Abstract
An micro electro mechanical sensor comprising: a substrate; and a sensor element movably mounted to a first side of said substrate; wherein a second side of said substrate has a pattern formed in relief thereon. The pattern formed in relief on the second side of the substrate provides a reduced surface area for contact with the die bond layer. The reduced surface area reduces the amount of stress that is transmitted from the die bond layer to the substrate (and hence reduces the amount of transmitted stress reaching the MEMS sensor element). Because the substrate relief pattern provides a certain amount of stress decoupling, the die bond layer does not need to decouple the stress to the same extent as in previous designs. Therefore a thinner die bond layer can be used, which in turn allows the whole package to be slightly thinner.
Claims
1. A micro electromechanical systems (MEMS) sensor that is an accelerometer or a gyroscope comprising: a substrate; and a sensor element movably mounted to a first side of said substrate; wherein a second side of said substrate has a pattern formed in relief thereon; and wherein the sensor element has a rotational symmetry and wherein the pattern has a rotational symmetry that substantially corresponds to the symmetry of the sensor element to minimize the effect of stress transfer on the sensor operation; and wherein the pattern comprises a set of radially oriented ribs.
2. A sensor as claimed in claim 1, wherein the pattern has a 4-fold or 8-fold symmetry.
3. A sensor as claimed in claim 1, wherein the ribs are arranged in a circle at equal angular intervals.
4. A sensor as claimed in claim 3, wherein the ribs comprise a first set of ribs and a second set of ribs, the ribs of the first set being longer than the ribs of the second set.
5. A sensor as claimed in claim 4, wherein the sensor has a rectangular shape and wherein the ribs of the first set extend closer to the diagonals of the rectangle than the ribs of the second set.
6. A sensor as claimed in claim 1, wherein the substrate is formed from glass and the sensor element is formed from silicon.
7. A sensor as claimed in claim 6, wherein the pattern is formed by powder blasting or wet etching.
8. A sensor as claimed in claim 1, wherein the substrate is formed from silicon and the sensor element is formed from silicon.
9. A packaged sensor comprising: a package; a die bond layer adhered to the package; and a sensor that is an accelerometer or a gyroscope and that includes: a substrate; and a sensor element movably mounted to a first side of said substrate; wherein a second side of said substrate has a pattern formed in relief thereon with the pattern of its second surface adhered to the die bond layer; and wherein the sensor element has a rotational symmetry; wherein the pattern has a rotational symmetry that substantially corresponds to the rotational symmetry of the sensor element to minimize the effect of stress transfer on the sensor operation; and wherein the pattern comprises a set of radially oriented ribs.
10. An micro electromechanical systems (MEMS) sensor system comprising: a package; a sensor that is an accelerometer or a gyroscope and that includes: a substrate; and a sensor element movably mounted to a first side of said substrate; wherein a second side of said substrate has a pattern formed in relief thereon; wherein the sensor element has a rotational symmetry; wherein the pattern minimizes effects of stress transfer on the sensor operation by having a rotational symmetry that substantially corresponds to the rotational symmetry of the sensor element; and wherein the pattern comprises a set of radially oriented ribs.
Description
BRIEF DESCRIPTION OF DRAWINGS
(1) One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:
(2) FIG. 1 shows an exploded cross-section view of a typical prior art construction;
(3) FIG. 2 shows an exploded cross-section view of a construction and mounting of a MEMS package according to certain examples of this disclosure;
(4) FIG. 3 shows one example of a pattern;
(5) FIG. 4 shows the pattern of FIG. 3 overlaid on some sensor structure;
(6) FIGS. 5a and 5b show a comparison of stress patterns for patterned and non-patterned substrates;
(7) FIG. 6 shows another example of a pattern; and
(8) FIG. 7 shows a further example of a pattern.
DETAILED DESCRIPTION
(9) FIG. 1 shows an exploded cross-section view of a typical prior art construction and mounting of a MEMS package with a thick die bond layer. The MEMS die 15 is formed in this example using Silicon on glass construction. A silicon layer 11 has the MEMS sensor formed therein, e.g. by etching. In this example, a gyroscope ring 12 is etched into the silicon layer 11. The silicon layer 11 is sandwiched between two glass substrate layers 13a, 13b. The silicon layer 11 is bonded to the glass layers 13a, 13b either by anodic bonding or frit bonding around the edges as indicated by arrows 14. Together the silicon layer 11 and glass layers 13a, 13b form the MEMS die 15.
(10) The MEMS die 15 is mounted within a package 16 via a thick die bond layer 17. A lid 19 is provided to seal the package in conventional manner. The die bond layer is typically around 250 μm thick and is of a compliant material so that it can adequately absorb the stresses that arise due to differing coefficients of thermal expansion in the MEMS die 15, the package 16 and the PCB 18 (or other alternative mounting surface if applicable). The bottom surface of the lower glass substrate 13b is in full contact with the die bond layer 17.
(11) FIG. 2 shows an exploded cross-section view of a construction and mounting of a MEMS package according to certain examples of this disclosure. The majority of the construction is the same as is shown in FIG. 1 and the same reference numbers are used. However the two key differences are that the bottom surface 20 of the lower substrate 13b is patterned with a relief pattern 21 and the die bond layer 17 is significantly thinner than the prior art construction of FIG. 1.
(12) The relief pattern 21 on the lower substrate 13b is of sufficient relief (i.e. the pattern extends proud from the rest of the surface 20 by a sufficient amount) that the die bond layer 17 does not contact the non-pattern part of the surface 20. Thus the stresses that arise between the PCB 18 and the package 16 and between the package 16 and the MEMS die 15 are transmitted only via the patterned part 21 of the surface 20.
(13) FIG. 3 shows one example of a pattern that may be used on the bottom surface 20 of substrate 13b. The shaded part of FIG. 3 shows the patterned part of surface 20, i.e. the part of surface 20 that projects out from the rest of the surface 20 and which is in contact with the die bond layer 17 in use.
(14) The pattern of FIG. 3 comprises a central circular projection 30 surrounded by sixteen radial ribs 31, each extending from a first inner radius to a second outer radius. In this particular example, all ribs 31 share a common first inner radius, but the ribs 31 can be divided into two sets, each of which has a different outer radius. The first set 31a are the longer ribs and extend to an outer radius that is radially further out than the outer radius of the second set 31b of shorter ribs. The rectangular (more specifically square in this example) outer perimeter of the substrate 13b is shown at 32. It can be appreciated that the pattern has four-fold rotational symmetry, matching the rotational symmetry of the substrate 13b. The pattern also has reflection symmetry about both the horizontal and vertical axes (as seen in FIG. 3) and around both diagonals of the square substrate 13b. In this example, the ribs 31a, 31b are arranged such that a long rib 31a is arranged either side of each diagonal (these being the longest radial lengths from the centre of the substrate 13b to the corners thereof) and a short rib 31b is arranged either side of each of the horizontal and vertical axes (these being the shortest radial lengths from the centre of the substrate 13b to the mid-points of each side thereof). The longer ribs 31a are thus closer to the diagonals than the shorter ribs 31b. This variation in rib length matches the stresses that arise due to the asymmetry of the substrate and thus reduces the effect of the asymmetry as residual stresses are coupled into the MEMS structure. It will be appreciated that the symmetries of the pattern correspond to symmetries of the sensor element (in the illustrated case this is a ring of a vibrational ring gyroscope, although this disclosure is not limited to such examples).
(15) It will be appreciated that in variations of the pattern shown in FIG. 3, the pattern could be arranged such that ribs 31 lie on the diagonals (and/or on the vertical and horizontal axes). The rotational and mirror symmetries would still be maintained. The number of long ribs 31a and short ribs 31b may be varied, e.g. with only one of each long rib, and/or only one of each short rib, and/or more than two of each long rib, and/or more than three of each short rib. There may also be more than two sets of ribs, i.e. more than three distinct lengths of rib. In this case, the longest ribs are still preferably arranged on or close to the diagonals with rib length decreasing towards the horizontal and vertical substrate axes.
(16) FIG. 4 shows the pattern of FIG. 3 overlaid on the structure of the MEMS device and transducers therefor. It can be seen that the ribs 31a, 31b are symmetrically arranged with respect to the MEMS device and transducers. Each of the sixteen ribs 31a, 31b overlays a transducer 40 of the MEMS device so that stresses are coupled in line with the transducers 40.
(17) FIG. 5a illustrates the asymmetrical stress pattern that arises in the MEMS die 15 in a typical prior art arrangement. The length and thickness of each arrow illustrate the amount of stress induced in the die 15. It can be seen that the largest stresses are in line with the longest radii of the substrate 13b, i.e. the diagonals, while the smallest stresses are towards the mid-points of each side (i.e. the shortest radii of the substrate 13b). It is this asymmetrical stress coupling that causes problems with the MEMS device, e.g. through deformation of the MEMS structure which causes asymmetrical changes in the alignment of the transducers, disruption of the alignment of the resonant modes and balancing of the mode frequencies, that can lead to bias, scale factor errors and performance degradation.
(18) FIG. 5b illustrates the symmetrical stress pattern that arises when the patterned substrate e.g. of FIG. 3 is used. It can be seen that the arrows now have the same thickness and length indicating that the stress has been coupled in more evenly with respect to the MEMS structure.
(19) FIG. 6 shows a perspective view of a substrate 13b with a pattern 21 on its bottom surface 20. The pattern is similar to that shown in FIG. 3, except that each of the ribs 31 is connected to the central hub 30. It can be seen that the hub 30 is larger in this example so is to avoid sharp etching angles between adjacent ribs 31.
(20) FIG. 7 shows an alternative pattern 21 that can be used on the bottom surface 20 of substrate 13b. The pattern 21 of FIG. 7 comprises two concentric annular rings, each of which stands proud of the rest of the surface 20. In this example two rings are shown; an outer ring 70 and an inner ring 71, although in other examples more than two rings may be used. The rings 70, 71 do not provide the same degree of control that is available with the ribs 31, but they still reduce the stress coupling by reducing the surface area of contact between the die bond layer 17 and the substrate 13b, thus reducing the amount of stress that is coupled into the MEMS die 15.
(21) In the case of all of the examples (FIGS. 3, 6 and 7), the pattern 21 provides support for the MEMS die 15 across substantially the whole width of the substrate 13b. This is achieved by providing contact points (i.e. parts of the relief pattern) at radially outer points of the substrate 13b (in particular more than a quarter of a side length from the centre of the substrate 13b, more preferably more than three eighths of a side length from the centre of the substrate 13b). Thus the MEMS die 15 is stably supported on the die bond layer 17, but still with reduced stress coupling therewith.
(22) By way of example and illustration, in a typical silicon on glass MEMS die structure, the MEMS die has a thickness of around 700 μm, a width of around 5000 μm and length of around 5000 μm. Package dimensions for this die are typically around 8 mm by 8 mm by 2.2 mm thick.
(23) The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application.
(24) The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof.
(25) While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.
