Method of fabricating micro-mirror switching device

Method of fabricating a micro-mirror switching device in single crystal silicon are described. The device is fabricated as three main elements: silicon mirror plate with metal-mirror, secondary actuator, and hinge/spring mechanism to integrate the mirror plate with the actuator. p-n junction is first formed on p-type silicon. Trenches are then etched in n-silicon to define the device element boundaries and filled with silicon dioxide. Three layers of sacrificial oxide and two structural poly-silicon layers are deposited and patterned to form device elements. Novel release processes, consisting of backside electrochemical etching in potassium-hydroxide, reactive ion etching to expose oxide-filled trenches from the bottom, and hydrofluoric acid etching of sacrificial oxide layers and oxide in silicon trenches, form the silicon blocks; those that are not attached to structural poly-silicon are sacrificed and those that are attached are left in place to hold together the switching device elements.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to the method of fabricating micro-machined micro-mirror switching device and more particularly to fabricate metallic mirrors on single crystal silicon substrate.

(2) Description of the Prior Art

After its first conception in the 80's as a display device, MEMS (micro electro-mechanical system) micro-mirrors have come a long way in revolutionizing the data transport in communication networks. This area has been of particular interest for people involved in MEMS, in the past ten years. Micro-mirror devices have applications in display devices as a pixel, scanner type of display device. Light beam steering using micro-mirrors is being exploited in optical communication networks. MEMS based optical cross connect devices are being used in long haul networks to select and switch the light data signals without converting them to electronic domain. A large number of micro-mirror designs and mechanisms have been proposed. Surface micro machining is the most commonly used method for fabricating micro-mirror devices. Thin films such as poly-silicon, silicon dioxide, silicon nitride, and metal films such as aluminum, gold, chromium, and titanium have been used to develop micro-mirror devices for various applications. Some other researchers have used silicon-on-oxide MEMS, deep RIE SCREAM process, and in some cases backside aqueous potassium hydroxide etching to fabricate micro-mirror devices. In most devices, torsion springs or free hinges together with comb-drive or gap closing electrostatic actuators have been used.

U.S Pat. No. 5,537,083 describes a micro mechanical filter having planar components and fabricated using integrated circuit micro fabrication techniques. The mechanical coupling between input and output transducers includes planar fixtures, displacement of the electrodes producing bending of the elements of the fixtures. Processes include depositing electrical signal processing circuitry on a substrate, depositing interface components between signal processing circuitry and a mechanical filter in a first layer; depositing in a second layer components of the mechanical filter.

U.S. Pat. No. 5,999,303 describes an optical head utilizing a micro machined element in combination with a light source and a lens to read and write data onto a storage disk. A micro-machined element may include a tethered steerable micro-machined mirror. A movement of the micro machined mirror alters a beam of laser light transmitted from the light source to the optical head and a reflected light beam from the storage disk.

U.S. Pat. No. 6,210,988 B1 relates to micro-electromechanical systems using silicon-germanium films. The process includes depositing a sacrificial layer of silicon-germanium (SiGe) onto the substrate; depositing a structural layer of SiGe onto the sacrificial layer, where the germanium (Ge) content of the sacrificial layer is greater than the Ge content of the structural layer; and removing at least a portion of the sacrificial layer.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to describe the design of single crystal silicon micro-mirror switching device.

Another object of the invention is to describe the design of micro-mirror switching device elements, mirror plate, secondary actuator, and spring/hinge mechanism.

A further object of the invention is to provide a method for forming the MEMS micro-mirror switching device.

Yet another object of the invention is to provide methods for forming the elements of micro mirror-switching device, micro-mirror, actuator, and hinge/spring mechanism.

Another object of the invention is a method for implementing the angular amplification design using silicon as both structural and sacrificial layers.

In accordance with these objectives, design and a method of fabricating the said micro-mirror switches are described. The switch has three elements: single crystal silicon (SCS) mirror plate, secondary actuator, and hinge/spring mechanism to integrate the mirror plate with secondary actuators. The actuator moves the micro-mirror through spring and hinge mechanism. The actuator in this invention is of the electrostatic type or of the thermal type. A p-n junction is first formed on p-type silicon by growing n-type silicon on p-type wafers. Trenches are then formed to define the closed loop rectangular boundaries for micro-mirror plate, actuator electrodes, and other components to be fabricated in SCS. After filling the trenches with silicon dioxide, the wafer surface is planarized using chemical mechanical polishing or other etch-back processes. The planarized wafer is then used to fabricate top actuator electrode, interconnects, metal pads etc. Next, a sacrificial silicon dioxide layer-1 is deposited and patterned. Protective layers of silicon nitride and poly-silicon are then deposited and patterned. Next, another sacrificial silicon dioxide layer-2 is deposited on which a layer of structural poly silicon layer-1 is formed and patterned. This step is followed by the deposition of another sacrificial silicon dioxide layer-3 that is patterned. Structural poly-silicon-2 layer is then deposited and patterned. In the next step, mirror metal is deposited and patterned to form mirror. Device formation is completed using the release processes to selectively cut desired structures in single crystal silicon, while sacrificing remaining SCS blocks.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The top view of the micro-mirror switching device with micro-mirror region10, secondary actuator region12, and hinge & spring region14, is shown in FIG.1. The hinge and spring element integrates the micro-mirror and the actuator elements. Electrostatic actuator consisting of upper cantilever-type moving poly-silicon electrode and lower stationary silicon electrode moves the mirror through hinge and spring mechanism. Several combinations or hinge and spring elements are used: free hinge and lever arm (not shown); free hinge only combination (FIG.2); flexure spring (not shown); flexure spring and free hinge (FIG.3); free hinge and flexure spring (not shown); torsion spring and flexure spring (FIG.4). Integration scheme according to the present invention implements the angular amplification concept for achieving large angular displacements for micro-mirrors with smaller bending of secondary cantilever-type actuators.

FIGS. 5athrough5hand6athrough6hdescribe the process steps for forming the micro-mirror switching device. All the elements of the device are formed simultaneously during the described process steps.

FIG. 5aandFIG. 6aare cross-sectional views of the device along axis5A and axis6B respectively (shown by dashed lines5A and6B inFIG. 1) after several process steps as described herein is completed. A p-n junction is first formed on p-type silicon20n-type silicon22is then formed either by diffusion or alternatively by depositing epitaxial silicon film. The depth of p-n junction is kept approximately at about 8-10 μm. Trenches24, approximately about 1-3 μm wide and about 4-8 μm deep, are etched in silicon with a deep reactive ion etching process, known in prior art (U.S. Pat. No. 5,501,893). The said trenches define the close-loop rectangular boundaries of the various regions10,12, and14(shown in FIG.1). Silicon dioxide26is then filled within the trenches and the wafer surface is planarized using plasma etch back or chemical mechanical polishing methods, known in prior art. Silicon dioxide28, later used as sacrificial layer-1 (sac-1) is deposited on the said planar surfaces and patterned. Resulting structures are shown inFIGS. 5aand6a.

Nitride30and poly-silicon32films, both used to protect field area oxide, are deposited next and patterned. Sacrificial silicon dioxide layer-2 (sac-2)34is then deposited but not patterned, as shown inFIGS. 5band6b. Structural poly-silicon layer-1 (poly-1)36is then deposited and patterned (shown only inFIG. 6b) to partly define spring/hinge elements.

Next , sacrificial silicon dioxide layer-3 (sac-3)38is deposited and patterned as shown inFIGS. 5cand6c.

Structural poly-silicon layer-2 (poly-2)40is deposited next and patterned, as shown inFIGS. 5dand6d. Poly silicon actuator and hinge/spring mechanism are fully defined at this process step. Mirror-metal is then deposited and the metal patterned to form the micro-mirror42as shown inFIG. 6d(only). At this step, all of the components are fabricated in single crystal silicon, but not separated or released to form working device elements.

The following steps describe the release process to cut the device structures in single crystal silicon and form the individual elements:i) Backside silicon20is etched electrochemically preferably in aqueous potassium hydroxide (KOH) solution to stop at the n-p junction with diffused n-type silicon22exposed. Other etching mediums can be substituted for the preferred aqueous KOH solution. These alternate chemicals include aqueous solution of ethylene diamine, pyrocatechol, and/or TMAH-isopropenol. A silicon window is opened from the backside of the wafer in this step. The resulting structure is shown inFIGS. 5eand6e.ii) Next, backside n-type silicon22is etched within said KOH-etched silicon window, using reactive ion etching (RIE) process in SF6 plasma, known in prior art, or using further KOH etching of silicon with time control, such that the silicon dioxide-filled trenches26are exposed at the bottom as shown inFIGS. 5fand6f.iii) Silicon dioxide-filled trenches26are then sacrificially etched in HF solution of concentration approximately 49% HF in water at room temperature. In this step, silicon dioxide is removed both from trenches and from under the micro-machined structures. The resulting structures are shown inFIGS. 5gand6g. The unsupported silicon blocks44get removed during this sacrificial etching step, while the silicon blocks46attached to structural poly-silicon layers remain in the final device. The final device structures are shown inFIGS. 5hand6g/6h. InFIG. 5h, lower fixed silicon electrode48, landing pad50, spring part52, and silicon/poly-silicon hinge54are shown. InFIG. 6h, mirror plate56, free hinge58, and silicon/poly-silicon hinge bar60are shown. The actuator shown inFIG. 5his of the electrostatic cantilever type with supporting blocks48and50.

When the actuator is of the thermal type, the cantilever would be formed of a bi-metal or generically of a bi-morph material; and the movement of the actuator takes place due to mismatch in the thermal coefficient of expansion of the two metals. The bi-morph cantilever is chosen from materials composed of silicon dioxide, silicon nitride, poly silicon, and/or gold and the heater material is chosen from aluminum, titanium nitride, aluminum-titanium alloy, and/or poly silicon. In terms of the process to incorporate the thermal actuator in the said micro-mirror device, the silicon blocks48and50are also sacrificed during etching. The remaining process steps are identical to those used in incorporating the electrostatic type of actuator.

The advantages of the present invention of forming the MEMS micro-mirror switching device over the prior art are as follows:1. Micro-mirror switches fabricated on single crystal silicon using conventional semiconductor device methods are flatter and therefore able to steer the light beam more precisely.2. The switching device is formed using both surface micro machining and bulk micro-machining methods, thereby retaining the advantages of both methods.3. Electrostatic, cantilever design of poly-silicon actuator allows higher angular rotation of the mirror with small displacement of the actuator tip.4. New angular amplification concept allows the use of minimum space for forming the actuator in the device for large angular bending.5. The invention method of fabrication uses a unique process of forming and using oxide-filled trenches (which are later sacrificially etched) that allows silicon to be used as both sacrificial and structural layers.6. A new release process is used where, after electrochemical etching of a window in backside silicon, reactive ion etching is used to expose SiO2-filled trenches and sacrificing the oxide through trenches and from the top.

While the preferred embodiment of the invention has been described, it will be understood by those skilled in the art that various modifications in form and details may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims: