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EP-3479475-B1 - MICROMECHANICAL RESONATOR AND METHOD FOR TRIMMING MICROMECHANICAL RESONATOR

EP3479475B1EP 3479475 B1EP3479475 B1EP 3479475B1EP-3479475-B1

Inventors

  • JAAKKOLA, ANTTI
  • PENSALA, TUOMAS
  • OJA, AARNE
  • PEKKO, PANU
  • DEKKER, James R

Dates

Publication Date
20260506
Application Date
20170629

Claims (13)

  1. A micromechanical resonator comprising a resonator element having a length, a width, and a thickness, the length and the width defining a plane of the resonator element, wherein the resonator element is a compound comprising a base layer structure (10, 30, 40, 50. 70) comprising a first layer made of a semiconductor material, and a second layer (31) on top of the base layer structure (10, 30, 40, 50. 70), characterized in that the linear temperature coefficients of frequency, TCF, of the first layer and the second layer (31) have opposite signs, and the second layer (31) comprises at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and at least one TCF-adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) in the plane of the resonator element, the at least one frequency-adjusting region and the at least one TCF-adjusting region of the second layer (31) having different thicknesses, wherein the thickness of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) is configured to be adjusted in response to a deviation of a resonator frequency with respect to a target value, and a difference between the thicknesses of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and the at least one TCF-adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) is configured to be adjusted in response to a deviation of a linear temperature coefficient of the resonator frequency with respect to a target value.
  2. A resonator according to claim 1, wherein the second layer (31) is made of molybdenum or wherein the second layer (31) is in the form of polysilicon deposited on top of the first layer.
  3. A micromechanical resonator comprising a resonator element having a length, a width, and a thickness, the length and the width defining a plane of the resonator element, wherein the resonator element is a monolithic silicon structure having a first layer and a second layer (31) that are in the form of layers with different doping levels within the monolithic silicon structure, characterized in that the linear temperature coefficients of frequency, TCF, of the first layer and the second layer (31) have opposite signs, the second layer (31) comprises at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and at least one TCF adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) in the plane of the resonator element, the at least one frequency-adjusting region and the at least one TCF-adjusting region of the second layer (31) having different thicknesses, wherein the thicknesses of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and the at least one TCF-adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) are configured to be adjusted in response to a deviation of a resonator frequency with respect to a target value and in response to a deviation of a linear temperature coefficient of the resonator frequency with respect to a target value.
  4. A resonator according to any one of preceding claims, wherein the at least one frequency-adjusting region comprise a frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) in a peripheral region (53, 63b, 63c, 73, 103, 113) of the resonator element in the plane of the resonator.
  5. A resonator according to claim 4, wherein a surface area of the frequency-adjusting region is 20 % or less of the total surface area of a cross section of the resonator in the plane of the resonator.
  6. A wafer comprising a plurality of micromechanical resonators, wherein the resonators comprise at least one micromechanical resonator according to any one of claims 1 to 5, and wherein at least one thickness of the frequency-adjusting regions in at least one resonator element of the resonators differs from a thickness of a corresponding frequency-adjusting region in another resonator so that the thicknesses of the regions are such that they produce essentially the same frequency and/or essentially the same linear temperature coefficients.
  7. A wafer according to claim 6, wherein the adjusted thicknesses of the frequency-adjusting regions (20, 54, 64b, 64c, 74, 104, 114) are adapted to set frequencies of the resonators are within +/- 100 ppm range from each other and/or the linear TCFs of the resonators are within 0,1 ppm/°C from each other.
  8. A method for trimming a micromechanical resonator comprising a resonator element having a length, a width, and a thickness, the length and the width defining a plane of the resonator, the resonator element being a compound comprising a base layer structure (10, 30, 40, 50. 70) comprising a first layer made of a semiconductor material and a second layer (11, 21, 31, 41,.51, 71) on top of the base layer structure (10, 30, 40, 50. 70), wherein the linear temperature coefficients of frequency, TCF, of the first layer and the second layer (11, 21, 31, 41,.51, 71) have opposite signs, characterized in that the method comprises forming at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and at least one TCF-adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) in the second layer (11, 21, 31, 41,.51, 71) in the plane of the resonator by changing a thickness of at least one portion of the second layer (11, 21, 31, 41,.51, 71), wherein the at least one frequency-adjusting region and the at least one TCF-adjusting region have different thicknesses, adjusting the thickness of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) in response to a deviation of a resonator frequency with respect to a target value, and adjusting a difference between the thicknesses of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and the at least one TCF-adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) in response to a deviation of a linear temperature coefficient of the resonator frequency with respect to a target value.
  9. A method according to claim 8, wherein a difference between the thicknesses of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and the at least one TCF-adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) are adjusted in response to a deviation of a linear temperature coefficient of the resonator element from a desired value.
  10. A method according to claim 9, wherein shapes and positions of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and the at least one TCF-adjusting region are selected such that an effect of the difference between the thicknesses of the at least one frequency-adjusting region and the at least one TCF-adjusting region on the resonator frequency is minimized.
  11. A method according to any one of claims 8 to 10, wherein the forming of the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) comprises forming a frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) in a peripheral region (53, 63b, 63c, 73, 103, 113) of the resonator element in the plane of the resonator, wherein a surface area of the frequency-adjusting region is selected such that said surface area is sufficiently large for reaching any deviation of the resonator frequency within a set deviation range by adjusting a thickness of the frequency-adjusting region.
  12. A method according to claim 9, wherein the method comprises removing matter from the second layer (11, 21, 31, 41,.51, 71) in order to form the at least one frequency-adjusting region (20, 54, 64b, 64c, 74, 104, 114) and at least one TCF-adjusting region (52, 53, 62c, 63c, 72, 73, 102, 103, 112, 113) of the resonator element.
  13. A method according to claim 12, wherein the removing of matter is performed by using laser ablation or ion beam trimming.

Description

FIELD OF THE INVENTION The present invention relates to micromechanical resonators and oscillators and more particularly to controlling the frequency and temperature coefficient of the frequency of the resonators and oscillators. BACKGROUND INFORMATION Because of manufacturing non-idealities (e.g. variations in thicknesses of material layers and non-uniformity of patterning and etching processes), frequency and temperature coefficient of frequency (TCF) of a resonator may deviate from their target values. For high accuracy timing applications, these properties may have to be corrected in some manner, with accuracy depending on the application. However, it may be difficult to reach these goals economically. Quartz resonators may be individually trimmed to the correct frequency by measuring the frequency, and by applying etching and/or by trimming of parts of gold electrode by a laser while monitoring the frequency. In silicon-based micromechanical resonators, the frequency and temperature behavior of the resonator may be recorded in memory, and a frequency synthesizer on an ASIC may be used for producing the desired oscillation frequency assuming that the non-accurate resonator frequency stays the same. However, a drawback of this approach is increased power consumption and degraded phase noise performance. US2012/248932 discloses a micro-machined vibrating element in the shape of a beam and having two patches arranged near the anchor areas. The resonance frequency of the vibrating element is increased in relation to a same vibrating element not having the patches. Mohsen Shahmohammad et al: "Turnover Temperature Point in Extensional-Mode Highly Doped Silicon Microresonators", IEEE TRANSACTIONS ON ELECTRON DEVICES, IEEE SERVICE CENTER, PISACATAWAY, NJ, US, vol. 60, no. 3, 1 March 2013, pages 1213-1220, discloses a thin-film piezoelectric-on-substrate (TPoS) resonator consisting of a thin piezo-electric film (e.g., AIN) sandwiched between two metallic electrodes (e.g., Mo) stacked on top of the device layer of a silicon-on-insulator (SOI) substrate. Also US2011/273061 discloses a piezoelectric resonator having an AIN layer sandwiched between two metallic electrodes stacked on top of the SOI substrate. The thickness of one or two layers positioned in the stack of layers in the resonator may be changed in order to tune the resonator. US2012/286903 discloses a micro-mechanical device, such as a resonator, comprising a semiconductor element comprising at least two regions having different material properties. The relative volumes, oping concentrations, doping agents and/or crystal orientations of the regions are configured so that the temperature sensitivities of the stiffness are opposite in sign and the overall temperature drift of the stiffness of the semiconductor element is 50 ppm or less on a temperature range of 100° C. US2005/195049 discloses a method for trimming the resonance frequency of a packaged micro-mechanical resonator by directing electromagnetic energy to the resonator through a transparent portion of the resonator package. The energy removes mass at the point(s) of contact on the resonator thereby affecting its resonance frequency. BRIEF DISCLOSURE An object of the present invention is to provide a micromechanical resonator element and a method for trimming a resonator so as to alleviate the above disadvantages. The objects of the invention are achieved by a resonator and a method which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims. The frequency and TCF of a resonator can be adjusted independently from each other by removing material from the top of the resonator in a patterned way, i.e. so that material is removed from different places on top of the resonator. The TCF may be adjusted by removing material from a central, optimally-sized region(s) at the resonator center. By optimizing the sizing, the TCF can be altered without affecting the resonator frequency. On the other hand, the resonator frequency may be adjusted by removing material in a peripheral region in the vicinity of the resonator edges, in which case the TCF may not be affected at all. Patterned material removal can be based on laser ablation or masked ion beam etching, for example. The resonator element according to the present disclosure allows the resonance frequency and TCF to be adjusted independently from each other, e.g., with two successive trimming steps. This can significantly simplify the adjustment process. Further, the resonator element according to the present disclosure enables micromechanical resonators to be fabricated as fully passive components not requiring active PLL-based frequency synthesis. As a result, micromechanical resonators may be formed pin-to-pin compatible to quartz crystals. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail by means of preferred em