Abstract:
A method and system for a MEMS device is disclosed. The MEMS device includes a free layer, with a first portion and a second portion. The MEMS device also includes a underlying substrate, the free layer movably positioned relative to the underlying substrate. The first portion and second portion of the free layer are coupled through at least one stem. A sense material is disposed over portions of the second portion of the free layer. Stress in the sense material and second portion of the free layer does not cause substantial deflection of the first portion.

Description:
TECHNICAL FIELD 
     The present invention relates generally to microelectromechanical systems (MEMS) device and more particularly, to a MEMS device with a stress isolation structure. 
     DESCRIPTION OF RELATED ART 
     MEMS devices are formed using various semiconductor manufacturing processes. MEMS devices may have fixed and movable portions. MEMS force sensors have one or more sense material, which react to an external influence imparting a force onto the movable portions. The sense material can be the MEMS structural layer or a deposited layer. The MEMS force sensor may be configured to measure these movements induced by the external influence to determine the type and extent of the external influence. 
     Sometimes, large external acceleration or shock may impart undesirable movements of the movable portions. These undesirable movements may induce false measurements or introduce errors into the measurement capabilities of the MEMS device. It may be desirable to minimize the impact of extraneous forces or stress on operation of the MEMS device. 
     With these needs in mind, the current disclosure arises. This brief summary has been provided so that the nature of the disclosure may be understood quickly. A more complete understanding of the disclosure can be obtained by reference to the following detailed description of the various embodiments thereof in connection with the attached drawings. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a MEMS device is disclosed. The MEMS device includes a free layer, with a first portion and a second portion. The MEMS device also includes an underlying substrate, the free layer movably positioned relative to the underlying substrate. The first portion and second portion of the free layer are coupled through at least one stem. A sense material is disposed over portions of the second portion of the free layer. 
     In yet another embodiment, a method for providing a MEMS device is disclosed. MEMS device includes a free layer, with a first portion and a second portion. The MEMS device also includes a underlying substrate, the free layer movably positioned relative to the underlying substrate. The first portion and the second portion of the free layer are coupled through at least one stem. A sense material is disposed over portions of the second portion of the free layer. 
     This brief summary is provided so that the nature of the disclosure may be understood quickly. A more complete understanding of the disclosure can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of several embodiments are described with reference to the drawings. In the drawings, the same components have the same reference numerals. The illustrated embodiments are intended to illustrate but not limit the invention. The drawings include the following Figures: 
         FIG. 1  shows top view of a MEMS device, according to one aspect of the present disclosure; 
         FIG. 1A  shows a cross-sectional view of the MEMS device of  FIG. 1 , along an axis X-X′, according to one aspect of the present disclosure; 
         FIG. 1B  shows a cross-sectional view of the MEMS device of  FIG. 1 , along an axis X-X′, subjected to a stress on second portion of the free layer; 
         FIG. 2  shows an alternate top-view example of the MEMS device, according to one aspect of the present disclosure; 
         FIG. 2A  shows a cross-sectional view of the MEMS device of  FIG. 2 , along an axis Y-Y′, according to one aspect of the present disclosure; 
         FIG. 3  shows yet another alternate example of MEMS device, according to one aspect of the present disclosure; 
         FIG. 3A  shows a cross-sectional view of the MEMS device of  FIG. 3 , along an axis Y-Y′, according to one aspect of the present disclosure; 
         FIG. 4  shows an example top view of a MEMS device configured as a magnetic sensor, according to one aspect of the present disclosure; 
         FIG. 4A  shows another example top view of a MEMS device configured as a magnetic sensor, according to one aspect of the present disclosure; 
         FIG. 5  shows an example top view of a MEMS device configured as an acceleration sensor, according to one aspect of the present disclosure; 
         FIG. 5A  shows a cross-sectional view of the MEMS device of  FIG. 5 , according to one aspect of the present disclosure; and 
         FIG. 6  shows an example top view of a MEMS device configured as a resonant sensor, according to one aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     To facilitate an understanding of the adaptive aspects of the present disclosure, an exemplary MEMS device with an isolation structure is described. The specific construction and operation of the adaptive aspects of the isolation structure of the present disclosure are described with reference to the exemplary MEMS device. 
       FIG. 1  shows a MEMS device  100 , in accordance with an embodiment of this disclosure. The MEMS device  100  includes a free layer  102 , a underlying substrate  104  and an anchor  106  disposed over the underlying substrate  104 . A pair of springs  108   a  and  108   b  couple the free layer  102  to the anchor  106 , such that free layer  102  is movable relative to the underlying substrate  104 . 
     Free layer  102  includes a first portion  110  and a second portion  112 . A stem  116  couples the first portion  110  to the second portion  112 . In some examples, the stem  116  couples first portion  110  to the second portion  112 , along a first side  113 . For example, stem  116  couples the first portion  110  to the second portion  112 , along the first side  113 . The stem  116  acts as a stress-isolation structure by allowing second portion  112  to deform independent of the first portion  110 . Sometimes, stem  116  may be referred to as a stress-isolation structure. 
     One or more strips of sense materials  122  are disposed over the second portion  112 . Adjacent strips of sense materials are separated by a non-material portion  124 . In some examples, pairs of adjacent strips of sense materials are disposed over the paddle such that the non-material portion extends along a length of the stem that couples the second portion to the first portion. As an example, referring to stem  116 , adjacent strips of sense materials  122  and non-material portion  124 , we notice that the non-material portion  124  extends along a length of the stem  116 . For example, line X-X′ passes along the length of the stem  116  and along the non-material portion  124 . 
     The width Ws of the stem (in the Y direction) is typically less than ⅕ of the width Wp of the second portion in the same direction. The length Ls of the stem (in the X direction) is typically ½ to 2 times the width Ws of the stem. Typically, width Ws may be in the range of about 3 micrometers to about 10 micrometers. Typically, the length Ls may be in the range of about 2 micrometers to about 10 micrometers. 
     Now, referring to  FIG. 1A , MEMS device  100  will be further described.  FIG. 1A  shows a cross-sectional view of the MEMS device  100  of  FIG. 1 , along an axis X-X′.  FIG. 1A  shows that free layer  102  and underlying substrate  104  are separated by a gap. In some examples, sense materials react to an external force or influence and cause the free layer to move relative to the underlying substrate, thereby changing the gap G. Change in the gap is measured to determine the type and/or extent of the external influence. 
     In some examples, change in the gap is measured by constructing a sensor that is sensitive to the change in the gap. For example, a capacitor may be constructed, whose capacitance changes with change in the gap. As an example, a portion of the first portion  110  of the free layer  102  may be configured as a first electrode  130 . The underlying substrate  104  includes a third portion  132 . A second electrode  134  is disposed over a portion of the third portion  130 , such that the first electrode  130  and second electrode  134  define two electrodes of a capacitor  136 . The gap G between the electrodes form a dielectric layer (for example, with air or vacuum as a dielectric) for the capacitor  136 . When the free layer  102  moves relative to the underlying substrate  104 , the gap G between the first electrode  130  and second elector  134  changes, thereby changing the capacitance value of the capacitor  136 . This change in the capacitance value of the capacitor  136  may be measured to determine the type and/or extent of the external force or influence. 
     As one skilled in the art appreciates, in some sensor structures, the sense material  122  may have residual stress from a deposition process or from a temperature change after deposition. Compressive stress in the sense material  122  may be caused by the sense material  122  expanding faster than the substrate material, for example, material of free layer  102 , as the temperature raises. Tensile stress is caused by the sense material  122  expanding more slowly. If the sense material  122  is under compressive stress, it causes the free layer to bend downwards. Most of the bending of the free layer is in the vicinity of the sense material  122 , for example, in the second portion of the free layer. As the first portion and second portion of the free layer are coupled by a stem  116 , when the sense material  122  expands, the second portion of the free layer bends, but the first portion of the free layer substantially remains flat. Stem  116  acts as a stress isolation structure. For example,  FIG. 1B  shows a cross sectional view of the MEMS device  100 , along the axis X-X′, with bent second portion  112  of the free layer  102 . However, first portion  110  of the free layer  102  is substantially flat, thereby maintaining the gap G between the first electrode  130  and the second electrode  134  substantially constant. 
     As one skilled in the art appreciates, free layer may be a substrate, for example, a silicon substrate. Underlying substrate may be a silicon substrate. In some embodiments, one or more electronic circuits, for example, semiconductor circuits may be formed over the underlying substrate, by appropriate deposition techniques. 
     Now, referring to  FIG. 2 , another example embodiment of a MEMS device, for example, MEMS device  200  is disclosed. The MEMS device  200  may be similar to MEMS device  100 . However, in the MEMS device  200 , the second portion  112  of free layer  102  includes one or more paddles  114   a - 114   c . One or more stems couple the first portion  110  to the second portion  112 . In some examples, the stems couple first portion  110  to the second portion  112 , along a first side  113 . For example, stem  116   a  couples the first portion  110  to paddle  114   a , along the first side  113 . Similarly, stem  116   b  couples the first portion  110  to paddle  114   b , along the first side  113  and stem  116   c  couples the first portion  110  to paddle  114   c , along the first side  113 . Sense material  122  is disposed over the second portion  112 . For example, sense material  122  is disposed over the paddles  114   a - 114   c.    
     The width Ws of the stem (in the Y direction) is typically less than ⅕ of the width Wp of the corresponding paddle in the same direction. The length Ls of the stem (in the X direction) is typically ½ to 2 times the width Ws of the stem. Typically, width Ws may be in the range of about 3 micrometers to about 10 micrometers. Typically, the length Ls may be in the range of about 2 micrometers to about 10 micrometers. 
     In some examples, the MEMS device  200  includes a paddle connector  118 . One or more connector stems couple the paddle connector to the second portion  112 . In some examples, the connector stems couple the paddle connector to the second portion  112 , along a second side  115 , which is opposite to the first side  113 . For example, connector stem  120   a  couples the paddle connector  118  to paddle  114   a , along the second side  115 . Similarly, connector stem  120   b  couples the paddle connector  118  to paddle  114   b , along the second side and connector stem  120   c  couples the paddle connector  118  to paddle  114   c , along the second side  115 . The paddle connector  118  forces the paddles  114   a - 114   c  to move together, for example, if a force is applied along a direction shown by line Y-Y′. This prevents the paddles  114   a - 114   c  from colliding with each other, due to for example an external force. 
     In some examples, the connector stem that couples the paddle to the paddle connector and the stem that couples the paddle to the first portion are disposed substantially along an axis passing through the length of the stem. For example, stem  116   b  that couples the paddle  114   b  to first portion  110  and connector stem  120   b  that couples the paddle  114   b  to paddle connector  118  is disposed substantially along an axis represented by the line X-X′, which passes along the length of the stem  116   b.    
     Now, referring to  FIG. 2A , a cross-sectional view of the MEMS device  200 , along the axis Y-Y′ is shown.  FIG. 2A  shows that the paddles  114   a - 114   c  are bent, for example, due to compressive residual stress from the deposition process or from a temperature change after deposition of the sense material  122 . However, due to the isolation of the first portion and the second portion, the first portion of the free layer remains substantially flat, even though the second portion may bend. 
     Now, referring to  FIG. 3 , yet another example of a MEMS device, for example, MEMS device  300  is disclosed. MEMS device  300  is similar to MEMS device  200 . However, in this example, one or more strips of sense material  122  are disposed over the second portion  112 . For example, one or more strips of sense material  122  are disposed over the paddles  114   a - 114   c . Adjacent strips of sense materials are separated by a non-material portion  124 . In some examples, pairs of adjacent strips of sense material are disposed over the paddle such that the non-material portion extends along a length of the stem that couples the paddle to the first portion. As an example, referring to paddle  114   b , stem  116   b , adjacent strips of sense materials  122   a  and  122   b  and non-material portion  124   a , we notice that the non-material portion  124   a  extends along a length of the stem  116   b . For example, line X-X′ passes along the length of the stem  116   b  and along the non-material portion  124   a.    
     In this example, as previously described with reference to MEMS device  200 , MEMS device  300  includes a paddle connector  118  that is coupled to paddles  114   a - 114   c  through one or more connector stems. The connector stem that couples the paddle to the paddle connector and the stem that couples the paddle to the first portion are disposed substantially along an axis passing through the length of the stem. For example, stem  116   b  that couples the paddle  114   b  to first portion  110  and connector stem  120   b  that couples the paddle  114   b  to paddle connector  118  is disposed substantially along an axis represented by the line X-X′, which passes along the length of the stem  116   b . As one skilled in the art appreciates, the non-material portion  124   a  is also disposed along the same axis represented by line X-X′. 
     Now, referring to  FIG. 3A , a cross-sectional view of the MEMS device  300 , along the axis Y-Y′ is shown.  FIG. 3A  shows that the paddles  114   a - 114   c  are bent, for example, due to residual stress from the deposition process or from a temperature change after deposition of the sense material  122 . However, due to the isolation of the first portion and the second portion, the first portion of the free layer remains substantially flat, even though the second portion may bend. 
     Example Sensor Implementations: 
     A MEMS device described in this disclosure may be configured to perform as a sensor, based upon appropriate selection and configuration of the sense material that reacts to an external force or influence. In one example, the sense material may be configured as a permanent magnet and the MEMS device may be configured as a magnetic sensor, to sense an external magnetic field that selectively moves the free layer with reference to underlying substrate. An example MEMS device configured as a magnetic sensor is described with reference to  FIG. 4 . 
     Now, referring to  FIG. 4 , a MEMS device  400  configured as a magnetic sensor is described. The MEMS device  400  may be similar to MEMS device  300 . The sense material  122  disposed over the second portion  112  of free layer  102  of MEMS device  400  is configured as permanent magnets. For example, strips of sense materials  122  may be configured as permanent magnets oriented along an axis shown by arrow  402 . In one example, this axis corresponds to the X axis, as shown by line X-X′. With the permanent magnets oriented along the X axis, any change in an external magnetic field along the Z axis (which is orthogonal to the X-axis and shown as magnetic field Bz) will move the free layer with reference to the underlying substrate. This movement of the free layer  102  with reference to the underlying substrate  104  changes the gap between the first electrode and the second electrode of the MEMS device  400 , as previously described with reference to MEMS device  300 . As previously described, a change in the gap can be measured by measuring the change in the capacitance value of the sense capacitor. As one skilled in the art appreciates, the sense material  112  may be a metal or a metal alloy that may be magnetized as a permanent magnet. Some possible sense materials are samarium-cobalt (SmCo) or neodymium-iron-boron (NdFeB) alloys; or cobalt-iron (CoFe) or nickel-iron (NiFe) alloys with magnetic pinning layers situated above and below to create permanent magnets. 
     Now, referring to  FIG. 4A , another example MEMS device  420  configured as a magnetic sensor is described. The MEMS device  420  may be similar to MEMS device  400 . However, the sense material  122  disposed over the second portion  112  of free layer  102  of MEMS device  420  is oriented in a different direction. In some examples, pairs of adjacent strips of sense material are disposed over the paddle such that the non-material portion extends along a length of the stem that couples the paddle to the first portion. As an example, referring to paddle  114   b , stem  116   b , adjacent strips of sense materials  122   c  and  122   d  and non-material portion  124   b , we notice that the non-material portion  124   c  extends along a length of the stem  116   b . For example, line X-X′ passes along the length of the stem  116   b  and along the non-material portion  124   c.    
     The sense material  122  is configured as permanent magnets. For example, strips of sense materials  122  may be configured as permanent magnets oriented along an axis shown by arrow  422 . In one example, this axis corresponds to the Y axis, as shown by line Y-Y′. With the permanent magnets oriented along the Y axis, any change in an external magnetic field along the X axis (which is orthogonal to the Y-axis and shown as magnetic field Bx) will move the free layer with reference to the underlying substrate. However, this movement of the free layer with reference to the underlying substrate will be in-plane with reference to the underlying substrate. 
     A second sense capacitor  424  with a third electrode  426  and a fourth electrode  428  may be configured to measure this movement, by measuring a change in a gap G 2  between the third electrode  426  and the fourth electrode  428 . For example, the third electrode  426  may be formed on the free layer  104  and the fourth electrode  428  may be formed on the underlying substrate  104 . For example, the fourth electrode  428  may be formed over a second anchor  430  disposed over the underlying substrate  104 . And, the third electrode  426  is disposed over the free layer  102  such that any in-plane movement of the free layer  102  changes the gap G 2 . 
     For example, with the permanent magnets oriented along the Y axis, any change in an external magnetic field along the X axis (which is orthogonal to the Y-axis and shown as magnetic field Bx) will move the free layer with reference to the underlying substrate. This movement of the free layer  102  with reference to the underlying substrate  104  changes the gap G 2  between the third electrode and the fourth electrode of the MEMS device  420 . As previously described, a change in the gap can be measured by measuring the change in the capacitance value of the second sense capacitor. 
     As one skilled in the art appreciates, the sense material  112  may be a metal or a metal alloy that may be magnetized as a permanent magnet. Some possible materials are samarium-cobalt (SmCo) or neodymium-iron-boron (NdFeB) alloys; or cobalt-iron (CoFe) or nickel-iron (NiFe) alloys with magnetic pinning layers situated above and below to create permanent magnets. 
     Now, referring to  FIGS. 5 and 5A , a MEMS device may be configured as an acceleration sensor. An example MEMS device  500  configured as a Z axis acceleration sensor is described with reference to  FIGS. 5 and 5A . The construction of the MEMS device  500  is similar to the construction of the MEMS device  200 . The MEMS device  500  includes a free layer  102 , a underlying substrate  104  and an anchor  106  disposed over the underlying substrate  104 . A pair of springs  108   a  and  108   b  couple the free layer  102  to the anchor  106 , such that free layer  102  is movable relative to the underlying substrate  104 . A set of paddles  114   a - 114   c  are formed at a first end  502  of the free layer  102 . The set of paddles  114   a - 114   c  are coupled to the first end  502  of the free layer  102  by a plurality of stems  116   a - 116   c . Further, a set of sense materials  122   x  are disposed over the first set of paddles  114   a - 114   c.    
       FIG. 5A  shows a cross-sectional view of the MEMS device  500 , along a line X-X′ shown in  FIG. 5 . Now, referring to  FIG. 5A , a sense capacitor  506  is formed by a pair of electrodes  508   a  and  508   b . The sense material  122   x  add weight to the free layer  102  at the end of the free layer  102 . 
     When the MEMS device  500  is moved along a positive Z axis, as shown by arrow  514 , for example, due to an external force, the first end  502  of the free layer  102  tilts towards the underlying substrate  104 . This tilt causes a first gap G 1  between the first pair of electrodes  508   a  and  508   b  to reduce, thereby increasing the capacitance value of the first sense capacitor  506 . This change in the capacitance value of the first sense capacitor  506  may be measured to measure acceleration in the Z direction of the acceleration of the MEMS device  500 . As one skilled in the art appreciates, the first sense material  122   x  may be a metal or a metal alloy. Some possible materials are tungsten, gold, iridium, osmium, or any other high-density material. 
     Now, referring to  FIG. 6 , MEMS device  600  may be configured as a chemical sensor. Sometimes, the chemical sensor may be referred to as a resonant sensor. The MEMS device  600  may be similar to MEMS device  200 . For example, the sense material  122   p  disposed over paddles  114   a - 114   c  may be configured to absorb an external material, which would slightly increase the mass of the sense material  122   p . In a resonating sensor, this increase in mass of the sense material  122   p  may cause the free layer  102  to resonate at a lower frequency. The external material may be a fluid. In some examples, the external material may be a liquid. In some examples, the external material may be a gas. In some examples, based on the characteristics of the external material, the change in the gap between the first electrode and the second electrode may be different for different external materials, thereby giving different capacitance values. This difference in capacitance value, for example, may be used to determine the type of external material present. The sense material  122   p  may be a polymer that selectively absorbs a target chemical, such as water vapor. 
     In a different use of the sensing material, the sensing material may be an anti-sticking material, for example titanium nitride, silicon carbide, or octadecyltrichlorosilane (OTS). The anti-sticking material prevents sticking of the second portion to other portions of the device with which it may come in contact with. 
     While embodiments of the present invention are described above with respect to what is currently considered its preferred embodiments, it is to be understood that the invention is not limited to that described above. To the contrary, the invention is intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims.