Etching a substrate: processes – Etching of semiconductor material to produce an article...
Reexamination Certificate
1999-09-30
2002-01-08
Powell, William A. (Department: 1765)
Etching a substrate: processes
Etching of semiconductor material to produce an article...
C216S033000, C216S084000, C216S092000, C438S748000
Reexamination Certificate
active
06337027
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to micro electromechanical systems (MEMS) devices and more particularly to a manufacturing process for manufacture of MEMS devices and application of such MEMS devices for a particular use.
(2) Description of Related Art
Micro electromechanical MEMS devices are free-standing structural elements integrated on a substrate. MEMS devices are useful for many sensor or actuator applications such as electrical signal isolators, micro switches, or tuning fork gyroscopes, by way of example. A typical MEMS device has structural elements such as cantilevered beams, suspended platforms, capacitor plates, or other elements displaced from the supporting substrate. The size of these structural elements is typically on the order of millimeters.
The manufacturing process for MEMS devices shares many of the same processing steps employed in the manufacture of integrated circuits, particularly patterning and etching steps. Unlike surface MEMS devices or LIGA devices, a typical bulk MEMS device includes a base substrate that supports the structural element and a sacrificial silicon substrate from which the structural element is obtained. The base substrate may be a Pyrex glass substrate having electrodes and conductive traces deposited thereon. The base substrate may also be etched to include a plurality of pedestals for anchoring the structural elements above the surface of the glass substrate.
The sacrificial silicon substrate has a doped epilayer in which an image of the MEMS device is imprinted using well-known semiconductor lithographic imaging techniques. Portions of the epilayer are then selectively etched using a plasma dry etch, to define the structural elements. The sacrificial silicon substrate and the glass substrate are then aligned and anodically bonded together to form a composite structure with the structural elements of the MEMS device mounted on the pedestals.
Unique to the process for manufacturing bulk MEMS devices, large amounts of sacrificial silicon substrate must then be removed to release the structural elements of the MEMS device. One process for removing the sacrificial portions of the silicon substrate is referred to as a wafer dissolution process. In the dissolution process, the composite structure is immersed in a container of heated solvent to remove the sacrificial silicon substrate. One solvent capable of removing the silicon is a mixture of ethylene diamine and pyrocathecol, commonly referred to as EDP. The doped epi layer has a significantly lower etch rate in EDP compared to the undoped silicon substrate so the silicon substrate is etched at a much faster rate than either the epi or glass substrate. The dissolution method requires that the composite structure remain immersed in the solvent for several hours, depending on etch conditions and substrate size or diameter, to completely remove the sacrificial silicon substrate. Once the sacrificial silicon is removed, the structural elements defined in the epi layer are left suspended above the substrate, but attached to the pedestals.
During the immersion period the solvent is agitated to bathe the composite structure and maintain a high concentration of active solvent in contact with the structure. Unfortunately, the dissolution or dissolving of the substrate in the toxic solvent presents significant environmental and manufacturing problems. For example, since the agitated solvent is heated to about 100° C. toxic and corrosive fumes are generated. Thus, containment of the fumes is a necessity for the safety of the manufacturing personnel and provisions must be made to safely vent the fumes from the manufacturing area in a manner that is consistent with environmental and safety concerns. Also, since the composite structure is fairly large, a significant volume of the solvent is required to completely submerge the composite structure. After processing, the spent solvent must be disposed. Clearly, what is needed is a manufacturing method that eliminates the generation of toxic fumes and that minimizes the amount of solvent that is necessary to remove the sacrificial silicon substrate and release the structural elements of the MEMS device.
Another problem with the solvent used in the dissolution method is that endpoint detection requires a visual analysis but visual detection is not possible while the composite structure is immersed because the EDP solvent, in large quantities, is highly opaque. Further, characterizing the etch rate is difficult since the etch rate varies as a function of the concentration of the unspent solvent. Therefore, the time to completely remove the sacrificial substrate will increase as a function of the amount of silicon previously etched. For these reasons, it is necessary for an operator to periodically remove the composite structure from the solvent to visually monitor the etch process. However, this is a noxious process that requires great care on the part of the operator and increases the probability of injury to the operator. Moreover, determining the endpoint of the etch process must be done very quickly before spent solvent coating the partially etched device forms precipitates on the device surface. If the inspection is not performed very rapidly, the precipitates will render the device irreparably damaged and the entire wafer will have to be scrapped. To avoid the formation of precipitates, it is common for the composite structure to be left in the solvent for a longer than optimal period of time before the inspection is performed. Although the risk of precipitate formation is reduced, the extended etch time often results in an over-etched MEMS device that will not function properly. What is needed is a process that permits timely detection of the etch process so that high volumes of composite structures may be completely etched (but not over-etched) regardless of the concentration of the solvent.
After the etch process is complete, the etched composite structure must be cleaned to remove residual solvent adhering to the composite structure. If the solvent is not quickly removed, crystal residue will form as the solvent evaporates. The residual contamination could render the device defective. Accordingly, the dissolution process also includes a cleaning process. The cleaning process requires that the composite structure be immersed in a vat of hot de-ionized (DI) water heated to about 100° C. This immersion process subjects the operator to the risk of potential injury from scalding water if the composite structure is not carefully handled.
After the cleaning process, the suspended structural elements are often found to adhere to the glass substrate due to surface tension or stiction (static friction). To overcome the stiction, the dissolution process further includes a vacuum release step where the composite structure is place in a vacuum chamber in an attempt to separate the suspended element from the glass substrate. Often, the vacuum step is not successful, affecting device yields. It has been found that minimizing the amount of the surface area of the glass substrate that could contact the suspended elements, stiction yield loss can be further reduced. For this reason, the prior art dissolution process includes process steps where a plurality of metal stand-offs are formed in the metal under the suspended structural elements. The stand-offs reduce the amount of surface area of the glass substrate that can come in contact with the suspended structural element. Thus, after the DI water clean, the composite structure is immediately placed in the vacuum chamber to rapidly dry and separate the suspended structural element from the electrode since these elements will typically adhere to the glass substrate after the immersion steps. If there is significant delay in removal of the Dl water, the stiction force will permanently maintain the suspended portion in contact with the electrode rendering the MEMS device defective. Although providing the stand-offs require additional processing steps, the impr
Powell William A.
Rockwell Science Center LLC
Shinners Craig E.
LandOfFree
Microelectromechanical device manufacturing process does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Microelectromechanical device manufacturing process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Microelectromechanical device manufacturing process will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2848329