Patent Application
20050101045
Micro-electromechanical systems (MEMS) is a technology that combines processors and circuits with tiny mechanical devices such as pressure and temperature sensors, vibration sensors, valves, gears, mirrors, and actuators embedded in semiconductor chips. Typical MEMS combine sensing, processing and/or actuating functions to alter the way that the physical world is perceived and controlled. They typically combine two or more electrical, mechanical, biological, magnetic, optical or chemical properties on a single microchip.
Integrated circuit and MEMS packaging can be very expensive. As used herein, “packaging” refers to any post-fabrication processing to prepare a MEMS for incorporation into a larger system or to actually incorporate the MEMS into a larger system.
Typically, MEMS are fabricated at one facility and then moved to another location or another facility for packaging. Packaging MEM systems is costly and complex and is currently not done in fabrication facilities. This two-location process, however, dramatically increases the expense of the product.
The accompanying drawings illustrate various embodiments of the present invention and are a part of the specification. The illustrated embodiments are merely examples of the present invention and do not limit the scope of the invention.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.
A recent trend has been to try developing methods for packaging MEM systems at the same time and location that the MEM systems are fabricated. This will be referred to as “in-fab packaging.” Packaging that is done within the fabrication facility can drastically decrease the end cost of the product. Moreover, with in-fab packaging, the MEM system can be environmentally sealed, protected from particles, contamination, and moisture in a controlled environment.
The present specification describes a process of packaging or sealing a MEM system by depositing material to seal voids in the MEMS structure. In one embodiment, a film is deposited on top of a completed MEMS structure. The purpose of this film is to reduce the size of the opening that was used to isotropically etch out the cavity in which the MEM system is formed. The film is then anisotropically etched to remove excess material on the top of the structure, to control thickness, and to remove material that was deposited at the bottom of the vertical opening. A subsequent film layer is then deposited to completely fill the opening and passivate the entire device. The deposited film may be, for example, a dielectric or metal material.
Referring now to
A layer of materials (107) formed on the substrate (106) under the cavity (102) may include, for example, an oxide layer (e.g., 200 Å) over an aluminum copper layer (e.g., 1,200 Å) over a titanium nitride layer (e.g., 500 Å) over a thermal oxide layer (e.g., 1,000 Å).
To create the cavity (102) that accommodates the MEM system and any movement of system components, the material (105) is isotropically etched and then removed to form the cavity (102). The material (105) may be, for example, amorphous silicon. The isotropic etchant removes the sacrificial material (105) through a release hole or opening (101) in the top film (109). The top film (109) may be, for example, an oxide of approximately 10,000 Å in depth.
The MEM system can then be formed in the cavity (102), after which the unit (100) is ready for packaging. As part of the packaging process, the opening (101) is sealed. Sealing the opening (101) is performed to protect the potentially fragile MEM system from particles, moisture and other sources of contamination. The opening (101) that needs to be sealed may have a width (103) of two microns, for example. By comparison, the cavity (102) may have a height (104) of one micron, for example.
As described herein, sealing the opening (101) can be performed as part of an in-fab packaging process in which a film is deposited on top of the completed MEMS unit (100). The purpose of this first deposited film is to partially fill or reduce the size of the opening (101) that was used for the isotropic etch.
The deposited film is then anisotropically etched to remove excess material on the top film (109), to control thickness, and to remove material that was deposited at the bottom of the opening (101) in the cavity (102). A subsequent film layer is then deposited to completely fill the opening (101) and to passivate the entire device (100). The deposited film may include, for example, a dielectric or metal material. As shown in
The two depositions described in this example will adequately fill a one-micron opening (101), thereby sealing the cavity (102). However, the deposition-etch-deposition process can be repeated subsequent times to fill a larger opening. Alternatively, if the opening (101) is smaller, a single deposition without any subsequent etching may be sufficient to seal the cavity (102). These depositions and etches can all be done with minimal impact to the MEM system in the cavity (102).
The top layer (202) has increased in size due to second deposition of sealing material to sealing the opening. For example, the new top layer (202) may have another 10,000 Å of oxide above the previous top layer (109,
Depending on the size of the opening being filled, this first deposition (step 401) may completely seal the opening and the process ends (determination 400). Assuming, however, that the first deposition only partially fills the opening (determination 400), following the deposition of this first layer of sealing material, the structure is etched (step 402). For example, the structure is anisotropically etched to remove excess dielectric from the top of the structure, to control thickness, and to remove dielectric that was deposited at the bottom of the opening in the MEMS cavity.
Lastly, another layer of sealing material is deposited (step 403) to completely fill the opening into the MEMS cavity. In this way, the MEM system is hermetically sealed.
A lamp-heated oxide Chemical Vapor Deposition (CVD) chamber with a bubbler-style vapor delivery of tetraethylorthosilicate (TEOS) may be used. TEOS is the precursor used to form the oxide. Both depositions, first and second, can use the same Plasma Enhanced CVD (PECVD) process parameters, varied only by the time given to obtain the desired deposition thickness.
In a particular example with an opening (101,
In this example, top down Scanning Electron Microscopy (SEM) images of a test wafer illustrated a decreased opening diameter of less than 650 nm after the first deposition. Cross sectional SEM images confirmed that the sidewalls of the opening were covered with oxide. Additionally, 250 nm of oxide were initially deposited on the silicon at the bottom of the release hole. With an etch rate of 625 nm/min, 40 seconds was sufficient time to remove all of the oxide from the surface of the oxynitride and the bottom of the cavity, with an ample oxide thickness remaining on the sidewalls of the opening. Over-etching may also result in removal of some oxynitride as well as silicon at the bottom of the opening, which may prove to be beneficial. The diameter of the opening after the etching increased to 725 nm. The final deposition then sealed the opening. Cross sections of wafers after the final deposition confirm net increase of less than 250 nm remaining on the silicon inside the cavity (102,
If the opening is not sealed, the steps of depositing a sealing material (step 401), etching that material (step 402) and depositing another layer of material (step 403) can be repeated until the opening is sealed (determination 405). When the opening is sealed, the process ends.
By tuning the deposition thickness and incorporating multiple deposition-etch-deposition cycles, it is possible to seal any sized opening or release hole with only the slightest additional film remaining on the bottom of the cavity (102,
Following the deposition of this first layer of sealing material, the structure is etched (step 402). For example, the structure is anisotropically etched to remove excess dielectric from the top of the structure, to control thickness, and to remove dielectric that was deposited at the bottom of the opening in the MEMS cavity.
Then, another layer of sealing material is deposited (step 406). However, this deposition does not completely fill the opening into the MEMS cavity, but rather leaves a very small channel, referred to as a pinhole, leading into the MEMS cavity.
Before the opening is completely sealed, the environmental conditions inside the MEMS structure, e.g., humidity, pressure, etc., are adjusted through the pinhole (step 407). The pinhole can be used to make any environmental adjustment desired including, for example, providing a desiccant in the sealed device. When the environmental conditions inside the MEMS structure have been adjusted as desired, a final deposition of sealing material is made to seal the channel (step 408).
By optimizing deposition/etch cycles to reduce the opening to a pinhole, various process conditions and film types can then be applied to season the chamber and finally seal the pinhole. In this way, the MEM system is hermetically sealed with the desired environmental conditions being established inside the MEMS structure. Alternatively, the seal provided can be non-hermetic depending on the desired characteristics of the internal MEMS environment.
It should be noted that depending on the initial size of the opening being sealed, in some cases, there may be two or more deposition-etch-deposition cycles to narrow the opening down to the desired pinhole. Any number of deposition-etch-deposition cycles can be performed as suits a particular application.
Depending on the pressure desired in the MEMS structure, a film that could be used to achieve 60 to 100 Torr pressure would be ozone-tetraethylorthosilicate (TEOS)-based oxide or Sub-Atmospheric (pressure) Chemical Vapor Deposited (SACVD) TEOS-based oxide. A brief deposition of such material will fill the pinhole, then a subsequent deposition can be made to cover the high pressure TEOS-based film with something more robust and less prone to moisture-absorption.
There are various possibilities regarding film composition. Silane-based oxides have greater sidewall coverage than TEOS oxide and may perform better as the initially deposited film. Though PECVD TEOS oxide can be deposited on the order of a Torr, pressures can be increased to near atmospheric levels using ozone TEOS-based oxides. Another example would be the use of nitride films if a better moisture barrier is needed. The in-fab packaging methodology described herein is flexible to the requirements of many MEMS applications.
If it is critical to maintain certain optical properties of the cavity (102), and the MEM system formed therein, oxide films having a low index of refraction (i.e., a low n value) can be used, for example, oxide films having an index of refraction of 1.6 or less can be used. However, other dielectrics can be used for MEM systems that are not optically sensitive.
As indicated above, MEM systems are historically difficult and expensive to package. The advantages of this in-fab packaging method include controlling the MEMS environment, for example, humidity and pressure. Moreover, in-fab packaging makes the MEM system less sensitive to subsequent packaging because of the hermetic seal provided by closing the opening (101,
In-fab packaging is also simpler than out-of-fab packaging because in-fab packaging combines passivation with packaging of the device. In addition to passivation, in-fab packaging can also be used to deposit a covering layer that can cover areas of the device that may have a poor appearance or undesirable optical outputs. In cases where the MEM system includes sensitive optics or moving parts that may be damaged by transportation, in-fab packaging makes it possible to package and passivate the device despite the device's low tolerance for movement.
A particular application of the principles described herein would be to provide simple and cost effective packaging for a Digital Light Device (DLD) MEMS. In the DLD manufacturing process, a chamber (e.g., cavity (102)) is created to allow for electromechanical movement of a pixel plate suspended in the chamber by flexures above a bottom capacitor plate (BCP).
As with other MEM systems, openings or release holes through the pixel plate are needed to remove the sacrificial material and create the chamber. The openings are then hermetically sealed, but need to be sealed without inhibiting the functional motion of the corresponding pixel plate. Additionally, an optically critical MEM system such as DLD needs to be sealed to prevent air intrusion. Air negatively impacts the performance by reducing the frequency response of the capacitor plate. As indicated above, in-fab packaging or “in-situ encapsulation” permits controlled tuning of the MEMS internal environment, including moisture content and pressure.
The challenge of sealing the chamber for a DLD is that the 1000 nm opening is relatively large and must be sealed without mechanically connecting the ceiling of the chamber (i.e., the pixel plate) to the floor (the BCP). Such bridging would obstruct the device operation.
In a DLD, the desired depth of the distance between the pixel plate and BCP is approximately 500 nm. The diameter of the opening is twice this distance. For such conditions, where the bridging distance is substantially less than the gap dimension, standard plasma enhanced chemical vapor deposition (PECVD) is inadequate to seal the release hole.
As described, the in-fab packaging methodology described herein incorporates a deposition-etch-deposition sequence to seal the chamber without bridging. The purpose of the initial deposition and etch is to reduce the diameter of the opening to be sealed without leaving any substantial deposition on the surface of the pixel plate or the exposed bottom capacitor plate. The second deposition seals the reduced opening and passivates the device.
The preceding description has been presented only to illustrate and describe embodiments of invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the following claims.
