Abstract:
The disclosed fabrication methodology addresses the problem of creating low-cost micro-electro-mechanical devices and systems, and, in particular, addresses the problem of delicate microstructures being damaged by the surface tension created as a wet etchant evaporates. This disclosure demonstrates a method for employing a dry plasma etch process to release encapsulated microelectromechanical components.

Description:
FIELD OF THE INVENTION 
     This invention relates to the process of manufacturing micro-electro-mechanical systems (MEMS), and, in particular, to MEMS devices having sealed cavities or encapsulated movable parts. 
     BACKGROUND OF THE INVENTION 
     This disclosure builds on prior art describing a method for constructing micro-electro-mechanical systems by using multiple sacrificial thin film layers removed using a liquid etch. 
     It is known in the prior art to create sealed cavities on an integrated circuit for a variety of applications, for example, as a speaker or microphone. It is also known to encapsulate movable mechanical components on an integrated circuit within a sealed cavity. The encapsulation of micro-electro-mechanical structures in a sealed cavity is desirable for several reasons. First, the tolerance of the structures to ambient conditions, such as high humidity, is greatly improved. Second, the dicing and packaging of the MEMS devices is greatly facilitated. Third, when the cavity is at a low or very low ambient pressure, the Brownian noise due to the motion of gas molecules can be significantly reduced. 
     Processes to create sealed cavities on the surface of a silicon wafer using only thin film deposition and etching techniques have already been developed to create MEMS microphones and speakers for sound and ultrasound. Starting with a silicon substrate, which could in principle have CMOS devices and interconnects already patterned onto it, a protective layer is placed over the entire wafer. Next a sacrificial layer is deposited. Then, the sacrificial layer is patterned to remove it over all parts of the wafer that are not going to be microencapsulated. Next, an encapsulating layer is deposited over the entire wafer. Very small holes are then patterned and etched through the encapsulating layer at selected positions over the sacrificial layer, and the wafer is immersed in a liquid chemical bath containing an etchant that is highly selective, to dissolve the sacrificial layer while not attacking the encapsulating layer or the protective layer. Finally, an insulating or conducting layer that will act to seal the membrane must be deposited onto the wafer. The etch access hole can be sealed off either by material accumulating up from or by material depositing laterally on the sides of the hole growing inward and sealing off the hole. In either case, the final layer serves to both plug the etch holes and to seal the cavity created when the sacrificial material was etched away. 
     It is also known to create MEMS microstructures within sealed cavities such as the one described above. See, for example, U.S. Pat. Nos. 5,285,131 and 5,493,177 (both to Muller, et al.) in which methods to create an incandescent lamp and a vacuum tube respectively are disclosed. The method disclosed is as follows. A silicon substrate is covered with a non-etchable protective layer that is selectively removed, thereby exposing the silicon wafer in the region to be encapsulated. Then, a layer of poly-silicon is deposited and patterned to cover the exposed silicon window and extending up onto the silicon nitride protection layer in selected positions that will be used as entry points for the liquid etching agents. Non-etchable conductors are then deposited and patterned on top of both the non-etchable mask layer and on top of the silicon substrate in the window. Next, a sacrificial layer is deposited and etched so that it only covers the structures in the region to be encapsulated. The encapsulation process proceeds with the deposition of an encapsulation layer and the etching of small holes in the encapsulation layer located over the poly-silicon above the protection layer that will guide the etching agents into the cavity. In this case, the etching step requires two different liquid etchants—the first one to selectively etch away silicon and poly-silicon and a second one to etch away the sacrificial layer. The encapsulation process is completed by depositing a seal layer to seal up the etch entry holes in the diaphragms. 
     It is desirable to use a wet etchant, in many cases hydro-fluoric acid, because of its high degree of selectivity, that is, the ability to selectively etch away the sacrificial layers, leaving behind the microstructure and cavity walls. However, one unfortunate problem when working with a “wet” etchant is that the surface tension generated as the liquid evaporates can be strong enough to bend or even break delicate MEMS microstructures. Therefore, the use of liquid etching agents severely limits the complexity of the MEMS microstructures that can be released from sacrificial layers in the cavity because only simple MEMS microstructures can tolerate the surface tension forces exerted by typical liquid etching agents as the surface is drying. MEMS devices having suspended structures have been developed using a wet release etch. However, the structures were quite simple, for example, wires supported at both ends with a small number of meanders. However, in order to create a wide range of MEMS devices, for example, acceleration sensors, quite flexible MEMS structures are necessary. These flexible structures would most likely be destroyed by the surface tension effects of a wet etch. 
     It is therefore desirable to create MEMS structures with out the wet etch step, eliminating potential damage to delicate MEMS structures by the surface tension created by the wet etchant. Such a process would improve yields of the devices, thereby making their production more economical. Additionally, more complex structures could be developed. 
     SUMMARY OF THE INVENTION 
     The disclosed invention specifies improvements to the known MEMS fabrication process by selecting a combination of layers for the MEMS structural layers, seal layer and the sacrificial layers that allows release of the microstructures using a dry plasma etchant. Ideally, the dry etchant would have a high etch rate for the material composing the sacrificial layers and a low etch rate with respect to the material composing the structural and seal layers. This eliminates the undesirable liquid surface tension inherent in the wet etch process. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show a top view and a side cross-sectional view respectively of the silicon CMOS wafer used as the base of the MEMS micro-encapsulated structure. 
         FIGS. 2A and 2B  show a top view and a cross-sectional view respectively of the wafer of the  FIG. 1A  with a sacrificial layer deposited thereon. 
         FIGS. 3A and 3B  show a top view and a cross-sectional view respectively of the wafer of  FIGS. 2A and 2B  having a structural layer added thereon. 
         FIGS. 4A and 4B  show a top view and a cross-sectional view respectively of the wafer of  FIGS. 3A and 3B  having a second sacrificial layer deposited thereon. 
         FIGS. 5A and 5B  show a top view and a cross-sectional view respectively of the wafer of  FIGS. 4A and 4B  having a seal layer applied thereon. 
         FIGS. 6A and 6B  show a top view and a cross-sectional view respectively of the wafer of  FIGS. 5A and 5B  having etch holes drawn therein. 
         FIGS. 7A ,  7 B and  7 C show a top view, a cross-sectional view along line  7 B and a cross-sectional view along line  7 C respectively of the wafer having the sacrificial layers removed by an enchant. 
         FIGS. 8A and 8B  show a top view and a cross-sectional view respectively of the wafer having a second seal layer applied thereon, thereby sealing the etch holes. 
         FIGS. 9A and 9B  show a top view and a cross-sectional view respectively of the second seal layer having been removed from the contact pad on the base wafer. 
     
    
    
     DETAILED DESCRIPTION 
     In general, the invention disclosed refers to gas phase release of any number of microstructure layers whose movement is independent or coupled and which are encapsulated in the thin film seal layer. However, in order to explain the invention, one specific embodiment will be described in detail below, namely a microstructure that can be utilized as a Z-axis accelerometer. This device consists of a paddle shaped MEMS microstructure anchored at one point by a thin supporting member such that it can move vertically within the sealed cavity. 
       FIGS. 1–9  illustrate the sequence of steps comprising the fabrication of the proposed encapsulated integrated microstructure CMOS process. We start by obtaining or fabricating a silicon CMOS wafer  2  coated with a layer of silicon nitride  4  and having metal pads interfacing to the original CMOS integrated circuit  6 ,  8  and  10  present as shown. Openings appear in the silicon nitride layer  4  to allow access to metal pads  6  and  8 . In the preferred embodiment, the metal pads would be aluminum, but may alternatively be copper or any other conductive material. 
     To begin the fabrication process, a sacrificial layer  12  is deposited on top of the passivation layer of the standard CMOS wafer  2 , which in this case is silicon nitride layer  4 . The MEMS device fabrication steps are all performed at low temperature on top of the complete CMOS wafer  2 , leaving the circuitry therein undisturbed. Cuts in the passivation layer  4  are left during the CMOS IC design and sacrificial layer  12  is removed over these cuts if access to the metal contacts is desired. The exposed metal contacts  6  and  8  are then used to make connections between the MEMS microstructure and the CMOS circuitry in silicon CMOS wafer  2  below. This is illustrated in  FIG. 2A . 
     In the preferred embodiment the microstructure may be composed of any metal, for example, Al, W, Ti, Ta, Cu, Ni, Mo, etc., but in the preferred embodiment would be made of aluminum. The selection of material for a particular microstructure layer is dictated by two factors. First, how much residual stress gradient in the material is acceptable for a particular application and, second, by the availability of a selective etchant that removes the portions of the microstructure which are undesired, while stopping on or having a low etch rate for the silicon nitride passivation layer  4  and the sacrificial layer  12 . 
     The deposition of the MEMS layer is shown in  FIG. 3A  and in cross section in  FIG. 3B . MEMS microstructure  14  is deposited by methods known by those with ordinary skill in the art and the undesirable portions are etched away, thereby leaving the desired shape of the microstructure behind. The top view of  FIG. 3B  clearly shows the shape of the microstructure as being a paddle having a long thin beam attached to an anchor point, which in this case is metal contact  8 . 
     Next, as shown in  FIG. 4A , and in cross section in  FIG. 4B , a second sacrificial layer  16  is deposited over the microstructure. It can be seen from the top view that portions of the top sacrificial layer  16  will come into contact with portions of the bottom sacrificial layer  12 , in particular, those areas near the edges of the paddle-shaped main body of the microstructure and those areas on either side of the thin connecting beam portion of the microstructure. 
     In the preferred embodiment, and, if possible as dictated by the shape of microstructure  14 , sacrificial layers  12  and  16  will be of the same material and will have a connection to each other, such that when etchant is introduced, both layers will be etched away without the need to etch additional etchant entry holes. Alternatively, sacrificial layers  12  and  16  may be of different materials. Although not necessary in the construction of the microstructure of this example, more complex microstructures, or multiple microstructures in the same cavity may require etching away of various sacrificial layers at different times, making it necessary to use different materials for the sacrificial layers and different etchants. 
     The preferred material for sacrificial layers  12  and  14  is photoresist. Photoresist is chosen for this reason because it can be etched with an oxygen plasma gas, which is not destructive of aluminum microstructure  14 , silicon nitride passivation layer  4  or seal layer  18 .  FIGS. 4A and 4B  show the deposition of second sacrificial layer  16 . 
     If sacrificial layers  12  and  16  are of different materials it is possible to etch them separately by selecting an etchant that is selective to one and not the other. It is even possible that a wet etch could be used with one of the sacrificial layers. For example, sacrificial layer  16  may be phosphorous-doped glass and the etchant may be hydrofluoric acid. This may be desirable because the wet etchants are generally faster acting than the dry etchants. As long as the microstructure is held in place by one or more other sacrificial layers, the surface tension problem will be avoided. In this case, it is only necessary that the last sacrificial layer binding the microstructure in place be removed using the dry-etchant process. 
       FIGS. 5A and 5B  show the deposition of seal layer  18 . This layer may be composed of an insulator or a conductor, depending on the desired electrical operation of the microstructure. Additionally, the seal layer must have a low enough residual stress and must be thick enough that the membrane created by the seal layer  18  will not buckle after the sacrificial layers  12  and  16  have been removed. In the preferred embodiment, seal layer  18  is the same metal as was chosen for the microstructure layer  14 , but in alternate embodiments may be made of any material resistant to the etchant chosen. In the event an insulating material is chosen for seal layer  18 , it may be patterned and removed to give access to the non-MEMS parts of the integrated circuit, such as bond pads  6  and  8 . If seal layer  18  is a conductor, it may be contacting one or both of bond pads  6  or  8 . 
     Next, one or more etchant access holes  20 , shown in  FIGS. 6A and 6B  are etched into seal layer  18  such that communication can be established with sacrificial layers  12  and  16 . This etch is done by any means well known to anyone of ordinary skill in the art. Preferably etch holes  20  will be as far away as possible from the actual MEMS microstructure. Next, as shown in  FIGS. 7A ,  7 B and  7 C, the etchant is introduced into holes  20  and sacrificial layers  12  and  16  are etched away, leaving void  22 .  FIG. 7B  shows a cross-sectional view of the device through the center, while  FIG. 7C  shows a cross-sectional view through one of the etchant access holes. A dry plasma etchant is used to avoid problems created by the surface tension of a wet etchant. In the preferred embodiment, the etchant is oxygen plasma. Oxygen plasma was chosen because it is highly selective with respect to the etching sacrificial layers  12  and  14 , which may be photoresist or other organic polymers, while having an extremely low etching rate for a wide variety of metals and insulators. 
     At this point, microstructure  14  is able to move vertically within the cavity created by open space  22  previously occupied by sacrificial layers  12  and  16 , with beam  15  acting as a spring and contact pad  8  acting as an anchor point. Depending upon the distance from the etch access holes to the furthest point of sacrificial layers  12  or  16  to be removed, etching time using the oxygen plasma may be quite long. It is preferred for this reason that a barrel etcher be used in the etching process such that a plurality of wafers may be etched at the same time. 
     The final step, shown in  FIG. 8A , is the application of a second seal layer  26  to seal etch holes  20 . In the preferred embodiment, seal layer  26  is the same metal as seal layer  18  and MEMS microstructure  14 . As shown in  FIG. 9 , if the second seal layer is not a conductor then it may be etched away using well known methods from the area over contact pad  6 , or it may be left in electrical contact with compact pad  6 . Final seal layer  26  may be etched away from contact pad  6 , or, if seal layer  26  is composed of a conductor, may be left in place. 
     A simple microstructure that could be utilized as a Z-axis accelerometer has been described to show the general process of creating a microstructure in a sealed cavity using a dry-etch process. However, as realized by one or ordinary skill in the art, and as contemplated by the scope of this patent, the process may be used to build microstructures of more complexity, involving many combinations of sacrificial and structural layers, as long as the last sacrificial layers binding any microstructure component are removed with the dry-etch process. Additionally, alternative combinations of material may be utilized for the dry etchant and sacrificial layer combinations, as long as the etchant selected has a low etch rate with respect to the microstructure material and the material utilized for the passivation and seal layers. Additionally, movable structures consisting of many layers of stacked sacrificial and structural materials are within the scope of this invention.