Abstract:
A method of forming a MEMS device by encapsulating a MEMS element with a sacrificial layer portion deposited over a substrate arrangement, the portion defining a cavity for the MEMS element, forming at least one strip of a further sacrificial material extending outwardly from the portion, forming a cover layer portion over the sacrificial layer portion, the cover layer portion terminating on the at least one strip, removing the sacrificial layer portion and the at least one strip, the removal of the at least one strip defining at least one vent channel extending laterally underneath the cover layer portion and sealing the at least one vent channel. A device including such a packaged micro electro-mechanical structure.

Description:
This application claims the priority under 35 U.S.C. §119 of European patent application no. 10195471.7, filed on Dec. 16, 2010, the contents of which are incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to a method of forming a MEMS device. 
     The present invention further related to a device comprising such a MEMS structure. 
     BACKGROUND OF THE INVENTION 
     The ongoing miniaturization of feature sizes in semiconductor manufacturing processes has facilitated the formation of microscopic structures, i.e. structures that have feature sizes in the micron and submicron, e.g. nanometer domain, on substrates such as silicon substrates. A prime example of such a microscopic structure is a microelectromechanical system (MEMS) structure. Such structures are sometimes also referred to as micromachines. 
     MEMS structures can be used for a wide range of applications in different fields of technology, e.g. electronics, medicine, pharmacy and chemistry. Applications in the filed of electronics for instance include accelerometers, gyroscopes, sensors, and so on. The MEMS structures may be made from any suitable material, e.g. silicon, polymer, metals amongst others. 
     Typically, the MEMS structure requires a certain degree of translational freedom in order to perform its function. To this end, the MEMS structure is packaged such that the structure is located in a sealed cavity. 
       FIG. 1  shows a process flow of a typical state of the art MEMS structure manufacturing process including a packaging step. In step (a), a silicon substrate  10  carrying an oxide layer  12  and a patterned silicon layer  14  including resonant structures  16  is provided. In step (b), the resonator trenches are resealed with oxide  18 , which is etched to provide contact regions to the underlying silicon layer  14 . In step (c), a thin (e.g. 2 micron) layer  20  of silicon is deposited over the substrate stack in which micron-scale vent channels (not shown) are etched to allow HF vapor access for release of the resonant structures  16  in step (d), in which the cavities  22  each including a resonant structure  16  are formed. In step (e), a silicon encapsulation layer  24  is deposited at 950° C. followed by a CMP planarization and etching of the encapsulation layer  24  to define the electrical contacts. In step (f), oxide  26  is deposited to seal the trenches and is etched to allow the electrical contact to the underlying encapsulation layer  24 , after which aluminum contacts  28  are formed in step (g) by deposition and etching. 
     This process flow has several drawbacks. For instance, the etching of the micrometer sized vent channels requires an additional processing step, thus adding to the cost of the overall manufacturing process. Also, to avoid contamination of the cavity containing the MEMS structure with capping material, the vent channels should not be located directly over the cavity, thus limiting the flexibility of the manufacturing process. Moreover, relatively thick capping layers are required to effectively seal the cavity, thus further adding to the cost of the manufacturing process. 
     In the paper “Wafer Level Encapsulation Technology for MEMS Devices using a HF-Permeable PECVD SIOC Capping Layer” by G. J. A. M. Verheijden et al. in MEMS 2008, IEEE 21 st  conference on Micro Electro Mechanical Systems 2008, pages 798-801, an alternative technology for the encapsulation of MEMS devices using a porous capping material is disclosed, which overcomes many of the problems associated with the process of  FIG. 1 . 
     The capping material consists of a low temperature PECVD layer of SiOC (trade name Black Diamond) and is shown to be permeable to HF vapor and H 2 O and therefore allows for the removal of a SiO 2  sacrificial layer and the formation of a cavity underneath the capping layer, with the capping material being permeable enough to allow for the evacuation of the SiO 2  reaction products. The cavity underneath the capping layer allows for high-Q operation of a MEMS resonator. A sealing layer can be deposited on the capping layer without significantly contaminating the cavity. However, it has been found that residual contamination of the cavity is difficult to avoid, especially when a sealing layer consisting of relatively small molecules is formed. In addition, there are no suitable process steps available to seal the porous layer at high temperature and low pressure, such that it is difficult to obtain a cavity with a high quality vacuum using this method. 
     In R. He et al., Journal of Micro-electromechanical Systems, Vol. 16, pages 462-472 (2007), a porous polysilicon cap layer is obtained over a MEMS structure by electrochemical etching of a polysilicon layer. Sealing of this layer is performed with the deposition of a further polysilicon layer or an LPCVD silicon oxide layer. The drawback of this approach is that the electrochemical etching of the polysilicon layer to obtain the porous polysilicon cannot be readily achieved in standard manufacturing tools, thus requiring an additional investment. In addition, the packaging method proposed in this paper requires the inclusion of a critical point drying step after the cavity formation for avoiding the MEMS structures sticking to the cavity walls. Furthermore, the required background pressures of the proposed sealing techniques are such that the preferable low pressure in the packaged cavity cannot be achieved. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a cost-effective method of packaging a MEMS structure, preferably in a low pressure cavity on a substrate. 
     The present invention further seeks to provide a device comprising such a packaged MEMS structure. 
     In accordance with a first aspect of the present invention, there is provided a forming a MEMS device, comprising encapsulating a MEMS element with a sacrificial layer portion deposited over a substrate arrangement, said portion defining a cavity for the MEMS element; forming at least one strip of a further sacrificial material extending outwardly from said portion; forming a cover layer portion over the sacrificial layer portion, said cover layer portion terminating on the at least one strip; removing the sacrificial layer portion and the at least one strip, the removal of the at least one strip defining at least one vent channel extending laterally underneath the cover layer portion; and sealing the at least one vent channel. 
     The present invention has been based on the insight that lateral vent channels can be formed that extend from the cavity to be formed in a lateral direction underneath the cover of the cavity, such that upon sealing of the vent channels, contamination of the cavity can be avoided. The channels are preferably kept very small, i.e. by providing one or more strips with a thickness in the range of 5-50 nm to prevent significant contamination of the cavity and to allow for the vent channels to be effectively sealed in the sealing step. 
     It is noted that the above embodiment of the present invention is particularly suitable for the formation of relatively small cavities. For larger cavities, if the sacrificial materials are to be removed by etching, e.g. a hydrogen fluoride (HF) etch, the etch duration required to fully remove the sacrificial layer portion can cause damage to other parts of the device. 
     For larger cavities, it is advantageous to convert the cover layer portion into a porous cover layer portion by chemical treatment of the cover layer portion, said chemical treatment simultaneously removing the sacrificial layer portion and the at least one strip such that the removal of the sacrificial layer portion can mainly take place through the porous cover layer portion, thereby reducing the time it takes to form the cavity, such that other components of the device are less likely to be damaged during the removal of the sacrificial layer portion. 
     It may appear that when a material is used that can be converted into a porous material by such chemical treatment the presence of the vent channels becomes superfluous. However, it has been surprisingly found by the present invention that this is not the case. This is because the material made porous by the chemical treatment can become very reactive, such that upon completion of the chemical treatment the porous material immediately seals again by reaction with ambient reactants such as water and air, which can cause residual gases to become trapped inside the cavity, which makes it difficult to set the pressure inside the cavity to the desired value. The presence of the vent channels ensures that such residual gases can still escape from the cavity upon completion of the chemical treatment step. 
     In a preferred embodiment, the cover layer portion comprises SIPOS (semi-insulating polycrystalline silicon), and wherein said chemical treatment step comprises treating the SIPOS with a hydrogen fluoride (HF) etch recipe, as SIPOS may be temporarily made porous by treatment with HF, which furthermore is a suitable etchant for removing sacrificial materials such as silicon oxide (SiO 2 ) and TEOS (tetraethyl-orthosilicate). 
     In an embodiment, the method further comprises forming a patterned support layer over the sacrificial layer portion, said patterned support layer defining etching holes on top of the sacrificial layer portion, wherein the step of forming the cover layer portion comprises forming the cover layer portion over the sacrificial layer portion including the patterned support layer, and wherein the cover layer portion is separated from the at least one strip by a portion of the patterned support layer. This has the advantage of improving the structural integrity of the cavity as well as defining etch holes on top of the sacrificial layer portion and defining the length of the vent channels to be formed. The support layer preferably is a polysilicon layer. 
     In an embodiment, the sealing step comprises depositing a capping layer over the resultant structure, which may be performed at a vacuum pressure, e.g. in the range of 10 −4 -10 −6  Torr to ensure that the pressure inside the cavity is set to the desired (near-)vacuum pressure. 
     In accordance with another aspect of the present invention, there is provided a device comprising a substrate covered by a cover layer portion, a cavity in between the substrate and the cover layer portion, said cavity including a micro electro-mechanical structure; at least one vent channel underneath the cover layer portion, said at least one vent channel extending laterally from said cavity; and a sealing layer covering the cover layer portion and sealing the at least one vent channel. Such a device is the resulting product of the method of the present invention and therefore benefits from a MEMS device in a cavity that is substantially free from contamination and which has a well-defined internal pressure. 
     In a preferred embodiment, the cover layer portion comprises hydrogen fluoride-treated SIPOS. The HF-treated SIPOS enables the formation of a device having a relatively large cavity as previously explained as during the HF treatment the SIPOS becomes porous and facilitates the removal of the sacrificial material defining the cavity without causing damage to other parts of the device. 
     In a preferred embodiment, the device comprises a patterned support structure covered by the cover layer portion to improve the structural integrity of the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are described in more detail and by way of non-limiting examples with reference to the accompanying drawings, wherein: 
         FIGS. 1   a - g  depict a known method of manufacturing a MEMS device; and 
         FIGS. 2   a - q  depict a method of manufacturing a MEMS device in accordance with an exemplary embodiment of the present invention. 
     
    
    
     It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A first embodiment of a packaging method of the present invention is described in more detail with the aid of  FIG. 2 . In  FIG. 2   a , a substrate  100 , which may be any suitable substrate, e.g. a silicon substrate, a silicon-on-insulator (SOI) substrate, a SiGe substrate and so on, is provided with a first layer  110  of a sacrificial material, onto which a layer of a suitable material  120 , e.g. silicon, is deposited and subsequently patterned, e.g. by means of wet chemical etching or reactive ion etching (RIE) to form the outline of MEMS structure  122 , such as a MEMS resonator or another suitable MEMS device as shown in  FIG. 2   b . The patterning step should be selective to the underlying sacrificial layer  110  if present. Such processing steps are well-known to the skilled person and will not be explained in further detail for reasons of brevity only. Any suitable sacrificial material may be used. Non-limiting examples of suitable sacrificial materials include TEOS (tetraethyl-orthosilicate) and SiO 2 , e.g. a LPCVD (low-pressure chemical vapor deposition) silicon dioxide. In an alternative embodiment, the sacrificial material layer  110  may be omitted in case the MEMS device  122  is to be anchored on the substrate  100 . After patterning of layer  120  to form the MEMS device  122 , ion implantations into the layer  120  may be performed if necessary. 
     In  FIG. 2   c , a layer  130  of a further sacrificial material is deposited over the patterned material layer  120  such that the MEMS device  122  is fully covered by the sacrificial material in the cross-section of the device shown in  FIG. 2 . In a preferred embodiment, the sacrificial layer  130  is of a material that can be removed in the same process step as the removal of the material of the optional sacrificial layer  110 . For instance, the material of the sacrificial layer  130  may be the same as the material of the sacrificial layer  110 . 
     In  FIG. 2   d , the further sacrificial layer  130  is patterned in any suitable manner, e.g. by photolithography and subsequent etching, such as wet chemical etching or RIE, thereby forming a portion of the sacrificial layer  130  that covers the MEMS device  122  and defines at least part of the cavity to be formed around the MEMS device  122 . The remainder of the cavity to be formed may be defined by a portion of the sacrificial layer  110 . The patterning of the further sacrificial layer  130  preferably is performed by an etching step that is selective to the underlying layer  120 , e.g. a silicon layer. 
     The method proceeds as shown in  FIG. 2   e  with the growth or deposition of a second further sacrificial layer  132 , e.g. a TEOS or SiO 2  layer over the resultant structure. The second further sacrificial layer  132  is preferably much thinner than the further sacrificial layer  130 , e.g. in the range of 5-50 nm compared to a typical thickness of the further sacrificial layer  130  in the micrometer domain. The further sacrificial layer  130  preferably has a thickness ranging from a few hundred nanometer to several micrometer as it has been found that a further sacrificial layer  130  having a thickness in this range is sufficiently thick to prevent sticking of the cap layer  135  to the MEMS device  122  during its release, whilst being thin enough to avoid complications in the further processing of the MEMS device  122 . 
     The second further sacrificial layer  132  is subsequently patterned as shown in  FIG. 2   f  in any suitable manner, e.g. using photolithography followed by an etching step such as a wet chemical etching step or a RIE step to form at least one strip of the second further sacrificial layer  132  that lies against the portion of the further sacrificial layer  130  covering the MEMS device  122 , with one or more strips of the second further sacrificial layer  132  extending laterally from this portion. A top view of this structure is shown in  FIG. 2   g . The strips  132  will act as vent channels for the degassing of the cavity of the MEMS device  122  as will be explained in more detail later. The number of strips  132  may be determined by the desired degassing rate, with higher rates requiring more strips  132 . However, it should be understood that in principle a single strip  132  may suffice, in particular when the volume of the cavity to be formed is relatively small. 
     The method may proceed as shown in  FIG. 2   h  with the deposition and subsequent patterning of a support layer  135 , which is intended to strengthen the lid of the cavity to be formed against collapsing. Any suitable material may be used for the support layer  135 , such as polycrystalline silicon (poly-Si). The patterning of the support layer  135  may be achieved in any suitable manner, e.g. by etching, which etching step should be selective to the underlying further sacrificial layer  130  and the second further sacrificial layer  132 . The patterning of the support layer  135  may optionally include the formation of etch holes on top of the portion of the underlying further sacrificial layer  130  covering the MEMS device  122 . This optional embodiment is for instance useful if a cover layer to be formed is to be made porous to assist the degassing of the cavity to be formed, as will be explained in more detail later. A top view of the resultant structure after the patterning of the support layer  135  is shown in  FIG. 2   i , which includes the etch holes exposing the underlying portion of the further sacrificial layer  130 . 
     It is noted that in an alternative embodiment, the formation of the support layer  135  is omitted from the method of the present invention, such that the method proceeds from the step shown in  FIG. 2   f  directly to the step shown in  FIG. 2   j . The presence of the support layer  135  is however preferred as it strengthens the cavity of the MEMS device  122  as previously explained. 
     The method proceeds as shown in  FIG. 2   j , in which a cover layer  140  is formed over the resultant structure. The cover layer  140  is subsequently patterned as shown in  FIG. 2   k , such that the cover layer  140  covers the portion of the further sacrificial layer  130  that covers the MEMS device  122  and the patterned support layer  135  if present. In addition, the patterning of the cover layer  140  ensures that the end portions of the strips  132  are exposed, i.e. that the cover layer  140  terminates on the strips  132 . In case of the presence of the patterned support layer  135 , a portion of the patterned cover layer  140  over a strip  132  may be separated from the strip  132  by a portion of the patterned support layer  135 . A top view of the resulting structure is shown in  FIG. 2   l.    
     The cover layer  140  may be a permanently non-porous layer, in which case the subsequent removal of the sacrificial materials, e.g. materials  110 ,  130  and  132  is to be achieved through vent channels to be formed by the removal of the strips  132 . This may be a feasible embodiment if the cavity to be formed is small enough such that the complete removal of the respective sacrificial materials may be achieved quickly enough such that other parts of the device are not damaged by the chemicals used for the removal of these sacrificial materials. 
     In a preferred embodiment, the cover layer  140  comprises a material that can be made porous by chemical treatment such that the removal of the sacrificial materials for defining the MEMS cavity and the vent channels can also take place through the pores of the material of the cover layer  140 . A preferred embodiment of such a material is SIPOS, which is an oxide-rich poly-Si, which may be formed by the co-deposition of SiO 2  and silicon or by depositing poly-Si in a (slightly) oxidizing atmosphere. The SIPOS layer  140  may be patterned in any suitable manner, e.g. by means of a wet chemical etch or by means of a RIE step. 
       FIG. 2   m  shows an alternative embodiment of  FIG. 2   k , in which the cover layer  140  is patterned simultaneously with the support layer  135 , i.e. both layers are patterned by a single patterning step such as a wet chemical etch or a RIE step. This leads to a subsequently vertical structure on the one or more strips  132 , said vertical structure formed by a portion of the patterned support layer  135  and a portion of the cover layer  140 . In this embodiment, if the support layer  135  should further comprise etch windows on top of the portion of the further sacrificial layer  130  that covers the MEMS device  122 , these etch windows should be formed prior to the deposition of the cover layer  140 . 
     The method subsequently proceeds from the resultant structure shown in  FIG. 2   k  or from the resultant structure shown in  FIG. 2   m  as shown in  FIG. 2   n , in which the MEMS device  122  is released from the substrate  100  by the simultaneous removal of the sacrificial materials, e.g. a portion of the sacrificial layer  110  if present, the portion of the further sacrificial layer  130  and the one or more strips formed by the patterning of the second further sacrificial layer  132 , thereby forming a cavity  150 . As previously mentioned, in a preferred embodiment, the cover layer  140  is a SIPOS layer, in which case the removal of the sacrificial materials is performed by HF etching, which simultaneously removes an SiO x  component (0&lt;x≦2) from the SIPOS cover layer  140 , thereby rendering the SIPOS cover layer  140  porous, such that the removal of the sacrificial materials is mainly achieved through the porous SIPOS cover layer  140 . The exposure to the etch recipe, e.g. the HF etch recipe should be long enough to ensure that the strips  132  are fully removed, thereby forming the vent channels  134  laterally extending from the cavity  150  to the outside world. The vent channels  134  are formed between the material layer  120  and a portion of cover layer  140  or a portion of the support layer  135  if present. The height of the vent channels  134 , i.e. the distance between the material layer  120  and the opposite portion of cover layer  140  or the portion of the support layer  135  if present preferably lies in the range of 5-50 nm. Below 5 nm degassing through the vent channels  134  becomes cumbersome, and above 50 nm the subsequent sealing of the vent channels  134  becomes cumbersome. 
     Following the termination of the etch step, the porous SIPOS cover layer  140  will rapidly react with ambient reactants, e.g. water or air, which will effectively seal the SIPOS layer  140 . The method next proceeds to step  2   o , in which a sealing or capping layer  160  is deposited over the resultant structure to seal the cover layer  140  if necessary but more importantly to seal the vent channels  134 . The sealing layer  160  preferably is an electrically insulating layer. Suitable materials for the sealing layer  160  include SiO x  (0&lt;x≦2) and Al 2 O 3 . The sealing layer  160  may be deposited in any suitable manner, e.g. by physical vapor deposition. In a preferred embodiment, the deposition is performed at a near-vacuum pressure, e.g. a pressure in the range of 10 −4 -10 −6  Torr (mm Hg). This ensures that the pressure in the cavity  150  is set to the operating pressure of the deposition process, as any overpressure in the cavity  150  can escape through the one or more vent channels  134  prior to the effective sealing of these channels. Upon sealing of the vent channels  134 , the cavity  150  has become hermetically sealed. 
     The method may further include post-packaging processing steps, such as the formation, e.g. etching, of a contact region for the MEMS structure  122  through the sealing layer  160  to the underlying material layer  120 , as shown in  FIG. 2   p , after which a conductive contact  180  may be formed in the contact region as shown in  FIG. 2   q . The conductive contact  180  may be a metal contact, e.g. an Al contact, and may be formed in any suitable manner, e.g. by depositing a metal layer over the resultant structure of  FIG. 2   p  and patterning this layer to form the metal contact. Alternatively, a selective deposition process may be used. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.