# Patent

## Title
Through silicon via with reduced shunt capacitance

## Abstract

This document refers to apparatus and methods for a device layer of a microelectromechanical system (MEMS) sensor having vias with reduced shunt capacitance. In an example, a device layer can include a substrate having a pair of trenches separated in a horizontal direction by a portion of the substrate, wherein each trench of the pair of trenches includes first and second vertical layers including dielectric, the first and second vertical layers separated by a third vertical layer including polysilicon.

## Background
CLAIM OF PRIORITY
This application is a U.S. National Stage Filing under 35 U.S.C. 371 from International Patent Application Serial No. PCT/US2011/052417 filed on Sep. 20, 2011, and published on Mar. 29, 2012 as WO 2012/040245 A2 and republished on Jun. 7, 2012 as WO 2012/040245 A3, which claims the benefit of priority under 35 U.S.C. §119(e) to Bryzek, et al., U.S. Provisional Patent Application Ser. No. 61/384,319, entitled “TSV WITH REDUCED SHUNT CAPACITANCE”, filed Sep. 20, 2010, each of which are hereby incorporated by reference herein in its entirety.


BACKGROUND
Microelectromechanical system (MEMS) chips can include multiple layers including a via layer. The via layer can include dielectric trenches used to form electrodes for sensing characteristics of the MEMS device within the MEMS chip. Such trenches are referred to in U.S. Pat. No. 7,539,003. The dielectric trenches can have significant shunt capacitance that can degrade performance of the MEMS device, such as MEMS devices used for capacitive sensing or radio frequency (RF) applications.
OVERVIEW
In certain examples, a microelectromechanical system (MEMS) sensor can include vias with reduced shunt capacitance. In an example, a device layer can include a substrate having a pair of trenches separated in a horizontal direction by a portion of the substrate, wherein each trench of the pair of trenches includes first and second vertical layers including a dielectric, the first and second vertical layers separated by a third vertical layer including polysilicon.
This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application.



BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.
FIG. 1 illustrates generally a cross-section of an example MEMS device.
FIG. 2 illustrates generally a layout view of an example via layer.
FIG. 3A illustrates generally an equivalent circuit 300 of an example TSV structure of a via layer.
FIG. 3B illustrates generally an alternative layout equivalent circuit of an example TSV structure of a via layer.
FIG. 4 illustrates generally a comparison of impedance as a function of frequency of various trench structures and a sense capacitance.
FIG. 5 illustrates generally a comparison of sense capacitor impedance to shunt impedance for four trench configurations as a function of frequency.



DETAILED DESCRIPTION
The inventors have discovered structures and methods for forming electrodes in a via layer of a MEMS device that significantly reduces shunt capacitance associated with existing designs. In certain examples, a layer of an integrated MEMS device can include through silicon via (TSV) structures with multiple high-resistivity, poly filled layers interleaved with multiple dielectic layers. The TSV structures can couple the high resistivity poly in series with the dielectric, thus, reducing the capacitive shunting effect on a sense capacitor of the MEMS device. In certain examples, an operating frequency of the the electronics associated with the MEMS device can be selected such that the impedance of the sense capacitor is larger than the impedance of the shunt capacitance, thus, reducing the effect of the shunt capacitance. In an example, a dielectric trench including low-K dielectric or a combination of low-k dielectric and other materials, such as thermal oxides, doped oxides, or other dielectric materials can lower shunt capacitance, for example, where the low-k dielectric includes a dielectric constant that is lower than oxides used in the trench. In an example, the low-K dielectric can have a dielectric constant that is about half of the dielectric constant of oxide.
In certain examples, a trench can be partially filled with poly and under negative pressure, e.g., vacuum. The dielectric constant of the area of the trench under vacuum can be four times lower than oxide, thus reducing the shunt capacitance with respect to an oxide filled trench.
FIG. 1 illustrates generally a cross-section of an example MEMS device 100. In certain examples, the MENS device 100 can include three layers, a cap layer 101, a device layer 102, and a via layer 103. The device layer 102 can include moveable portions 104 supported by an anchor 105. In certain examples, the moveable portions 104 of the MEMS device 100 can be etched from a silicon wafer. The etched device layer 102 can include structures that allow sensing of in-plane and out-of plane movement of the moveable portions 104. In certain examples, the moveable portions 104 can be driven to oscillate at a particular frequency to allow sensing of angular acceleration of the MEMS device 100.
The cap layer 101 can provide at least a portion of an environmental enclosure of the moveable portions 104 of the device layer 102. In certain examples, the cap layer 101 can include a recess (not shown) to accommodate the movement of the moveable portions 104 of the device layer 102. In an example, the cap layer 101 can restrict the movement of the moveable portions 104 of the device layer 102, such as when the MEMS device 100 is mechanically shocked. In certain embodiments, the cap layer 101 is bonded to the device layer 102 such that a vacuum can be maintained about the moveable portions 104 of the device layer 102.
The via layer 103 can also provide a portion of an environmental enclosure of the moveable portions 104 of the device layer 102. In certain examples, the via layer 103 can include electrodes 106, 107, 108 for sensing or driving the moveable portions 104 of the device layer 102. In an example, one or more of the device layer electrodes, for example a drive electrode 107 can provide a drive signal to the device layer 102. In an example, one or more sense electrodes 106, 108 of the via layer 103 can provide sensing information indicative of movement of the moveable portions 104 of the device layer 102. In certain example, the sensing information can include changes in sense capacitance related to the movement of the moveable portions 104 of the device layer 102. In an example, trenches 109, 110 within the via layer 103 can provide a degree of electrical isolation of the sense electrodes.
FIG. 2 illustrates generally a layout view of an example via layer 203 including trench, or via, structures that can provide reduced shunt capacitance compared to existing structures. The layout of the via layer 203 includes areas for multiple electrodes. In an example, the layout of the via layer 203 can include a drive electrode 207 near the center and sense electrodes 206, 208 surrounding the drive electrode 207. In certain examples, the sense electrodes can be isolated from the drive electrode 207, as well as, an outer area of silicon of the via layer 203, by dielectric trenches, or via structures 209, 210. Referencing FIG. 1, but extendable to the example of FIG. 2, in an example, the dielectric trenches 109, 110 can include a first trench region 113 including a vertical layer of polysilicon 114 positioned between vertical layers of dielectric 115. In an example, a second trench region 117, separated from the first trench region by a vertical layer of monocrystalline silicon 116, can include a second vertical layer of polysilicon 118 positioned between layers of dielectric 115. With regard to FIG. 1, a drive signal can be applied to the moveable portions 104 of the device layer 102 through a section of the via layer 103, e.g., anchor electrode 107, electrically isolated from a sense electrode 106, 108 by a quadruple dielectric trench 110. In an example, the other side of the sense electrode 106, 108, opposite the drive electrode 107, can be isolated from a grounded part of the via layer 103 through another quadruple dielectric trench 109.
In an example, not shown, an isolating trench can surround the sensing electrode, and a separate trench can be formed around the anchor electrode for connecting drive voltage. As the drive voltage is not sensitive to shunt capacitance, a simple single trench can be used for isolating the anchor electrode.
A sensor of the MEMS device can include a sense capacitance coupled to the sense electrode 106, 108. Movement of the moveable portions 104 of the device layer 102 can change the sense capacitance. The sense electrode 106, 108, including low resistance monocrystalline in certain examples, can be used to measure the changes of the sense capacitance.
FIG. 3A illustrates generally an equivalent circuit 300 of an example TSV structure of a via layer. In certain examples, the equivalent circuit can include a drive electrode 307, a sense electrode 306, a sense capacitance 326, a first TSV network 310 and a second TSV network 309. The sense capacitance 326 can vary with movements of the device layer. In an example, the first TSV network 310 can represent an isolating trench between the drive electrode 307 and the sense electrode 306. The second TSV network 309 can represent an isolating trench between the sense electrode 306 and a perimeter portion of the via layer. Each network 309, 310 can include four capacitive elements 315 and three resistive elements 314, 316. The capacitive elements 315 can be associated with four vertical dielectric layers discussed above with respect to FIGS. 1 and 2. Two of the three resistive elements 314 can be associated with the two vertical layers of polysilicon. In an example, the third resistive element 316 can be associated with the vertical layer of low resistance monocrystalline silicon. The relatively high resistive vertical layers of polysilicon 314 coupled in series with the shunt capacitance 315 of the dielectric can reduce the shunting effect on the sensor improving the performance of the sense capacitance 326.
FIG. 3B illustrates generally an alternative layout equivalent circuit of an example TSV structure of a via layer. Because the drive voltage is not substantially sensitive to shunt capacitance, for example, due to a low source impedance, degradation of performance of the MEMS sensor can occur due to loading of the sense capacitor output. In examples employing the alternative layout, the equivalent circuit 330 shows the shunt capacitance of the TSV structures 331, 332 loads both sides of the sense capacitance 326.
Additional improvements to reduce shunt capacitance can be achieved by using different combinations and types of polysilicon and dielectric. For example, when a vertical dielectric layer includes two materials such as thermal oxide, with dielectric constant about 3.9 to ensure hermeticity, and low k dielectric, with dielectric constant about 2 to provide structural strength during fabrication, additional shunt capacitance reduction can be achieved. In certain examples, low-k dielectric materials can be porous, thus not capable of maintaining vacuum. Therefore, some low-k dielectrics are not suitable for 100% trench fill. A combination dielectric layer can maintain vacuum and provide benefits associated with a low-k dielectric.
In an example, shunt capacitance reduction, X, can depend on the amount of oxide (O %) and low K dielectric (D %) as a function of the trench length and each dielectric constant, K1 and K2 respectively. For example,

X=(K2*D+K1*O)/K1

If oxide represents 20% of the trench and low k dielectric 80% and K1=3.9, K2=2, then,

X=(2*0.8+3.9*0.2)/3.9=0.61,

representing 39% reduction of shunt capacitance in comparison to a trench filled with oxide only.

If the trench length is 4.0 mm, via layer 200 um, trench area is 0.8 mm2, resulting in oxide capacitance of about 27.6 pF for 1 um thick dielectric. Filling 80% of trench with low k dielectric material can reduce the capacitance to about 16.8 pF.
Partially filing the non-dielectric vertical layers of a trench can also reduce shunt capacitance. In certain examples, if part of the polysilicon fill is removed (etched) during fabrication of the TSV, the shunt capacitance can be further reduced. The reduction impact can be similar to that of using low-k dielectric. The reduction impact can be greater when the polysilican layer is several time thicker than the dielectric layer and the trench is exposed to vacuum which can have a dielectric constant of about 1, which is twice as low as some low-k dielectrics. For the previous example trench, if about 80% of the polythickness is removed, the shunt capacitance can drop from about 27.6 pf to about 6.1 pf.
FIG. 4 illustrates generally a comparison of impedance as a function of frequency of various trench structures and a 1 pf sense capacitance 405. The various trench structures include, single 401, double 402 and quadruple 403 trench structures with high resistivity poly fill, and a quadruple trench structure 404 employing a low-k dielectric and 80% polysilicon etch back.
Degradation of the signal to noise ratio, thus loss of available performance, can be proportional to the ratio of sense capacitor impedance to shunt impedance. FIG. 5 illustrates generally a comparison of sense capacitor impedance to shunt impedance for four trench configurations as a function of frequency. The four configurations include, single 501, double 502 and quadruple trench structures 503 with high resistivity poly fill, and a quadruple trench structure 504 employing a low-k dielectric and 80% polysilicon etch back. Selecting an operating frequency of electronics where the ratio is lower than 1 enables drastic reduction of shunting impact of TSV capacitance. High resistance poly silicon can provide a low signal to noise ratio at sufficiently low to avoid excessive power consumption. For example, the comparison shows that the single trench structure did not see the ratio of sense capacitor impedance to shunt impedance get below 1 over the illustrated frequency range. In contrast, the quadruple trench structure with low-k dielectric and 20% poly fill had the ratio of sense capacitor impedance to shunt impedance below 1 at DC and below 0.1 at about 639 kHz. In general, the lower the operating frequency of the MEMS device the lower the power consumption, which can be provide significant advantages when using the MEMS device in a mobile system with a limited energy supply source.
ADDITIONAL NOTES AND EXAMPLES
In Example 1, a method includes etching at least a pair of trenches in a first side of a silicon substrate, forming an oxide layer upon the first side of the silicon substrate, forming a polysilicon layer on the oxide layer, etching back the polysilicon layer within each trench of the pair of trenches to a predefined depth, and forming a dielectric layer on the sidewalls of the trenches, the dielectric layer including a portion of the oxide layer and a second dielectric material;
In Example 2, the method of Example 1 optionally includes forming a nitride layer atop the oxide layer.
In Example 3, the forming the nitride layer of any one or more of Examples 1-2 optionally includes removing a portion of the polysilicon layer to the upper surface of the oxide layer at the trench, wherein removing a portion of the polysilicon layer includes leaving a portion of the poly silicon layer within the trenches.
In Example 4, the forming a nitride layer atop the oxide layer of any one or more of Examples 1-3 optionally includes includes forming a resist layer atop the nitride layer.
In Example 5, the method of any one or more of Examples 1-4 optionally includes patterning the resist layer to define boundaries for a recess.
In Example 6, the method of any one or more of Examples 1-5 optionally includes removing a portion of the nitride to further define the boundary of the recess.
In Example 7, the method of any one or more of Examples 1-6 optionally includes etching the recess into the silicon substrate.
In Example 8, the etching the recess of any one or more of Examples 1-7 optionally includes removing a portion of the oxide layer from sidewalls of the trenches.
In Example 9, the forming the oxide layer of any one or more of Examples 1-8 optionally includes forming an oxide layer on sidewalls of the trenches.
In Example 10, the method of any one or more of Examples 1-9 optionally includes bonding a portion of the first side of the silicon substrate to a device layer of a MEMS sensor.
In Example 11, the method of any one or more of Examples 1-10 optionally includes grinding a second side of the silicon substrate to expose a first end of the trenches.
In Example 12, a via layer for a MEMS device can include a substrate having a pair of trenches separated in a horizontal direction by a portion of the substrate, wherein each trench of the pair of trenches includes first and second vertical layers including a first material, the first and second vertical layers separated by a third vertical layer including a second material, wherein the first material includes a dielectric.
In Example 13, the second material of any one or more of Examples 1-12 optionally includes polysilicon.
In Example 14, the third vertical layer of any one or more of Examples 1-13 optionally includes a volume including less than 80% polysilicon.
In Example 15, the third vertical layer of any one or more of Examples 1-14 optionally includes a volume including less than 20% polysilicon.
In Example 16, the first and second vertical layers of any one or more of Examples 1-16 optionally includes thermal oxide.
In Example 17, each of the first and second vertical layers of any one or more of Examples 1-16 optionally includes thermal oxide and a third material having a dielectric constant lower than the dielectric constant of the thermal oxide.
In Example 18, the first material of any one or more of Examples 1-17 optionally includes a thermal oxide and the second material of Example 17 optionally includes a dielectric having a dielectric constant lower than the dielectric constant of the thermal oxide.
In Example 19, a sensor can include a cap layer, a device layer, coupled to the cap layer, including a proof mass, and a via layer coupled to the device layer, wherein the device layer can include a silicon substrate having a pair of trenches separated in a horizontal direction by a portion of the silicon substrate, and wherein each trench of the pair of trenches includes first and second vertical layers including a dielectric separated by a third vertical layer including polysilicon.
In Example 20, the third vertical layer of any one or more of Examples 1-19 optionally includes a volume including less than 80% polysilicon.
In Example 21, the third vertical layer of any one or more of Examples 1-20 optionally includes a volume including less than 20% polysilicon.
In Example 22, the first and second vertical layers of any one or more of Examples 1-21 optionally includes thermal oxide.
In Example 23, each of the first and second vertical layers of any one or more of Examples 1-22 optionally includes thermal oxide and a second material having a dielectric constant lower than the dielectric constant of the thermal oxide.
The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.
In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.
The above description is intended to be illustrative, and not restrictive. In other examples, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

## Claims

1. A via layer for a MEMS device, the via layer comprising;
a substrate having a pair of trenches separated in a horizontal direction by a portion of the substrate, wherein each trench of the pair of trenches includes first and second vertical layers including a dielectric, the first and second vertical layers separated by a third vertical layer including polysilicon, wherein each of the first and second vertical layers include thermal oxide and a third material having a dielectric constant lower than the dielectric constant of the thermal oxide.

2. The via layer of claim 1, wherein less than about 80% of the volume of the third vertical layer includes polysilicon.

3. The via layer of claim 1, wherein less than about 20% of the volume of the third vertical layer includes polysilicon.

4. The via layer of claim 1, wherein the first and second vertical layers include thermal oxide.

5. A sensor comprising:
a cap layer;
a device layer, coupled to the cap layer, including a proof mass; and
a via layer coupled to the device layer, wherein the device layer includes:
a silicon substrate having a pair of trenches separated in a horizontal direction by a portion of the silicon substrate,
wherein each trench of the pair of trenches includes first and second vertical layers including a dielectric separated by a third vertical layer including polysilicon; and
wherein each of the first and second vertical layers include thermal oxide and a second material having a dielectric constant lower than the dielectric constant of the thermal oxide.

6. The sensor of claim 5, wherein less than about 80% of the volume of the third vertical layer includes polysilicon.

7. The sensor of claim 5, wherein less than about 20% of the volume of the third vertical layer includes polysilicon.

8. The sensor of claim 5, wherein the first and second vertical layers include thermal oxide.