MEMS structure with graphene component

A microelectromechanical systems (MEMS) structure includes a substrate, an epitaxial polysilicon cap located above the substrate, a first cavity portion defined between the substrate and the epitaxial polysilicon cap, and a first graphene component having at least one graphene surface immediately adjacent to the first cavity portion.

FIELD OF THE DISCLOSURE

This disclosure relates to microelectromechanical systems (MEMS) structure.

BACKGROUND

Microelectromechanical systems (MEMS), for example, gyroscopes, resonators and accelerometers, utilize micromachining techniques (i.e., lithographic and other precision fabrication techniques) to reduce mechanical components to a scale that is generally comparable to microelectronics. MEMS typically include a mechanical structure fabricated from or on, for example, a silicon substrate using micromachining techniques.

With surface micromachining, a MEMS device structure can be built on a silicon substrate using processes such as chemical vapor deposition. These processes allow MEMS structures to include layer thicknesses of less than a few microns with substantially larger in-plane dimensions. Frequently, these devices include parts which are configured to move with respect to other parts of the device. In this type of device, the movable structure is frequently built upon a sacrificial layer of material. After the movable structure is formed, the movable structure can be released by selective wet etching of the sacrificial layers in aqueous hydrofluoric acid (HF). After etching, the released MEMS device structure can be rinsed in deionized water to remove the etchant and etch products.

Due to the large surface area-to-volume ratio of many movable structures, a MEMS device including such a structure is susceptible to interlayer or layer-to-substrate adhesion during the release process (release adhesion) or subsequent device use (in-use adhesion). This adhesion phenomenon is more generally called stiction. Stiction is exacerbated by the ready formation of a 5-30 angstrom thick native oxide layer on the silicon surface, either during post-release processing of the MEMS device or during subsequent exposure to air during use. Silicon oxide is hydrophilic, encouraging the formation of water layers on the native oxide surfaces that can exhibit strong capillary forces when the small interlayer gaps are exposed to a high humidity environment. Furthermore, Van der Waals forces, due to the presence of certain organic residues, hydrogen bonding, and electrostatic forces, also contribute to the interlayer attraction. These cohesive forces can be strong enough to pull the free-standing released layers into contact with another structure, causing irreversible latching and rendering the MEMS device inoperative.

Various approaches have been used in attempts to minimize adhesion in MEMS devices. These approaches include drying techniques, such as freeze-sublimation and supercritical carbon dioxide drying, which are intended to prevent liquid formation during the release process, thereby preventing capillary collapse and release adhesion. Vapor phase HF etching is commonly used to alleviate in-process stiction. Other approaches are directed to reducing stiction by minimizing contact surface areas, designing MEMS device structures that are stiff in the out-of-plane direction, and hermetic packaging.

An approach to reducing in-use stiction and adhesion issues is based upon surface modification of the device by addition of an anti-stiction coating. The modified surface ideally exhibits low surface energy by adding a coating of material, thereby inhibiting in-use adhesion in released MEMS devices. Most coating processes have the goal of producing a thin surface layer bound to the native silicon oxide that presents a hydrophobic surface to the environment. In particular, coating the MEMS device surface with self-assembled monolayers (SAMs) having a hydrophobic tail group has been shown to be effective in reducing in-use adhesion. SAMs have typically involved the deposition of organosilane coupling agents, such as octadecyltrichlorosilane and perfluorodecyltrichlorosilane, from nonaqueous solutions after the MEMS device is released. Even without anti-stiction coating, native oxide generation occurs on silicon surfaces.

In spite of these various approaches, in-use adhesion remains a serious reliability problem with MEMS devices. One aspect of the problem is that even when an antistiction coating is applied, the underlying silicon layer may retain various charges. For example, silicon by itself is not a conductor. In order to modify a silicon structure to be conductive, a substance is doped into the silicon. The realizable doping-level is limited, however, due to induced stress in the functional silicon layer. Accordingly, during manufacturing process, charges are deposited on the silicon surfaces of sensing elements and the charges do not immediately migrate. The charges include dangling bonds due to trench forming processes used to define various structures. In capacitive sensing devices those charges may cause a reliability issue since they are not all locally bound. Some charges have a certain mobility and may drift as a function of temperature or aging. This can lead to undesired drift effects, e. g. of the sensitivity or offset of the capacitive sensor. Therefore, a highly conductive working layer (not possible w/silicon) or at least a highly conductive coating on top of the structures in order to not accumulate surface charges would be desirable.

Moreover, the limited conductivity of silicon may result in unacceptable RC time constants in electronic evaluation circuits including capacitive sensors. A sensor element with, e. g., a 10 pF total capacitance (C) and 10 kOhm total resistance (R) may be limited to operation below frequencies of about 1 MHz. Operation at higher frequencies is desired in certain applications, however, since higher frequency operation may lead to a better signal to noise performance of the sensor. Therefore, increased conductivity in MEMS devices which enable achievement of lower RC time constants would be beneficial.

Thus, there remains a need for a reliable structure for MEMS devices that is compatible with MEMS fabrication processes that can be used to reduce stiction forces, surface charges, and/or the resistivity of MEMS structures.

SUMMARY

In accordance with one embodiment of the disclosure, there is provided a microelectromechanical systems (MEMS) structure including a substrate, an epitaxial polysilicon cap located above the substrate, a first cavity portion defined between the substrate and the epitaxial polysilicon cap, and a first graphene component having at least one graphene surface immediately adjacent to the first cavity portion.

In one or more embodiments, the first cavity portion extends vertically within the MEMS structure, and the at least one graphene surface includes a vertically extending wall defining a vertical wall of the first cavity portion.

In one or more embodiments a MEMS structure includes a second horizontally extending cavity portion opening to the first cavity portion, and a second graphene component defining a lower portion of the second horizontally extending cavity portion.

In one or more embodiments, the at least one graphene surface is a scalloped vertically extending wall.

In one or more embodiments a first surface of the at least one graphene surfaces is immediately beneath the first cavity portion, and a second surface of the at least one graphene surfaces is immediately above a second cavity portion.

In one or more embodiments, the first graphene component is movable within a cavity including the first cavity portion and the second cavity portion.

In accordance with one embodiment of the disclosure a method of forming a microelectromechanical systems (MEMS) structure includes providing a substrate, forming a first portion of an epitaxial polysilicon cap above the substrate, forming a first cavity portion above the substrate by vapor release through at least one vent extending through the first portion of the epitaxial polysilicon cap, converting a silicon carbide portion immediately adjacent to the first cavity portion to graphene using a hydrogen bake, and sealing the at least one vent with a second portion of the epitaxial polysilicon cap after converting the silicon carbide portion.

In accordance with one or more embodiments, providing the substrate comprises providing a silicon on insulator (SOI) wafer, forming the first cavity portion comprises exposing a silicon portion of the SOI wafer immediately adjacent to the first cavity portion, and the method further comprises conformally depositing the silicon dioxide portion on the exposed silicon portion.

In accordance with one or more embodiments, forming the first cavity portion includes deep reactive ion etching a trench completely through a silicon layer of the SOI wafer, filling the trench with a sacrificial oxide portion after conformally depositing the silicon dioxide portion, and using a hydrofluoric acid vapor to expose the silicon dioxide portion.

In accordance with one or more embodiments, conformally depositing the silicon dioxide portion includes conformally depositing the silicon dioxide portion on a scalloped surface of the exposed silicon portion, and converting the silicon carbide portion comprises converting the silicon carbide portion to a scalloped graphene portion.

In accordance with one or more embodiments, the hydrogen bake is conducted in an epitaxial reactor, and the second portion of the epitaxial polysilicon cap is deposited in the epitaxial reactor.

In accordance with one or more embodiments, providing the substrate comprises providing a silicon carbide layer on an insulator layer, and the silicon carbide portion is a portion of the silicon carbide layer.

In accordance with one or more embodiments, forming the first cavity portion includes deep reactive ion etching a trench completely through the silicon carbide layer, filling the trench with a sacrificial oxide portion, and using a hydrofluoric acid vapor to expose the portion of the silicon carbide layer.

In accordance with one or more embodiments, converting the silicon carbide portion comprises completely converting the segment of the silicon carbide layer to graphene.

In accordance with one or more embodiments, forming the first cavity portion comprises releasing a segment of the silicon carbide layer.

DESCRIPTION

FIG. 1depicts a simplified MEMS structure100. The MEMS structure100in this embodiment is depicted as a silicon on insulator (SOI) wafer including a substrate in the form of a silicon handle layer102, a buried silicon oxide layer104, and a silicon device layer106. Above the device layer106is a second buried silicon oxide layer108and an epitaxial polysilicon cap layer110.

Within the device layer106a working component112is defined by a trench114. The working component112is connected to a contact116through a connector118. The trench114extends vertically completely through the device layer106and connects an upper cavity portion120and a lower cavity portion122to form a cavity124. An electrode126is spaced apart from the working component112and connected to a contact128through a connector130. The connector130is electrically isolated from the connector118by a spacer132.

The electrode126and working component112are at least partially coated with graphene. A horizontally extending graphene portion140extends across the upper surface of the working component112immediately adjacent to the upper cavity portion120and vertically extending graphene walls142/144extend along and immediately adjacent to the trench114. A second horizontally extending graphene portion146extends along the upper surface of the electrode126immediately adjacent to the upper cavity portion120.

The graphene portions140/142/144/146provide reduced resistance and reduced possibility for stiction. Specifically, graphene is an allotrope of carbon wherein every carbon atom is bonded to three other carbon atoms in plane and bonded to a hydrogen atom perpendicular to the plane. Graphene exhibits high electrical conductivity, high electron mobility, high sustainable currents, low mechanical friction, high light transmission, and high thermal conductivity. Graphene is thus desirable in applications wherein reduced electrical resistance is desired. Graphene also has a very low surface energy due to very weak Van der Waals forces and as a result is a very good anti-stiction layer.

Returning toFIG. 1, the electrode126and working component112are part of an in-plane motion sensor148. The MEMS structure100in this embodiment further includes a pressure sensor160. The pressure sensor160is electrically separated from the in-plane motion sensor148by a spacer162. The pressure sensor160includes a lower electrode164in the form of a horizontally extending graphene portion166. The horizontally extending graphene portion166is electrically connected to a contact168through a connector170which extends through the epitaxial polysilicon cap layer110. An upper electrode172is defined in the epitaxial polysilicon cap layer110by the spacer162and a spacer174. The upper electrode172is spaced apart from the lower electrode164by a cavity176immediately adjacent to the graphene portion166. A contact178is provided for the upper electrode172.

The incorporation of graphene into the MEMS structure100is easily accomplished without excessive modification of known manufacturing techniques and processes. By way of exampleFIG. 2depicts an SOI wafer200that is used in one embodiment to form the MEMS structure100. The SOI wafer includes a substrate layer202, a buried oxide layer204, and a device layer206. In some embodiments, the layers204and206are formed during a manufacturing process of the MEMS structure while in some embodiments the SOI wafer200is previously formed. As depicted inFIG. 2, trench208is then etched completely through the device layer using a deep reactive ion etch process. This process results in scalloped edges of the trench208.

Turning toFIG. 3, a silicon carbide layer210is formed on the exposed surfaces by conformal deposition of silicon carbide. Deposition of silicon carbide may be accomplished using any desired conformal deposition process such as LPCVD, PECVD, ALD, epitaxial deposition, etc.

An oxide layer212(FIG. 4) is then deposited on the upper surface of the silicon carbide layer210. The oxide layer also fills the trench208with segment214. A trench is then formed through the silicon carbide layer and the device layer206and a silicon nitride layer is deposited and patterned to fill the trench with a silicon nitride spacer portion212.

The oxide layer214and the silicon carbide layer212are then patterned and etched resulting in the configuration ofFIG. 4wherein portions220,222, and224of the device layer206are exposed. The upper surface of the silicon nitride spacer portion212is also exposed.

A first portion230of an epitaxial polysilicon cap is then formed on the upper surface of the remaining oxide layer212, the nitride spacer portion216, and the portions220,222, and224(FIG. 5). The first portion230is then trenched and a second silicon nitride layer is deposited and patterned resulting in silicon nitride spacer portions232,234,236, and238. Vent holes242are formed through the first portion230of the epitaxial polysilicon cap (FIG. 6). A hydrofluoric acid is then used to vapor etch all of the exposed oxide portions through the vent holes242resulting in the configuration ofFIG. 7.

InFIG. 7, cavities244and246have been formed exposing portions of the silicon carbide layer210. The cavity244includes an upper cavity portion248and a lower cavity portion250which are joined by a trench portion252. A segment254of the device layer206is thus released from the remainder of the device layer206with the exception of an anchor portion (not shown).

The structure is now subjected to a hydrogen bake. The hydrogen bake is conducted in an epitaxial reactor. The temperature is controlled to be above 1050° C., and preferably between 1050° C. and 1300° C. At this temperature, all of the organic and other impurities from the cavities including any native silicon dioxide are removed resulting in a very clean environment.

The high temperature of the hydrogen bake also sublimates silicon from the exposed silicon carbide layer thereby precipitating layers of graphene beginning at the outer surface. A sufficiently long bake will convert the entire layer of silicon carbide210to graphene256as depicted inFIG. 8.

Advantageously, the silicon carbide protects the underlying silicon from the bake. Specifically, the DRIE process creates a scalloped surface. In a normal bake, the silicon reflows resulting in a smooth vertical wall surface and smooth surfaces increase the potential for stiction issues. In contrast, the formation of graphene from silicon dioxide prevents the underlying silicon from reflowing. Accordingly, the graphene is formed with a scalloped surface which reduces the potential for stiction. Additionally, the graphene surface increases the efficiency of electrostatic transduction in the MEMS structure by several orders of magnitude.

Once the hydrogen bake has been maintained for the desired amount of time, an epitaxial polysilicon cap portion258is formed using the same epitaxial reactor used to form the graphene. This hermetically seals the MEMS structure in a pure, high vacuum environment typically of about 1-10 Pascals. This assists in keeping the graphene pristine to optimize the quantum, electronic, and thermal properties of the graphene since graphene is easily contaminated.

Once the MEMS structure is sealed, electrical isolation spaces and electrical contacts are formed as desired resulting in the configuration of the MEMS structure100inFIG. 1. While the process for forming an in-plane sensor along with a pressure sensor has been discussed above, other embodiments form only a single sensor or device within the MEMS structure. Such methods will typically reduce the number of process steps. Additionally, the above described method can be easily modified to provide other types of sensors and other combinations of sensors. Such sensors include in-plane accelerometers, gyroscopes, out-of-plane accelerometers, combined in-plane/out-of-plane accelerometers, pressure sensors, microphones, resonating structures, magnetic field sensors, angular rate sensors etc.

Additionally, while the description above provided silicon dioxide by way of a conformal coating on silicon, the silicon dioxide can be provided in other ways. By way of example,FIG. 9depicts a simplified depiction of a MEMS structure270which includes a substrate layer272, a buried oxide layer274, a device layer276, an oxide layer278, and an epitaxial polysilicon cap280. The MEMS structure270differs from the MEMS structure100primarily in that the device layer276is provided as a monolithic silicon dioxide layer directly positioned on the buried oxide layer274.

Accordingly, once subjected to a hydrogen bake as described above, the working portion282and electrodes284/286of the device layer276have been completely converted to graphene using a modified form of the process described above while portions288of the device layer276remain silicon dioxide. Accordingly, the graphene working portion282is immediately above a lower portion290of a cavity292. Thus, forming the cavity292results in release of the graphene working portion282.

Consequently, by using a silicon carbide on insulator wafer a suspended all-graphene device (MEMS or otherwise) can be realized. This allows for the manufacture of graphene membranes for pressure sensors, microphones etc., and even resonating microstructures using the process described above. The structures can further be used in electronic and photonic devices.