US20110303638A1

US20110303638A1 - Method for Fabricating a Micromirror

Info

Publication number: US20110303638A1
Application number: US13/161,241
Authority: US
Inventors: Xiaojing Zhang; Youmin Wang; Ting Shen; Karthik Kumar
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-08-20
Filing date: 2011-06-15
Publication date: 2011-12-15

Abstract

A method for fabricating a micromirror is disclosed. A set of coarse features is formed in a low-temperature oxide (LTO) layer deposited on a front side of a wafer. A set of fine features is then formed in a photosensitive material layer deposited on top of the LTO layer. After removing a portion of the LTO layer to align the width of the coarse features with the width of the fine features, the first silicon dioxide layer and the first and second silicon device layers are etched to form stator comb fingers and rotor comb fingers accordingly. Finally, a portion of the substrate on a back side of the wafer is removed, and the silicon dioxide layers are removed from the front and back sides of the wafer to form a rotatable mirror.

Description

PRIORITY CLAIM

The present application is a continuation-in-part of U.S. Non-provisional application Ser. No. 12/193,501, filed on Aug. 18, 2008, published on Feb. 10, 2010 under Pub. No. US 2010/00039687, which claims priority to U.S. Provisional Application No. 60/965,417, filed on Aug. 20, 2007, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field
The present invention relates to microscanners in general, and, in particular, to a method for fabricating a micromirror.
2. Description of Related Art
Microscanners are essential components for the miniaturization of optical diagnostic equipments such as endoscopes. For example, silicon-based microscanners have been integrated into confocal and other instruments for providing images. However, conventional microscanners have small reflection angle and high actuation power. In addition, conventional microscanners require complex fabrication procedures that are not compatible with standard silicon fabrication techniques used in the industry.
Consequently, it would be desirable to provide an improved method for fabricating microscanners.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, a set of coarse features is formed in a low-temperature oxide (LTO) layer deposited on a front side of a wafer having a substrate, a first and second silicon device layers separated from each other by a first and second silicon dioxide layers. Next, a set of fine features is formed in a photosensitive material layer deposited on top of the LTO layer. The fine features are constrained laterally within the coarse features. After removing a portion of the LTO layer to align the width of the coarse features with the width of the fine features, the first silicon dioxide layer and the first and second silicon device layers are etched to form stator comb fingers and rotor comb fingers accordingly. Finally, a portion of the substrate on a back side of the wafer is removed, and the silicon dioxide layers are removed from the front and back sides of the wafer to form a rotatable mirror.
All features and advantages of the present invention will become apparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is an isomeric view of a micromirror, in accordance with a preferred embodiment of the present invention;

FIG. 2 shows a set of combdrive actuators within the micromirror from FIG. 1, in accordance with a preferred embodiment of the present invention; and

FIGS. 3 a-3 i graphically illustrates a method for fabricating the micromirror from FIG. 1, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings and in particular to FIG. 1, there is depicted a diagram of a micromirror (or microscanner), in accordance with a preferred embodiment of the present invention. As shown, a micromirror 174 has two axes, and electrostatic vertical combdrives can be utilized to provide fast, high-torque rotary actuation about the two axes of micromirror 174. For example, two sets of staggered vertical combdrive actuators 260, 262 can be utilized to rotate rotatable minor 210 along each of the two axes. The movements of combdrive actuators 260, 262 can be controlled by the application of appropriate electrical biases via pads V¹ _inner, V¹ _outer, V² _inner, V² _outerand Ground. Combdrive actuators 260, 262 include rotor and stator comb fingers. The thickness and spacing between rotor and stator comb fingers are preferably fixed at approximately 8 μm.
The performance of micromirror 174 is characterized by its response to various electrical signal inputs. For example, one input can be a sinusoidal variable-frequency voltage with suitable offset (to ensure the applied voltage was always positive) between ground and one of combdrive actuators 260, 262 of each rotation axis. Optical scan angles of 22° and 12° on the inner and outer axes are achieved for frequency values around 2.81 kHz and 670 Hz on the inner and outer rotation axes, respectively. On the other hand, for a static voltage applied between ground and one of combdrive actuators 260, 262 on each rotation axis, off-resonance actuation using only one combdrive actuator results in single-sided deflection. The total optical deflection angle can be doubled by making use of both combdrive actuators 260, 262 on either side of the torsion bars forming the rotation axis. In this respect, off-resonance operation differs significantly from driving at resonant frequency. Optical scan angles of about 5° and 4.5° can be achieved by applying static voltages up to 240 V on the inner and outer axes, respectively.
With reference now to FIG. 2, there is illustrated a detailed diagram of combdrive actuators 260 from FIG. 2, in accordance with a preferred embodiment of the present invention. As shown, combdrive actuators 260 include rotor comb fingers 306 and stator comb fingers 308. Preferably, each of stator comb fingers 308 has a width between about 0.5 μm and 50 μm, each of rotor comb fingers 306 has a width between about 0.5 μm and 50 μm, and a target gap spacing g ranges between 0.5 μm and 50 μm. As their names imply, rotor comb fingers 306 are capable of being rotated, while stator comb fingers 308 remain stationary throughout.
In response to a voltage being applied at stator comb fingers 308, rotor comb fingers 306 rotate about a torsion bar 304. Specifically, when a voltage is applied at stator comb fingers 308, an electrostatic torque is experienced by rotor comb fingers 306, which subsequently rotates rotor comb fingers 306 because they are constrained primarily to rotary motion by torsion bar 304. Rotor comb fingers 306 are capable of being rotated to a maximum rotation angle of θ_max. As rotor comb fingers 306 are being rotated, a shear stress is developed within torsion bar 304 due to twisting, and the shear stress offers a mechanical restoring torque against such twisting. The rotation of rotor comb fingers 306 reaches an equilibrium at a rotation angle at which the electrostatic torque exactly matches the mechanical restoring torque.
With reference now to FIGS. 3 a-31, there are illustrated a method for fabricating a micromirror, such as micromirror 174 from FIG. 1, in accordance with a preferred embodiment of the present invention. The process begins with a fusion bonded double silicon-on-insulator (DSOI) wafer 400. Wafer 400 includes a substrate 420 having two <100> silicon device layers 412, 414 separated from each other by two silicon dioxide layers 416, 418, as shown in FIG. 3 a. Each of silicon device layers 412, 414 is approximately 30 μm thick, and each of silicon dioxide layers 416, 418 is approximately 1 μm thick.
Before further processing of wafer 400, pre-fabrication of any complementary-metal-oxide semiconductor (CMOS) circuitry can be performed at this point, if necessary. For example, CMOS circuitry may include control electronics and sensors to adaptively correct for aberrations in a micromirror.
Following the CMOS circuitry pre-fabrication (if performed), wafer 400 is cleaned by immersing wafer 400 in a 9:1 solution of H₂SO₄:H₂O₂for approximately 8 minutes. After rinsing with de-ionized water, wafer 400 is spun dry. The above-mentioned cleaning process is commonly known as Piranha clean.
Next, wafer 400 is placed into a furnace in which a low-temperature oxide (LTO) layer 422 is deposited on top of silicon device layer 412 via a low-pressure chemical vapor deposition (LPCVD) process at a low temperature (450° C.) in order to reduce thermal budget. LTO layer 422 is preferably a silicon dioxide layer having a thickness between about 50 nm and about 1.5 μm. LTO layer 422 serves to protect any CMOS circuitry and to act as a hard mask for the deep trench etching to be performed to create vertical comb finger structures.
A first photolithography step is performed on LTO layer 422 to etch a set of coarse features 424, 426 of vertical comb finger structures on top of silicon device layer 412. The photolithography step involves coating a layer of hexamethyldisilazane (HMDS) on LTO layer 422, which serves as an adhesion promoter between LTO layer 422 and a photosensitive material to be added. Coarse features 424, 426 are etched in LTO layer 422 via a reactive ion etching (RIE) step using CHF₃and O₂gases, as shown in FIG. 3 b.
A photosensitive material layer 428, such as Shipley SPR 220-3 positive photoresist, is then spun on LTO layer 422. A second photolithographic step is then performed on photosensitive material layer 428 to etch a set of fine features 430, 432 of vertical comb finger structures on top of LTO layer 422. Fine features 430, 432 are constrained laterally within respective coarse features 424, 426, as shown in FIG. 3 c.
The misalignment tolerance for the second photolithography step, which includes a self-alignment step, is half of the gap spacing between stator comb fingers and rotor comb fingers. A significant advantage of the second photolithography step is that if the alignment is deemed to be unsatisfactory on inspection after the second photolithography step, the photoresist can be removed by a Piranha clean, and the self-alignment step can be repeated as many times as necessary. This flexibility eliminates the uncertainty in determining whether or not self-alignment has been achieved, as may happen when the self-alignment is performed to a layer buried deep within a material stack. The minimum comb gap spacing achievable can be determined by the maximum aspect ratio that a silicon deep reactive ion etching (DRIE) tool used in subsequent steps can achieve.
Next, a second RIE step is utilized to remove exposed LTO layer 422 in order to trim coarse features 424, 426 within LTO layer 422 to match the widths of corresponding fine features 430, 432 within photosensitive material layer 428, in order to complete the self-alignment process, as illustrated in FIG. 3 d.
Using coarse features 424, 426 within LTO layer 422 and fine features 430, 432 within photosensitive material layer 428 as masks, a DRIE is utilized to remove a portion of silicon device layer 412 (stopped on silicon dioxide layer 416) to form stator comb features 438 and rotor comb features 440 on top of silicon dioxide layer 416, as shown in FIG. 4 e. The DRIE is preferably performed in an inductively-coupled plasma generator using SF₆/O₂and C₄F₈gases in a pulsed scheme (commonly known as a Bosch process).
A third RIE step is then utilized to remove silicon dioxide layer 416, using coarse features 424, 426 within LTO layer 422 and fine features 430, 432 within photosensitive material layer 428 as masks. Photosensitive material layer 428 is subsequently removed, leaving rotor comb features 440 unprotected by any masking element, while stator comb features 438 are still protected by LTO layer 422, as illustrated in FIG. 3 f.
A second DRIE step is utilized to remove portions of silicon device layers 412 and 414 (stopped on silicon dioxide layer 418) to define rotor comb fingers 444 in silicon device layer 414. After the completion of the second DRIE step, rotor comb features 444 reside only in silicon device layer 414, while stator comb features 442 reside in both silicon device layers 412 and 414, as illustrated in FIG. 3 g.
The lower section of stator comb features 442 (portions located in silicon device layer 414) is redundant from an actuation perspective, but they do not affect the operation of a micromirror.
A third photolithographic step using a photoresist layer 446 is then performed on a backside of wafer 400 using a third photomask to align to the features on the front side of wafer 400. Preferably, photoresist layer 446 is approximately 15 μm thick and is capable of protecting the underlying silicon through a substrate DRIE step. Photoresist layer 446 can be, for example, Shipley SPR 220-7 positive resist. The third photomask contains the outline of a rotatable mirror structure 445 and is used to remove all silicon directly beneath rotatable mirror structure 445, as illustrated in FIG. 4 h.
Since the feature on the third photomask is relatively large (comparable to the size of the entire device), a significant amount of misalignment can be tolerated. Wafer 400 is bound by photoresist to a second silicon substrate (not shown) serving as a mechanical handle in preparation for the backside substrate DRIE step on substrate 420. The backside DRIE step releases the devices and creates dicing lines to facilitate cleaving of wafer 400 into individual chips.
Wafer 400 can be separated from its handle wafer by soaking wafer 400 in acetone, following which a fourth RIE is performed on the front and back sides of wafer 400 to remove any remaining exposed hard mask in LTO layer 422 and silicon dioxide layer 418. The result is an optical scanning device having multiple bond pads 448, stator comb fingers 450, rotor comb fingers 452, and a rotatable mirror 454, as illustrated in FIG. 3 i.
As a final step, an E-beam evaporation is used to coat a thin film (approximately 500-1000 Å) of metal on the surface of mirror 454 to improve reflectivity. The metal can be aluminum, gold, chromium, etc. Preferably, mirror 454 has a circular shape with a 500 μm diameter to facilitate illumination at artibrary angle of incidence by a 500 μm diameter laser beam, which allows for uncomplicated optical paths and easy integration into an imaging system.
As has been described, the present invention provides an improved method for fabricating a micromirror. The self-alignment of the coarse silicon oxide layer and fine photoresist layer ensures the fine and convenient alignment for the comb-drive structure. The entire fabrication process requires only three photomasks, and the three-mask fabrication process allow simpler processing flow with reduced cost.
While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims

1. A method of fabricating a micromirror, said method comprising:

forming a set of coarse features in a low-temperature oxide (LTO) layer deposited on a front side of a wafer having a substrate, a first and second silicon device layers separated from each other by a first and second silicon dioxide layers;

forming a set of fine features in a photosensitive material layer deposited on top of said LTO layer, wherein said fine features are constrained laterally within said coarse features;

removing a portion of said LTO layer to align the width of said coarse features with the width of said fine features;

etching said first silicon dioxide layer and said first and second silicon device layers to form stator comb fingers and rotor comb fingers; and

removing a portion of said substrate on a back side of said wafer, and said silicon dioxide layers from said front and back sides of said wafer to form a rotatable minor.

2. The method of claim 1, wherein said wafer is a double silicon-on-insulator (DSOI) wafer.

3. The method of claim 1, wherein said etching further includes

etching said first silicon device layer by using said LTO layer and said photosensitive material layer as a mask;

etching said first silicon dioxide layer by using said LTO layer and said photosensitive material layer as a mask; and

removing said photosensitive material layer.

4. The method of claim 1, wherein said etchings are performed by a Deep Reactive Ion Etching (DRIP process.

5. The method of claim 1, wherein said method further includes coating a thin metal film on a surface of said micromirror.