Patent Publication Number: US-7595539-B2

Title: Method for release of surface micromachined structures in an epitaxial reactor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a division of prior application U.S. Ser. No. 10/334,186 filed Dec. 30, 2002 now U.S. Pat. No. 6,939,809. 

   FIELD OF THE INVENTION 
   The present invention relates to the manufacturing of micromechanical devices, and relates in particular to a method for releasing micromechanical structures from adjacent structures during manufacture in an epitaxial reactor. 
   BACKGROUND INFORMATION 
   One method of depositing structural layers during manufacture of surface-micromachined devices involves the use of an epitaxial reactor. Epitaxy is a process for producing layers of monocrystalline layers of silicon over a single crystal substrate, and for forming polycrystalline silicon layers over other substrate materials, such as SiO 2  films on silicon substrates. Epitaxial reactors may be operated with precisely controlled temperature and atmosphere environmental conditions to ensure uniform deposition and chemical composition of the layer(s) being deposited on the target substrate. In addition to precise control, use of an epitaxial reactor can permit build-up of layers on a substrate at significantly higher rates than typically found with LPCVD systems. 
   From U.S. Pat. No. 6,318,175, for example, one approach to creating a micromachined device such as a rotation sensor is to apply a sacrificial SiO 2  layer to a monocrystalline silicon substrate at a position where one or more micromechanical deflection parts are to be formed. Windows through the SiO 2  layer to the silicon substrate may then be formed by applying a layer of photosensitive material, overlaying a mask pattern over the photosensitive layer and exposing the masked surface to light, and then using a developer to selectively remove the light-exposed portion of the photosensitive material and HF to etch the SiO 2  layer directly underlying these exposed portions. Following creation of the desired windows in the SiO 2  layer, an upper epitaxial layer of silicon may then be deposited on both the SiO 2  layer and the contact openings. The upper epitaxial layer grows in polycrystalline form on the SiO 2  layer, and in monocrystalline form on the contact window openings to provide a direct connection to the silicon substrate. The structural elements of the micromechanical device may be defined on the upper silicon layer using, for example, an anisotropic plasma etching technique. The etching may be performed through the polycrystalline portion of the epitaxial layer to the SiO 2  layer to form trenches around the structural limitations of the micromechanical parts. Finally, the SiO 2  layer may be removed from beneath the micromechanical parts in the upper silicon layer during an etching process to complete the formation of the micromechanical device. 
   The final step of releasing the micromachined structures formed in the upper silicon layer from the underlying sacrificial silicon dioxide layer may be problematic given the following: the geometry of the micromachined structures; the difficulty in ensuring complete etchant penetration through the sacrificial layer beneath the structures; and problems with device deformation and adhesion during the dry process. Release has been accomplished by etching using an HF vapor, as discussed in German Published Patent Application No. 19704454 and U.S. Pat. No. 5,683,591, or by application of liquid HF in combination with supercritical carbon dioxide (CO 2 ), to selectively release and evacuate the sacrificial SiO 2  from underneath the micromachined structural parts. 
   These processes, however, may have associated disadvantages. The chemically aggressive nature of HF may preclude its use in releasing micromachined devices created on substrates cohabited by integrated circuit portions. There may be potential damage due to liquid etchants impinging on delicate micromachined structures. There may be problems created by incomplete elimination of liquid etchants. There may be increased process complexity and expense associated with process steps requiring removal and/or reinsertion of the devices from the epitaxial reactor. There may be a need to maintain the supply and environmental control of materials in special states in an epitaxial reactor environment. 
   Therefore there is a need for a less-complex, more cost-effective method for releasing micromachined structures from their underlying substrates. 
   SUMMARY OF THE INVENTION 
   According to an exemplary embodiment of the present invention, a method for releasing a micromachined structure or device from its supporting substrate may begin with a partially formed micromachine device, which may comprise a substrate layer of, for example, monocrystalline silicon, a sacrificial oxide-bearing layer of, for example, deposited or grown SiO 2  on the substrate layer and etched to create a pattern of holes or open areas through to the substrate layer, and a functional layer of, for example, epitaxially deposited silicon which may be etched to define micromechanical structures or devices thereon and thereby expose the underlying sacrificial layer. 
   Once the elements of the micromechanical structure or device have been defined in the function layer, an exemplary embodiment of the present invention provides for in situ cleaning of the device within the epitaxial reactor with both hydrogen (H 2 ) to remove surface oxides, and with hydrochloric acid (HCl) to remove silicon residues and surface imperfections resulting from the trench etching process. Following the cleaning step, the structures may be released by exposure of the device to high temperature H 2 , which bonds to the sacrificial layer&#39;s oxide-bearing material. The resulting gas may be flushed from the device by the H 2  flow. Exposure to the H 2  flow may be continued until all the sacrificial layer material beneath the elements of the micromechanical structure or device is evacuated. 
   The release of the micromechanical elements from their underlying sacrificial layer supports in the foregoing manner may provide the following advantages: avoiding the need to apply high forces on delicate elements; preventing adhering of the micromechanical element release from the sacrificial layer (i.e., “sticking”); and reducing the need for highly aggressive etching agents such as HF or liquid release agents whose complete removal from the micromechanical structure or device may be problematic. The use of H 2  in this manner may have the further advantage of ready compatibility with an epitaxial environment and relatively convenient handling of materials as compared to other release substances such as acids, thereby simplifying process operations and enhancing epitaxial reactor production of the micromachined devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a  through  1   f  show cross-sections and plan views of various stages of production of an exemplary micromachined device. 
       FIGS. 2   a  through  2   c  illustrate the removal of sacrificial material to release a micromechanical element in accordance with an exemplary embodiment of the present invention. 
       FIG. 3  is a flowchart illustrating steps for releasing a micromechanical element in accordance with an exemplary embodiment of the present invention. 
       FIG. 4  shows a cross-section of an exemplary device having multiple layers of a silicon bearing compound. 
   

   DETAILED DESCRIPTION 
   According to an exemplary embodiment of the present invention, a method for releasing a micromachined structure or device from its supporting substrate begins with a partially formed device, which may be formed as follows. As shown in the cross-section view in  FIG. 1   a , the partially formed device is based on a substrate layer  1  of silicon, upon which a sacrificial layer  2  of SiO 2  is deposited or grown.  FIG. 1   b  shows a cross-section view of the substrate and sacrificial layer combination of  FIG. 1   a  after a pattern of holes or open areas  3  have been formed in sacrificial layer  2  using etching techniques, such as application of a photo-sensitive material over the sacrificial layer, applying a mask with the desired etching pattern over the photo-sensitive material, exposing the masked surface to light, and then applying an etchant to remove the exposed portions of the photo-sensitive material and the sacrificial SiO 2  underneath the exposed portions.  FIG. 1   c  shows a plan view of the partially formed device of  FIG. 1   b  showing hole  3  defined by the etching process through sacrificial layer  2 . The cross-section view in  FIG. 1   b  is taken through the line IB-IB of  FIG. 1   c.    
   The partially formed device may then receive an epitaxially deposited function layer  4  of silicon, as shown in cross-section view  1   d . The portions  5  of the function layer  4  formed on the SiO 2  have a polycrystalline structure, while the portions  6  of the function layer  4  formed on the monocrystalline silicon substrate layer  1  have a monocrystalline structure. Function layer  4  is then etched to define the micromechanical structures or devices in function layer  4 , with deep, narrow trenches  7  etched through the exposed portions of the photo-sensitive material and the underlying polycrystalline silicon of function layer  4 , as shown in  FIG. 1   e .  FIG. 1   f  is a plan view of the partially formed device showing micromechanical element  8  defined by etched trenches  7 . The cross-section views in  FIG. 1   d  and  FIG. 1   e  are both taken through the line IE-IE, which corresponds to line IB-IB of  FIG. 1   c . The deflection beam portion  9  of micromechanical element  8  is shown in  FIG. 1   e  extending from the base portion  10  of micromechanical element  8 . Base portion  10  is affixed to the silicon substrate  1 , while deflection beam portion  9  may rest upon, and may therefore be restrained by, an underlying column  11  of SiO 2  of sacrificial layer  2 . This column of sacrificial material must be removed to free beam  9  to deflect from its rest position during operation of the micromechanical device. 
   Next, the surfaces of the partially formed device may be cleaned in situ in the epitaxial reactor. In order to remove oxides on the surface of the device, H 2  at elevated temperature may be passed over the device, which may cause the oxide molecules to bond to the H 2  to form water and evaporate from the device surface. Following removal of any residual surface oxides, gaseous HCl may be used to remove any remaining silicon residues and surface imperfections from the surface of the device, such as silicon residues remaining on the device surface during the micromechanical element-defining trench etching process. 
   Following the cleaning operation(s), the device may be exposed to H 2  flowing at temperatures in the range of 800° C. to 1,400° C., as shown in  FIG. 2   a .  FIG. 2   a  illustrates the exposure of the SiO 2  of the sacrificial layer in communication with the trenches  7  defining micromechanical element  8  to the high temperature H 2  gas  12  flowing into and out of the trenches  7 . Upon reaching the exposed surfaces of the SiO 2  sacrificial layer around and underneath the micromechanical element, the gaseous H 2  may bond to oxygen in the oxide-bearing material at the surface of the sacrificial layer, forming water (H 2 O) and silicon monoxide (SiO). The water and silicon monoxide are gaseous, and accordingly may be immediately released from the exposed surface of the sacrificial layer into the flowing H 2  gas, which may sweep the water and silicon monoxide out of the device. As illustrated in  FIG. 2   b , the release and removal of the gaseous water and gaseous silicon monoxide from the device trenches  7  may expose additional SiO 2  in the sacrificial layer to the high temperature H 2  flow, causing additional SiO 2  to be released from the sacrificial layer. As illustrated in  FIG. 2   c , this process may continue until all the SiO 2  underlying the deflection beam portion  9  of micromechanical element  8  has been removed, and the beam is freed. 
   The foregoing method may have the following advantages: permitting removal of the sacrificial layer underlying the micromechanical elements without applying any significant impact force to the micromachined elements; ensuring complete removal of the sacrificial layer material under the micromechanical elements from the device. This method may avoid problems associated with incomplete drying of liquid agents from within the semi-conductor device following sacrificial layer removal. 
   In another alternative exemplary embodiment, an SOI (Silicon on Insulator) wafer may be used to construct the device, where the substrate layer, the sacrificial layer, and the function layer may collectively form the SOI wafer. 
   Increased production rates may result from higher etch rates and reduced handling of the wafer. Sacrificial layer removal rates may be further enhanced by, for example, increasing the temperature of the H 2  used to convert the SiO 2  to H 2 O and SiO, or by introduction of small amounts of gaseous germanium or gaseous silicon bearing compounds. Adding small amounts of silicon carrier during the H 2  exposure may also be useful to moderate the extent of silicon pitting at higher H 2  gas temperatures. The use of H 2  for SiO 2  removal in this exemplary embodiment may have the advantages of ready compatibility with existing epitaxial equipment and high temperature environments, and relatively convenient handling of materials. Accordingly, the method of the present embodiment may allow for simplified process operations, further enhancing epitaxial reactor production of micromachined devices. 
     FIG. 3  is a flowchart showing a detailed implementation of the foregoing steps for releasing a micromechanical element. The process method starts at step  100  with a device into which trenches have been etched to define an element of a micromechanical structure or device. In step  110 , the surface is chemically cleaned. This step may include removing residual materials from the trench etching process, removing residual oxides from the surface of the micromechanical device remaining following the trenching process. Step  110  is followed by step  120 , placing the substrate, wafer, and/or micromechanical device in an epitaxial reactor. Step  120  is followed by step  130 , removing exposed sacrificial layer material by flowing H 2  gas at high temperatures within the device long enough to convert the sacrificial layer SiO 2  to gaseous H 2 O and SiO and permit these to be removed from the sacrificial layer and be borne out of the device. The removing operation may be performed at a pressure between about 1 millitorr and 100 torr, and may preferably be performed at a pressure of about 10 torr. Step  140  follows step  130  and marks the end of the micromechanical element release portion of a micromachined device manufacturing process. 
   In an alternative exemplary embodiment, Step  110  may not be performed, and the process proceeds from Step  100  directly to Step  120 . In another alternative exemplary embodiment, an in situ cleaning step may be performed between Step  120  and Step  130 . The in situ cleaning step may be performed in the epitaxial reactor and may include removing residual oxides from the surface of the micromechanical device by exposing the surface of the device to H 2  gas and/or removing silicon residues by exposing the surface of the device to HCl. In a further exemplary embodiment, a step of epitaxial deposition may be performed following Step  130  before Step  140 . 
   In another alternative exemplary embodiment, an SOI (Silicon on Insulator) wafer may be used in which a top silicon layer of the SOI wafer is the function layer and an insulator layer of the SOI wafer is the sacrificial layer. 
     FIG. 4  shows handle wafer  41 , which may be a silicon wafer, with device layer  42  arranged above handle wafer  41  and defining cavity  44   a . Cavity  44   a  may represent a space which was previously occupied by a sacrificial layer. Encapsulation layer  43  is arranged on top of device layer  42  and includes vents  45  which access another cavity  44   b . In this manner, device  46  may be released from handle wafer  41  below, and from encapsulation layer  43  above, in one release step. In this manner, an exemplary device having multiple function layers and multiple sacrificial layers may be constructed. In an alternative exemplary embodiment, more sacrificial layers and more function layers may be arranged above encapsulation layer  43 . 
   In other alternative exemplary embodiments, there may be more than one silicon layer to be released. For example, two vertically stacked silicon layers with sacrificial oxide layers under and between them. In these more complex structures, the sacrificial oxide may be on top of and beside, as well as on the bottom of, the structures. 
   While the present invention has been described in connection with the foregoing representative embodiment, it should be readily apparent to those of ordinary skill in the art that the representative embodiment is exemplary in nature and is not to be construed as limiting the scope of protection for the invention as set forth in the appended claims.