Conductive etch stop for etching a sacrificial layer
In one embodiment, a micro device is formed by depositing a sacrificial layer over a metallic electrode, forming a moveable structure over the sacrificial layer, and then etching the sacrificial layer with a noble gas fluoride. Because the metallic electrode is comprised of a metallic material that also serves as an etch stop in the sacrificial layer etch, charge does not appreciably build up in the metallic electrode. This helps stabilize the driving characteristic of the moveable structure. In one embodiment, the moveable structure is a ribbon in a light modulator.
This application is a continuation of U.S. Ser. No. 10/891,916, filed on Jul. 15, 2004, which is a divisional of U.S. application Ser. No. 10/187,028, now U.S. Pat. No. 6,777,258, filed on Jun. 28, 2002, both of which are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates generally to micro devices, and more particularly, but not exclusively, to micromechanical system structures and manufacturing methods.
2. Description of the Background Art
A micro electromechanical system (MEMS) typically includes micromechanical structures that may be actuated using electrical signals. An example of a MEMS device is the Grating Light Valve™ (GLV) device available from Silicon Light Machines, Inc. of Sunnyvale, Calif. GLV-type devices are described in the following disclosures, which are incorporated herein by reference in their entirety: U.S. Pat. No. 5,311,360 to Bloom et al.; U.S. Pat. No. 5,841,579 to Bloom et al.; and U.S. Pat. No. 5,661,592 to Bornstein et al.
Generally speaking, a GLV-type device is a light modulator. It may be used in various applications including video, printing, and optical switching, for example. A GLV-type device includes an array of moveable structures referred to as “ribbons”. A ribbon typically includes a metallic layer formed over a resilient, suspended structure. Under the ribbon is a bottom electrode that works in conjunction with the metallic layer, which serves as a top electrode. An air gap separates the bottom electrode from the metallic layer. Applying a voltage difference between the ribbon and the bottom electrode generates an electrostatic field that pulls the ribbon towards the bottom electrode. Light that impinges on the reflective metallic layer may thus be modulated by reflection or diffraction by controlling the applied voltage.
The response of a GLV-type device to a control signal, such as an applied voltage, is also referred to as its “driving characteristic”. As can be appreciated from the foregoing, the better a device's driving characteristic, the better it can modulate light. Thus, it is desirable to have a GLV-type device that has a relatively stable driving characteristic.
SUMMARYIn one embodiment, a micro device is formed by depositing a sacrificial layer over a metallic electrode, forming a moveable structure over the sacrificial layer, and then etching the sacrificial layer with a noble gas fluoride. Because the metallic electrode is comprised of a metallic material that also serves as an etch stop in the sacrificial layer etch, charge does not appreciably build up in the metalic electrode. This helps stabilize the driving characteristic of the moveable structure. In one embodiment, the moveable structure is a ribbon in a light modulator.
These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.
DESCRIPTION OF THE DRAWINGS
FIGS. 2(a)-2(m) schematically show side cross-sectional views of a GLV-type device being fabricated in accordance with an embodiment of the present invention.
The use of the same reference label in different drawings indicates the same or like components. Drawings are not necessarily to scale unless otherwise noted.
DETAILED DESCRIPTIONIn the present disclosure, numerous specific details are provided such as examples of process parameters, materials, process steps, and structures to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.
Moreover, embodiments of the present invention are described herein in the context of a GLV-type device. However, it should be understood that the present invention is not so limited and may also be used in MEMS devices in general or in other micro devices, such as integrated circuits with moveable micromechanical structures. Additionally, it is to be noted that as used in the present disclosure, the terms “over” and “under” refer to the relative placement of two materials that may or may not be directly in contact with each other; that is, the two materials may be separated by another material or an air gap.
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Device 100 includes a ribbon 110 comprising a resilient structure 103 and a metal layer 102. Metal layer 102 is typically a layer of aluminum, while resilient structure 103 is typically a layer of silicon nitride (Si3N4). An air gap 101 separates ribbon 110 from a bottom electrode 107. Because ribbon 110 may be flexed towards bottom electrode 107, ribbon 110 may also be referred to as a “moveable structure.” Unlike metal layer 102, bottom electrode 107 is typically of a non-metallic material such as polysilicon. The polysilicon is heavily doped with an n-type dopant (e.g., phosphorous) so that it may be used as an electrode. Polysilicon is traditionally thought of as a good electrode material because it is compatible with standard integrated circuit fabrication processes. Additionally, polysilicon is stable at least up to 550° C., the temperature at which an amorphous silicon layer is deposited over bottom electrode 107, as discussed below.
Air gap 101 is typically formed by depositing amorphous silicon in the space occupied by air gap 101, and then isotropically etching the amorphous silicon with xenon difluoride (XeF2). The amorphous silicon is deposited over bottom electrode 107 using a low pressure chemical vapor deposition process at around 550° C. To protect a polysilicon bottom electrode 107 during the etching of the amorphous silicon, a thin silicon dioxide (SiO2) layer 104 is deposited over bottom electrode 107. That is, silicon dioxide layer 104 serves as an etch stop for the amorphous silicon etch. As shown in
The movement of ribbon 110 may be controlled by applying a voltage difference between ribbon 110 and bottom electrode 107. The voltage difference generates an electric field that pulls ribbon 110 towards bottom electrode 107. Because metal layer 102 is formed of a metallic material that is smooth to light of a particular wavelength or range of wavelengths, incident light of suitable wavelength may be diffracted or reflected off metal layer 102 depending on whether ribbon 110 is pulled or not.
Although device 100 is suitable for most applications, its driving characteristics may change over time. That is, applying the same voltage difference at different time periods may not result in the same movement of ribbon 110. The inventor has discovered that the use of a non-conductive etch stop (i.e., silicon dioxide layer 104) over bottom electrode 107 contributes to this problem. Because the non-conductive etch stop is a dielectric, random ionic charges may build up on the surface of bottom electrode 107 facing the etch-stop. The charge build up may block electric fields generated to move ribbon 110, thereby adversely affecting the driving characteristics of device 100. In an embodiment of the present invention, a bottom electrode is of a metallic material. The metallic material also serves as a conductive etch stop to prevent undesirable charge build up.
FIGS. 2(a)-2(m) schematically show side cross-sectional views of a GLV-type device being fabricated in accordance with an embodiment of the present invention. FIGS. 2(a)-2(m) are not drawn to scale.
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To help facilitate integration of metallic electrodes 207 in traditional integrated circuit factories, a metallic electrode 207 is preferably of a metallic material that is compatible with standard integrated circuit fabrication processes such as CMOS (Complementary Metal-Oxide Semiconductor). Likewise, a metallic electrode 207 is preferably formable using conventional patterning and etching techniques.
A metallic electrode 207 may be formed by (a) depositing a metal layer over a substrate, (b) patterning and etching the metal layer to form a metallic electrode, and (c) then changing the composition of the metallic electrode by reacting it with another material, referred to as a “source material”, at relatively high temperature.
In one embodiment, metallic electrodes 207 comprise titanium nitride (TiN). A metallic electrode 207 of titanium nitride may be formed on isolation layer 205 by first depositing titanium to a thickness of about 1000 Angstroms by physical vapor deposition. The titanium may then be patterned and etched to form a metallic electrode. The formation of the metallic electrode may then be followed by an ammonia-based rapid thermal processing (RTP) at around 1050° C. for about 30 seconds. The ammonia in the rapid thermal process serves as a source material, providing nitrogen to the metallic electrode of titanium. That is, the ammonia reacts with the titanium to form titanium-nitride.
Forming a metallic electrode 207 of titanium-nitride by first patterning and etching a titanium layer and then exposing the titanium to a nitrogen source has several advantages. For one, the resulting titanium-nitride reacts with an isolation layer 205 of silicon dioxide to form titanium-oxide at their interface. This creates good adhesion between a metallic electrode 207 and an isolation layer 205. Additionally, by patterning and etching the titanium before reacting the titanium with ammonia, all sidewalls of a metallic electrode 207 are exposed to the ammonia, making the sidewalls rich with titanium-nitride.
Depending on the application, a metallic electrode 207 of titanium-nitride may also be formed by reactively sputtering titanium in a process chamber containing nitrogen.
Titanium-nitride has several properties that make it suitable as a metallic electrode 207 in the example fabrication process of FIGS. 2(a)-2(m). Titanium-nitride has good conductivity (about 50-100 μΩ-cm), is stable at temperatures at least up to about 1200° C., does not react with silicon (the material used as sacrificial layer in one embodiment) at temperatures at least up to about 900° C., is compatible with standard integrated circuit fabrication processes, has good adhesion, is optically smooth to light in the ultra violet (UV) to infra-red (IR) wavelengths. Additionally, titanium-nitride is not appreciably etched by a noble gas fluoride etchant, such as a xenon difluoride etchant used to remove a sacrificial layer of amorphous silicon in one embodiment. Thus, a metallic electrode 207 of titanium-nitride may also serve as a conductive etch stop in the amorphous silicon etch. In light of the present disclosure, those of ordinary skill in the art can use other metallic materials for specific applications. For example, tungsten-silicide (WSi2) may also be used as a metallic electrode material.
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In step 304, a sacrificial layer (e.g., sacrificial layer 211) is deposited over the bottom metallic electrode.
In step 306, a moveable structure (e.g., ribbon 200) is formed over the sacrificial layer. The moveable structure may include a resilient structure (e.g., ribbon material 202) and a top metallic electrode (e.g., metal layer 222) over the resilient structure.
In step 308, the sacrificial layer between the moveable structure and the bottom metallic electrode is removed using a noble gas fluoride etchant. This results in an air gap that allows the moveable structure to flex towards the bottom metallic electrode upon application of a control signal, such as a voltage.
While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure. Thus, the present invention is limited only by the following claims.
Claims
1. A method of forming a micro device, the method comprising:
- forming an isolation layer over a substrate;
- forming a first metallic electrode over the isolation layer;
- forming a sacrificial layer over the first metallic electrode;
- forming a moveable structure over the sacrificial layer; and
- etching the sacrificial layer between the moveable structure and the first metallic electrode using the first metallic electrode as an etch stop protecting the isolation layer, the etching of the sacrificial layer forming an air gap between the first metallic electrode and the moveable structure.
2. The method of claim 1 wherein the first metallic electrode comprises titanium-nitride.
3. The method of claim 1 wherein the sacrificial layer is deposited at a temperature greater than about 500° C.
4. The method of claim 1 wherein the sacrificial layer comprises amorphous silicon.
5. The method of claim 1 wherein the sacrificial layer is etched using a noble gas fluoride etchant.
6. The method of claim 1 wherein forming the moveable structure over the sacrificial layer comprises:
- forming a ribbon material over the sacrificial layer; and
- forming a second metallic electrode over the ribbon material.
7. The method of claim 6 wherein the ribbon material comprises silicon nitride.
8. The method of claim 6 wherein the metal comprises aluminum.
9. The method of claim 1 wherein the first metallic electrode is of a material that is stable at least up to about 900° C.
10. The method of claim 1 further comprising:
- forming the first metallic electrode by depositing titanium over the substrate and exposing the titanium to an environment including ammonia.
11. A method of forming a micro device, the method comprising:
- forming an isolation layer over a substrate;
- forming a bottom metallic electrode over the isolation layer;
- forming a sacrificial layer over the bottom metallic electrode;
- forming a resilient structure over the sacrificial layer;
- forming a top metallic electrode over the resilient structure; and
- etching the sacrificial layer between the resilient structure and the first metallic electrode using the first metallic electrode as an etch stop protecting the isolation layer, the etching of the sacrificial layer forming an air gap between the first metallic electrode and the resilient structure.
12. The method of claim 11 wherein the sacrificial layer is etched using a noble gas fluoride etchant.
13. The method of claim 11 wherein the sacrificial material comprises amorphous silicon.
14. The method of claim 11 wherein the bottom metallic electrode comprises titanium-nitride.
15. The method of claim 11 wherein the resilient structure is part of a ribbon light modulator.
Type: Application
Filed: Jul 28, 2005
Publication Date: Nov 24, 2005
Inventor: James Hunter (Campbell, CA)
Application Number: 11/192,621