recess the step of performing anisotropic etching so as to form the cavity is preceded by a step of performing directional etching locally.

This allows for a second recess having more sides, one of which will be used to intersect with the lower recess section of the first recess provided with filler material. If the second recess is formed from the first main side of the wafer, and upper side of the second recess will intersect with the filler material.

The present invention will now be illustrated with reference to the following figures.

<FIG> show SEM photographs of a MEMS device <NUM> with increasing magnification, demonstrating the feasibility of the method according to the invention.

<FIG> shows a main body <NUM> (predominant gray area) provided with a protrusion <NUM> comprising a pyramidal base <NUM> and a hollow tip <NUM>.

<FIG> show cross-sectional views through a wafer, illustrating the method to demonstrate the manufacture a MEMS device <NUM> (<FIG>).

<FIG> shows a wafer <NUM> (thickness <NUM>) of single crystal silicon with a <<NUM>> crystallographic orientation.

On the silicon wafer a masking layer <NUM> is deposited (<FIG>), said masking layer <NUM> comprising a first masking layer <NUM>' of silicon nitride (thickness <NUM>) and a second masking layer <NUM>" of silicon oxide (thickness <NUM>) using Low Pressure Chemical Vapor Deposition (LPCVD). The silicon nitride layer <NUM>' is used for protection of the silicon substrate <NUM> during wet anisotropic etching of silicon. The second masking layer <NUM>" of silicon oxide is used later for protection of the silicon nitride layer during Deep Reactive Ion Etching of silicon used to form a cylindrical cavity (see <FIG>).

Patterning of the masking layer <NUM> is performed in a standard manner, using Reactive Ion Etching (RIE) (<FIG>), locally exposing <<NUM>> silicon of the substrate.

Where the silicon is locally exposed, pyramidal pits <NUM> are etched by wet anisotropic etching of silicon using <NUM>% KOH solution at temperature of <NUM>. The pyramidal pits <NUM> have a base of <NUM> x <NUM>.

To perform corner lithography, first a thin (<NUM>) stochiometric silicon nitride layer <NUM> is conformally deposited (<FIG>) into the pyramidal pit <NUM> using Low Pressure Chemical Vapor Deposition. The pyramidal pit <NUM> will be the upper section of a recess used as a mold for forming the protrusion.

The silicon nitride layer <NUM> is etched isotopically in hot phosphoric acid (<NUM> % H<NUM>PO<NUM> at <NUM>). The timed etching in hot H<NUM>PO<NUM> proceeds until the silicon nitride <NUM> is removed from all surfaces except from a small dot <NUM> of silicon nitride at the nadir of the pyramidal pit <NUM> (see "<NPL>) and "<NPL>)). The size of the remaining silicon nitride dot depends on the original thickness of silicon nitride layer and the etching time (see "<NPL>). In our case the size of the dot <NUM> is around <NUM>.

Exposed silicon of the substrate <NUM> is thermally oxidized (wet oxidation at <NUM> for <NUM> hour) resulting in a silicon oxide layer <NUM> (<FIG>). Silicon underneath the silicon nitride dot <NUM> is not oxidized (LOCOS-local oxidation of silicon). The dot <NUM> serves as a masking dot <NUM>.

The masking dot <NUM> is selectively removed by etching the silicon nitride in hot phosphoric acid (<NUM> % H<NUM>PO<NUM> at <NUM>). Thus a small opening <NUM> in the silicon oxide layer <NUM> at the nadir of the pyramidal pit <NUM> is created, exposing silicon of the substrate <NUM>.

A cylindrical cavity <NUM> (first recess comprises pit <NUM> and cylindrical cavity <NUM>) is created (<FIG>) in the silicon wafer <NUM> by Deep Reactive Ion Etching (DRIE). The silicon oxide layers <NUM>", <NUM> serve as an etching mask. The layer <NUM>" protects silicon nitride and thermally grown silicon oxide protects silicon in the pyramidal cavity.

The size of the cylindrical cavity (depth and the diameter) depends on the etching time and the properties of the DRIE process. Background information on the DRIE process can be found here: https://en. org/wiki/Deep_reactive-ion_etching.

The cylindrical cavity <NUM> will be the lower section of the recess used as a mold for forming the protrusion.

After the DRIE etching, the silicon oxide layers <NUM>", <NUM> are removed (<FIG>) in concentrated hydrofluoric acid (HF <NUM>%).

To create a MEMS device <NUM> with a hollow protrusion <NUM>, a layer <NUM> (thickness <NUM>) of silicon nitride is deposited by LPCVD into the silicon mold (<FIG>) formed by the pyramidal pit <NUM> and the cylindrical cavity <NUM>. That is, the cavity <NUM> is not filled entirely. Filling the cavity <NUM> entirely may be preferable for scanning probes, for example, AFM probes.

A layer of sacrificial material <NUM>, here polycrystalline silicon with a thickness of <NUM>, is deposited on the wafer using LPCVD (<FIG>).

The sacrificial polysilicon layer <NUM> is patterned by Reactive Ion Etching to provide the layout of the microchannel <NUM>' (<FIG>).

A further layer <NUM> of silicon nitride having a thickness of <NUM> is deposited, fully encapsulating the layer of sacrificial material <NUM> (<FIG>).

The silicon nitride layer <NUM> is subsequently patterned by Reactive Ion Etching to create the probe layout (<FIG>).

Next a layer <NUM> of silicon oxide with the thickness of <NUM> is deposited by LPCVD (<FIG>). This layer <NUM> is used as a masking for DRIE of silicon and for the protection of silicon nitride <NUM> during wet chemical etching in hot phosphoric acid.

The protective silicon oxide layer <NUM> is patterned using the standard optical lithographic method and wet chemical etching in buffered HF to obtain an access hole <NUM> in the silicon oxide layer <NUM> exposing the silicon substrate <NUM> (<FIG>).

Through the small opening <NUM> in the protective silicon oxide layer <NUM> a trench <NUM> of around <NUM> deep into the wafer substrate <NUM> is formed by DRIE of silicon (<FIG>).

The etching is further proceeded in hot Tetramethylammonium hydroxide (TMAH) (<NUM> % at <NUM> for <NUM> minutes) to form a silicon cavity <NUM> (second recess <NUM>) bounded by the slow etching (<NUM>) crystallographic planes (<FIG>). One of the (<NUM>) planes intersects with the filled cavity <NUM>. In this process silicon surrounding the lower part of the protrusion <NUM> is removed, thus exposing said lower portion of the protrusion <NUM>.

The exposed part of silicon nitride layer <NUM> at the distal end of the protrusion <NUM> is etched by wet chemical etching in hot phosphoric acid (<NUM>% at <NUM>) as shown in <FIG>.

Silicon nitride on the other parts of the wafer is preserved by the protective silicon oxide layer <NUM>.

The protective layer <NUM> is removed using wet chemical etching in BHF (<FIG>).

The wafer <NUM> is than anodically bonded to a glass cover (not shown in the figures) and released in hot TMAH solution. During the release the sacrificial polysilicon layer <NUM> is removed creating a hollow needle <NUM> (<FIG>).

<FIG> show cross-sectional views through a wafer, illustrating an alternative method to manufacture a MEMS device.

<FIG> corresponds substantially to <FIG>.

The stack of the structural silicon nitride layers <NUM>', <NUM>, <NUM> is patterned using standard Reactive Ion Etching to obtain an access hole <NUM>' in said stacked silicon nitride layers, exposing the silicon substrate <NUM> (<FIG>).

Using the small opening <NUM>', a pyramidal pit <NUM> bounded by the slow etching (<NUM>) crystallographic planes is formed by KOH etching (<NUM> % KOH at <NUM>). One side of the pyramidal pit <NUM> intersects with the filled cavity <NUM> (<FIG>.

A thin layer of TEOS <NUM> (<NUM>) is deposited (a protective layer) (<FIG>).

The bulk silicon of the substrate <NUM> around the pyramidal pit <NUM> is removed locally (after patterning) by TMAH etching (schematically shown in <FIG>).

The exposed part of silicon nitride layer <NUM> at the distal end of the protrusion to be formed that extends into the pyramidal pit <NUM> is etched by wet chemical etching in hot phosphoric acid (<NUM>% at <NUM>) (<FIG>).

The procedure may be continued with a similar step as described for <FIG>. Using a mask and RIE etching, the layer stack of silicon oxide and silicon nitride is patterned to create the probe layout.

Claim 1:
A method of manufacturing a MEMS device (<NUM>), said device (<NUM>) comprising
- a main body (<NUM>), and
- a protrusion (<NUM>) protruding from the main body (<NUM>); wherein the method comprises performing a series of steps on a monocrystalline wafer substrate (<NUM>), wherein the wafer substrate (<NUM>) comprises a first main side and a second main side opposite of the first main side; wherein the steps comprise
- creating a first recess in the wafer substrate (<NUM>) at the first main side, said first recess comprising an upper recess section (<NUM>) and a lower recess section (<NUM>), creating said first recess comprising
- creating the upper recess section (<NUM>) in the wafer substrate (<NUM>) using anisotropic etching,
- performing corner lithography, involving the steps of
- providing a mask (<NUM>) at the nadir of the upper recess section (<NUM>),
- growing a cover layer (<NUM>) inside the upper recess section (<NUM>) and outside the mask (<NUM>),
- removing the mask (<NUM>) at the nadir, and
- performing directional etching using the cover layer (<NUM>) as a mask so as to form the lower recess section (<NUM>);
- introducing filler material (<NUM>, <NUM>) in the first recess,
- forming a second recess in the wafer substrate (<NUM>) adjacent to the first recess and at a side chosen from the first main side and the second main side, wherein creating said second recess comprises the following steps
- performing anisotropic etching so as to form a cavity (<NUM>) with a wall of said cavity (<NUM>) intersecting the lower recess section (<NUM>) of the first recess provided with filler material (<NUM>, <NUM>); and
- using the plane of the wall to define a barrier and etching the filler material (<NUM>) using said barrier, and
- removing at least part of the wafer substrate material so as to expose the filler material (<NUM>, <NUM>) introduced in the first recess.