DE102006054348B4 - Layer thickness sensor and method for monitoring deposition processes - Google Patents

Layer thickness sensor and method for monitoring deposition processes

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Publication number
DE102006054348B4
DE102006054348B4 DE200610054348 DE102006054348A DE102006054348B4 DE 102006054348 B4 DE102006054348 B4 DE 102006054348B4 DE 200610054348 DE200610054348 DE 200610054348 DE 102006054348 A DE102006054348 A DE 102006054348A DE 102006054348 B4 DE102006054348 B4 DE 102006054348B4
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thickness sensor
resonator
film
layer
acoustic
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DE102006054348A1 (en
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Marc Loschonsky
Armin Dadgar
Leonhard Reindl
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Albert-Ludwigs-Universitaet Freiburg
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Albert-Ludwigs-Universitaet Freiburg
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    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/02Details
    • H03H9/02007Details of bulk acoustic wave devices
    • H03H9/02086Means for compensation or elimination of undesirable effects
    • H03H9/02102Means for compensation or elimination of undesirable effects of temperature influence
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01BMEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B7/00Measuring arrangements characterised by the use of electric or magnetic means
    • G01B7/02Measuring arrangements characterised by the use of electric or magnetic means for measuring length, width or thickness
    • G01B7/06Measuring arrangements characterised by the use of electric or magnetic means for measuring length, width or thickness for measuring thickness
    • G01B7/063Measuring arrangements characterised by the use of electric or magnetic means for measuring length, width or thickness for measuring thickness using piezo-electric resonators
    • G01B7/066Measuring arrangements characterised by the use of electric or magnetic means for measuring length, width or thickness for measuring thickness using piezo-electric resonators for measuring thickness of coating

Abstract

A layer thickness sensor for monitoring deposition processes, wherein the layer thickness sensor comprises an acoustic resonator with a sensitive region, which is arranged so that it is coated with the deposited layer during the deposition process, so that a resonant frequency of the layer thickness sensor depending on the thickness of the deposited layer changed, wherein the acoustic resonator is formed by a piezoelectric thin film resonator, characterized in that the acoustic resonator is formed so that at least two vibration modes with different frequencies and different temperature response for computational compensation of the temperature response of the film thickness sensor can be excited.

Description

  • The present invention relates to a film thickness sensor for monitoring deposition processes, and to a method for monitoring film deposition processes in microsystem technology and nanotechnology.
  • There are two main production processes for the generation of structures in both microsystem technology and nanotechnology, namely the "top-down" and the "bottom-up" principle. In a "top-down" technology, one structures the pre-deposited, different thickness functional layers according to the component requirements, in the so-called "bottom-up" technology, the functional layers are deposited on pre-structured geometries. Layer deposition plays an important role for both production principles as it determines the properties of the component layers and thus of the finished component. Due to the constant miniaturization, the layer thicknesses used are decreasing more and more. In order to obtain at least a constant relative layer thickness measurement accuracy, the absolute measurement resolution must increase inversely proportional to the layer thickness in the layer thickness measurement system.
  • The requirements for the measurement resolution of very small amounts of substance have therefore grown enormously in recent years for process technology issues. Although exist for the coating thickness measurement sophisticated measuring methods and equipment, such. As the scanning ion microscopy FIB (Focussed Ion Beam) with subsequent analysis in the scanning electron microscope SEM, the transmission electron microscope (TEM) cross preparation or, for the same materials, the atomic force microscope AFM (Atomic Force Microscope) in specially equipped laboratories. However, these methods are rarely suitable for controlling the deposition processes, since they can usually not be used during the deposition process.
  • In process technology, a quartz microbalance (BAW, or Quartz Micro Balance, QMB) has hitherto been predominantly used for process control, in which, as is known from "Use of Quartz Crystals for the Weighting of Thin Layers and for Microwaving", G. Sauerbrey, Zeitschrift für Physik 155, 206-222 (1959), it is known that an accumulated amount of substance Δm is converted into a frequency shift Δf according to the Sauerbrey context: Δf / f = - Δm / m (1)
  • The mass resolution Δm of the quartz microbalances results from the extremely high quality of the quartz, which allows a detectable frequency shift Δf of about 25 Hz. In the fundamental mode, the quartz thickness equals half the acoustic wavelength. At the top and bottom of the quartz plate occurs a total reflection due to the impedance jump. The production-related thickness of the quartz plate of about 50 microns limits the resonant frequency f of the fundamental mode to typical 10 MHz to max. 55 MHz. Quartz microbalances in these frequency ranges have been used successfully in process technology since the end of the 1960s for layer thickness measurements in the gas phase deposition process. The use of a quartz microbalance for coating thickness determination offers the following advantages:
    • 1. Continuous measurement during the coating production,
    • 2. suitability for dielectrics, semiconductors and metals,
    • 3. usability for a thickness range of 0.3 nm to 3 μm,
    • 4. Measurement of the thickness of dielectrics, semiconductors and metal layers in any order,
    • 5. Relatively robust mechanical construction.
  • The use of surface acoustic wave components (SAW) as mass-sensitive resonators basically offers the possibility of increasing the resonance frequency f and of significantly reducing the oscillating mass m. The resonant frequency of the SAW resonator is determined by the period of the finger structures, currently in a range of 50 MHz to 3.15 GHz. The total reflection takes place by means of bilateral acoustic Bragg gratings. Mass accumulation alters the properties of the SAW via a second-order effect ("mass and stress-loading"). The achievable with SAW resonators quality of max. 10,000 is well below that of quartz resonators. These two influences reduce the sensitivity of OFW microbalances to quartz microbalances. Microbalances based on SAW resonators for gas sensor applications are currently still in the research and development stage with only a small field of application. The maximum permissible accumulated layer thickness of SAW sensors is limited to a few percent of the acoustic wavelength. A use for layer thickness monitoring in process technology is therefore very limited.
  • Another field of application for SAW components is mobile communication technology. Their rapid progress has led in recent years to a dramatically increasing demand for high-frequency, steep-flanked, miniaturized filter components. Recently, acoustic thin-film resonators, known as FBARs (Thin Film Bulk Acoustic Resonators), are being used for this purpose. Also the FBARs are based on a bulk wave (BAW). However, in contrast to classical quartz microbalances, the resonant frequency of FBARs is determined by the thickness of an applied piezoelectric layer and can therefore be in the range from 500 MHz to well above 10 GHz. The total reflection at the top is made by the impedance jump in the transition from upper electrode to the environment (air or vacuum). For the total reflection at the bottom, two techniques are developed: the reflection to ambient (air or vacuum), so-called "membrane-type FBARs", or to a buried acoustic Bragg mirror, so-called "Solidly-Mounted Bulk Acoustic Resonators" (SBAR). ,
  • The technology of the FBAR and SBAR components has so far been optimized only with regard to the requirements of mobile communication technology. However, these differ in essential points from the requirements of a mass-sensitive resonator. Although a high quality is essential for both, however, a high electromechanical coupling factor is only necessary for filter applications. This high coupling factor was achieved mainly with longitudinally polarized modes on zinc oxide (ZnO) or aluminum nitride (AlN). For a layer thickness measurement, a high coupling factor is not essential as long as the material remains sufficiently piezoelectric. Also, the wave type used is of minor importance.
  • The existing Schichtdickenmesssysteme based on quartz crystals are limited in their mass resolution and Miniaturizierbarkeit due to their manufacturing resonator thickness and are therefore suitable only for a layer thickness to a minimum of about 0.2 nm.
  • The WO 00/7488 A1 refers to a high temperature resistant microbalance for monitoring chemical vapor deposition (CVD) or physical vapor deposition (PVD) deposition processes. As an alternative to single-crystalline piezoelectric materials, this document proposes to use multilayer constructions of suitable piezoelectric materials, such as Ca 3 Ga 2 Ge 4 O 14 materials, gallium phosphate or materials of the Al, GaN system.
  • In order to eliminate the effects due to temperature changes, both the body to be coated and the microbalance are raised to temperatures of at least 500 ° C and the temperature is recorded.
  • It is therefore an object of the present invention to provide an improved sensor for coating thickness measurement systems which makes it possible to determine layer thicknesses in the range of less than about 0.2 nm.
  • As an inventive solution, both longitudinally and transversely polarized, piezoelectric thin-film resonators are provided for this task.
  • Essential features for a high accuracy in a layer thickness measurement are
    • 1. A close control of the temperature effects, either by a. Using a temperature compensated material, or b. Computational compensation of the temperature cross influences.
    • 2. Suppression of disturbing secondary modes
    • 3. A high quality of the acoustic oscillation
    • 4. Good reproducibility of the acoustic and piezoelectric characteristics of the FBAR and SBAR layers
  • In principle, both thin-film resonator principles, FBAR and SBAR design, are suitable for an application for determining the location of growth processes in nanotechnology. The allowable layer thickness that can be applied to an FBAR may be quite wide without compromising the principal function, as the total reflection at the upper and lower boundary layers to air or vacuum is maintained. In an SBAR, the center frequency and bandwidth of the buried mirror is determined by its layer sequence and can not be influenced later. If the resonant frequency of the upper layers, piezoelectric layer, lower and upper electrode and the accumulated layer to be measured moves out of the bandwidth of the Bragg mirror due to the layer growth, then the quality of the acoustic oscillation and thus that of the resonator breaks down; a use of the resonator in an oscillator is then no longer possible. The allowable accumulated layer thickness range for a SBAR is thus significantly limited compared to an FBAR. A disadvantage of an FBAR can be its reduced mechanical stability and the higher outlay in processing by means of sacrificial layers.
  • GaN and AlN have become very important in the field of optoelectronics and electronics. In the field of BAW applications, sputtered AlN layers are used, sometimes with epitaxial layers. Such epitaxial c-planar AlN and GaN-BAW layers have been studied by several groups in recent years, and these studies demonstrate the excellent suitability of the materials for BAW devices. AlN and GaN layers are much more reproducible than z. B. ZnO and also show a higher quality, but so far no temperature-compensated layers are known in the literature.
  • Metal organic vapor phase epitaxy (MOVPE) is known for the production of GaN-based Bragg reflectors for optical but not acoustic applications. On silicon, however, the growth of such acoustic Bragg reflectors due to the required large layer thicknesses and the small differences in the acoustic refractive index of the possible materials is not feasible, because it inevitably leads to tearing of the layers. In turn, sputtered reflectors are suitable for this purpose, it being possible, for example, to sputter W-SiO 2 alternating layers which are then present in amorphous or polycrystalline form. However, in contrast to the usual sputtered materials of monocrystalline or heavily textured materials, a lower attenuation of the reflectors and thus a better quality is achieved. Such higher-order sputtered layers can be realized by a strong energy input during the sputtering process. On sputtered metal and oxide and nitride layers, with the help of these acoustic reflector layers can then be applied to a metal layer suitable as an electrode then z. B. by means of metal-organic vapor phase epitaxy c-planar or a-planar GaN or AlN can be constructed. Alternatively, the growth of r-planar GaN is possible, here lies the c-axis tilted and the acoustic waves assume an intermediate form. As a metal or highly conductive material on silicon in the literature so far only HfN is known on which high-quality GaN can be grown. Also metals such. B. Ag, Au, W, Mo, Ni, Pt, directly on Si z. B. applied or sputtered with an electron beam evaporator, the former usually causes a lower orientation of the layer compared to sputtering methods are suitable for this purpose. This is the basis for the combined production of SBAR structures by means of sputtering and epitaxy methods.
  • For the growth of layers for membrane-based resonators on silicon also a sufficient thickness and a low strain of the GaN or AlN layer must be ensured in order to prevent cracking of the membrane. The production of up to 7 μm thick crack-free GaN layers on silicon (111) and (001) is documented in the literature.
  • Advantageously, the growth of mutually strained layers to produce a greatly reduced temperature response. Advantageous for this is the control of the tension already during the layer growth z. B. by means of optical curvature measurements.
  • With reference to the advantageous embodiments shown in the accompanying drawings, the invention will be explained in more detail below. Similar or corresponding details of the subject invention are provided with the same reference numerals. Show it:
  • 1 a schematic representation of a film thickness sensor according to a first advantageous embodiment;
  • 2 a schematic representation of a film thickness sensor according to a second advantageous embodiment.
  • 1 schematically shows a layer thickness sensor 100 according to the above-described membrane type FBAR principle, in which on a substrate 1 a carrier membrane 2 is arranged, on one side the embedded between two electrodes piezoelectric layer 3 carries and communicates on the opposite side through an opening in the substrate with air or vacuum.
  • Alternatively, the in 2 shown SBAR layer thickness sensor 200 a buried acoustic Bragg mirror 4 on.
  • According to the present invention, there is provided a film thickness measurement system, in particular for gaseous phase film deposition processes, based on mass-sensitive, both longitudinally and transversely polarized, piezoelectric thin film resonators. For this purpose, piezoelectric layers optimized especially for process measuring technology are used. Both functional principles of the acoustic resonator, the Bragg-mirror principle, which in 2 is shown, or the membrane principle according to 1 , are suitable.
  • The minimization of the temperature response is achieved according to an advantageous development of the present invention by the excitation of two different modes with different temperature response for computational compensation of the temperature transverse effect.
  • These temperaturgangkompensierbaren thin-film resonators are constructed as gravimetric sensors in a low-noise, long-term stable oscillator circuit and finally integrated in a process-compatible microsystem overall system.
  • Some parameters of a possible system are listed below:
    Oscillating resonator mass: 7 ng
    Resonant frequency: 1.880 GHz
    SNR 20 dB
    Number of measuring points for an evaluation interval: N = 2500 at tA = 1 s
  • For the so-called Cramer-Rao-Lower-Bound (CRLB) it follows for the relationship described in (1):
    Figure 00090001
    Δf / f = 3.056E-13 Δm = 21.392E - 22g
  • The highest accuracy is achieved when, according to an advantageous embodiment of the present invention, the acoustic thin-film resonator is also used as a temperature sensor. This is achieved by using two or more resonant modes with significantly different temperature response. Furthermore, mutually strained Al (Ga) N / Ga (Al, In) N layers are used to compensate for the temperature variation.
  • The available methods of curvature measurement or in-situ stress determination allow an exact control of these parameters, which are to be further analyzed ex-situ by means of X-ray diffraction and reflection.
  • The excitation and the frequency characteristics of the higher modes and the parasitic secondary modes result in the presence of a rotational symmetry of a 2D model, otherwise from a 3D model. These parameters and the acoustic mode profiles that determine the later damping can be calculated from such models.
  • The thin-film resonators are characterized by the following properties for use in coating thickness measuring systems:
    • 1. maximum quality, minimum damping
    • 2. low temperature response
    • 3. maximum sensitivity
    • 4. additional strong excitation of at least one secondary mode with its own temperature response and different mass sensitivity as a reference, alternatively, an additional temperature sensor is integrated in the chip
    • 5. Suppression of all other secondary modes

Claims (14)

  1. A layer thickness sensor for monitoring deposition processes, wherein the layer thickness sensor comprises an acoustic resonator with a sensitive region, which is arranged so that it is coated with the deposited layer during the deposition process, so that a resonant frequency of the layer thickness sensor depending on the thickness of the deposited layer changed, wherein the acoustic resonator is formed by a piezoelectric thin-film resonator, characterized in that the acoustic resonator is designed so that at least two vibration modes with different frequencies and different temperature response for computational compensation of the temperature response of the film thickness sensor can be excited.
  2. The film thickness sensor according to claim 1, wherein the piezoelectric thin-film resonator has an ambient-level total reflection acoustic mirror on a back surface of the film thickness sensor.
  3. The film thickness sensor according to claim 1, wherein the piezoelectric thin-film resonator has a buried total refractive acoustic Bragg reflector on a back surface of the film thickness sensor.
  4. A film thickness sensor according to any one of the preceding claims, operable to monitor a gaseous phase film deposition process.
  5. Layer thickness sensor according to one of the preceding claims, wherein at least one vibration mode of the thin-film resonator is longitudinally polarized.
  6. Layer thickness sensor according to one of the preceding claims, wherein at least one vibration mode of the thin-film resonator is transversely polarized.
  7. Layer thickness sensor according to one of the preceding claims, wherein the acoustic thin-film resonator has at least one AlN and / or GaN layer.
  8. Layer thickness sensor according to one of the preceding claims, wherein the acoustic thin-film resonator having mutually strained Al (Ga) N / Ga (Al, In) N layers.
  9. Method for monitoring deposition processes with the following steps: Depositing a layer on a substrate to be coated; Operating a piezoelectric resonator as a film thickness sensor, which is arranged such that a sensitive region is coated with the deposited layer during the deposition process, such that a resonant frequency of the resonator changes as a function of the thickness of the deposited layer, wherein the resonator is replaced by a piezoelectric thin-film resonator is formed, characterized in that the method further comprises the steps of: exciting at least two different vibration modes with different frequencies and different temperature response; Determining sensor signals for each vibration mode; Computational compensation of the temperature response of the film thickness sensor using the different sensor signals.
  10. The method of claim 9, wherein the piezoelectric thin-film resonator comprises an ambient-related total reflection acoustic mirror on a back side of the film thickness sensor.
  11. The method of claim 9, wherein the piezoelectric thin film resonator comprises a buried total refractive acoustic Bragg reflector on a back surface of the film thickness sensor.
  12. Method according to one of claims 9 to 11, wherein the deposition of the layer takes place from a gaseous phase.
  13. The method of any one of claims 9 to 12, wherein at least one vibration mode of the thin-film resonator is longitudinally polarized.
  14. Method according to one of claims 9 to 13, wherein at least one vibration mode of the thin-film resonator is transversely polarized.
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EP20070725761 EP2027435A1 (en) 2006-06-02 2007-06-01 Mass-sensitive thin-film resonators for layer thickness measuring systems
PCT/EP2007/004881 WO2007140943A1 (en) 2006-06-02 2007-06-01 Mass-sensitive thin-film resonators for layer thickness measuring systems

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DE102010029072B4 (en) * 2010-05-18 2015-01-08 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Microelectromechanical translation vibrating system
US8438924B2 (en) * 2011-02-03 2013-05-14 Inficon, Inc. Method of determining multilayer thin film deposition on a piezoelectric crystal
CN104579233B (en) * 2013-10-23 2018-12-04 中兴通讯股份有限公司 A kind of production method and device of film Resonator

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WO2000077488A1 (en) * 1999-06-15 2000-12-21 Massachusetts Institute Of Technology High-temperature balance

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US5873154A (en) 1996-10-17 1999-02-23 Nokia Mobile Phones Limited Method for fabricating a resonator having an acoustic mirror
US5936150A (en) * 1998-04-13 1999-08-10 Rockwell Science Center, Llc Thin film resonant chemical sensor with resonant acoustic isolator
US6668618B2 (en) 2001-04-23 2003-12-30 Agilent Technologies, Inc. Systems and methods of monitoring thin film deposition
WO2004055982A1 (en) 2002-12-13 2004-07-01 Koninklijke Philips Electronics N.V. Electro-acoustic resonator

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WO2000077488A1 (en) * 1999-06-15 2000-12-21 Massachusetts Institute Of Technology High-temperature balance

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Title
Günter Sauerbrey: Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung. In: Zeitschrift für Physik 155, 1959, 206 - 222. *

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