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`(cid:44)(cid:49)(cid:55)(cid:40)(cid:47) EXHIBIT 100(cid:24)
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`US. Patent
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`Jan. 29, 2002
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`Sheet 1 0f 4
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`US 6,342,134 B1
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`135
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`Amy/,vAw
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`lss“
`Willi/[4.9
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`125
`L \\
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`US. Patent
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`Jan. 29, 2002
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`Sheet 2 0f 4
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`US 6,342,134 B1
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`FIG. 2
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`30°
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`280
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`US. Patent
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`Jan. 29, 2002
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`Sheet 3 0f 4
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`US 6,342,134 B1
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`I=LI(3.
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`5?
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`1,8
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`CHAMBER
`PRESSURE
`(mT)
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`1.3
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`15
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`17
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`19
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`21
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`23
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`25
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`NITROGEN GAS FLOW (sccml
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`CHAMBER
`PRESSURE
`(mT)
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`OXYGEN GAS FLOW (sccm)
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`US. Patent
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`Jan. 29, 2002
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`Sheet 4 0f 4
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`US 6,342,134 B1
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`FIG. 5
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`ROTATING MAGNET
`MAGNETRON
`
`CONDUCTIVE
`TARGET
`
`POSITION SUBSTRATE
`ONTO PLATEN
`
`FLOW NOBLE GAS
`
`BEGIN MAGNET ROTATION
`
`INCREMENT REACTIVE
`
`GAS 1 SCCMr
`ATTAIN CROSSOVER POINT
`
`
`INCREASE REACTIVE GAS
`FLOW RATE 3 SCCM
`
`
`
`DEPOSIT FILM ONTO
`MONITOR WAFER
`OF REFRACTIONILFINE ADJ.
`
`
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`ATTAIN DESIRED INDEX
`
`
`ADJUST P/S FREOUENCY
`FOR STABILITY
`
`PULSE WIDTH
`
`COARSE ADJ.
`REACTIVE FLOW GAS
`
`
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`
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`ATTAIN DESIRED
`TENSILE STRESS
`
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`ATTAIN DESIRED
`SURFACE ROUGHNESS
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`
`
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`
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`INCREASE
`
`POWER
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`
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`DEPOSIT HIGH QUALITY
`INSULATING FILM
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`INCREASE RF BIAS
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`REDUCE NOBLE
`GAS FLOW
`
`DECREASE
`PRESSURE
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`US 6,342,134 B1
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`1
`METHOD FOR PRODUCING
`PIEZOELECTRIC FILMS WITH ROTATING
`MAGNETRON SPUTTERING SYSTEM
`
`RELATED APPLICATIONS
`
`This application is related to US. patent application Ser.
`No. 09/502,868, now abandoned titled “Method for Produc-
`ing Devices Having Piezoelectric Films,” filed concomi—
`tantly herewith by inventors Bower, Pastalan, and Ritten-
`house and assigned to the present assignee (hereinafter the
`“Bower application”), which is incorporated herein by ref-
`erence.
`
`FIELD OF THE INVENTION
`
`The invention relates to a method for producing electronic
`devices containing a piezoelectric film comprising use of a
`rotating magnetron sputtering system. The invention is par-
`ticularly useful in fabricating acoustic resonators and semi—
`conductor devices.
`
`BACKGROUND OF THE INVENTION
`
`Communications systems typically include a variety of
`devices (e.g., filters, mixers, amplifiers, integrated circuits,
`and so forth). Communications systems are useful for trans-
`mitting information (e.g., voice, video, data) relayed by
`means of wireless links, twisted pair, optical fibers, and so
`forth. As wireless communications systems become more
`advanced, signals are being transmitted at higher frequen—
`cies (e.g., PCS, ISM, etc). As systems are continually
`developed in response to market pressures, the demand for
`increased performance and reduced size intensifies. Market
`forces demand increased integration and reduction of com-
`ponent size.
`Resonators such as Bulk Acoustic Wave (BAW) resona-
`tors are important components in the fabrication of bandpass
`filters and other related semiconductor devices. The BAW
`resonator is a piezoelectric resonator that essentially com-
`prises a film of piezoelectric material (e.g., a crystalline AlN
`film), deposited between at
`least
`two electrodes. Upon
`application of voltage to such a structure, the piezoelectric
`material Will vibrate in an allowed vibrational mode at a
`certain frequency. Piezoelectric resonators are thus useful in
`discriminating between signals based on frequency diversity
`(e.g., a bandpass filter), and in providing stable frequency
`signals (e.g., as in a frequency stabilizing feedback element
`in an oscillator circuit).
`Typically, the performance and resonant frequency of the
`piezoelectric resonator will depend upon the composition,
`thickness, and orientation of the piezoelectric material. The
`resonant frequency of a piezoelectric material is typically
`inversely proportional to its thickness; thus, for piezoelectric
`resonators to operate at high frequencies {e.g., frequencies
`greater than ~700 Megahertz (MHz) up to 10 Gigahertz (10
`GHz)},
`the thickness of the piezoelectric film must be
`reduced to a thin film (e. g., having a thickness ranging from
`about 500 nm to about 10 ,um). The coupling between
`electrical and mechanical energy of a piezoelectric resonator
`is dependent on the crystalline orientation of the atoms
`comprising the piezoelectric film. The induced strain (i.e.,
`stress wave) in a piezoelectric film in response to applied
`voltage (i.e., electric field) can only occur from the advan-
`tageous alignment of the crystalline axis within the piezo—
`electric film. An example of an advantageous film orienta-
`tion is <002> of AlN perpendicular to the substrate.
`Piezoelectric film quality may be affected by the method
`used to form the film. Typically, sputter deposition or
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`2
`reactive sputter deposition techniques have been used. Sput—
`er deposition involves a vacuum deposition process in
`which a sputtering target is bombarded with ions, and the
`atoms of the target material are mechanically ejected from
`he target and deposited onto a nearby substrate. In reactive
`sputtering, a reactive gas is introduced into the deposition
`chamber and reacts with the target material to produce a film
`hat is sputtered onto the substrate, either directly or upon
`urther reaction with freed target material. In DC reactive
`sputtering, a direct current electrical potential is applied
`within the sputtering chamber in which a reactive sputtering
`3rocess is carried out. However,
`typical sputtering and
`reactive sputtering techniques,
`including DC reactive
`sputtering, often do not provide adequate deposition rates. A
`3ulse DC sputtering method for efficiently depositing thin
`films of piezoelectric materials such as aluminum nitride
`(AlN), e.g., with improved control over the direction and
`delivery of the reactive gas,
`is described in US. patent
`application Ser. No. 09/145,323, to Miller et al., “Pulse DC
`Reactive Sputtering Method for Fabricating Piezoelectric
`Resonators,” filed Sep. 1, 1998, assigned to the present
`assignee and incorporated herein by reference. In Miller et
`al., the quality of the piezoelectric films is improved with the
`echniques used to deposit the films, i.e., the pulse Width of
`he positive portion of the applied voltage is adjusted based
`on its effect on the desired film constituency, stress, and
`exture.
`
`
`
`Magnetron sputtering systems are known in which
`magnetically—enhanced targets are used to confine the
`alasma discharge along a particular path and enhance the
`’ow of target material. See, e.g., US. Pat. No, 5,830,327 to
`Kolenkow, “Methods and Apparatus for Sputtering with
`RotatingMagnet Sputter Sources”; US. Pat. No. 5,693,197
`0 Lal et al., “DC Magnetron Sputtering Method and Appa-
`ratus”: and US. Pat. No. 5,378,341 to Drehman et at,
`“Conical Magnetron Sputter Source,” all of which are incor-
`3orated herein. Use of magnetron sputtering has been
`aroblematic, however, for depositing silicon dioxide films.
`Because silicon dioxide is a good insulator, a film suffi-
`ciently thick to cause arcing problems is rapidly formed at
`certain areas of the target, i.e., splats or regions of silicon
`dioxide may be deposited on the target surface so that it is
`not uniformly biased, and eventually, the target may become
`coated to the point where it is no longer conductive and the
`deposition may stop. Thus, magnetron reactive sputtering
`has not been conventionally used to deposit quality silicon
`dioxide films. See, e.g., US. Pat. No. 5,683,558 to Sieck et
`al., “Anode Structure for Magnetron Sputtering Systems,” at
`col. 1,
`lines 53—55 (“The arcing associated with silicon
`dioxide has prevented planar magnetron reactive sputtering
`from being efficiently utilized to deposit quality silicon
`dioxide films”). Additionally, previous methods of deposit-
`ing insulating films (including piezoelectric films) have
`involved use of RF sputtering utilizing fixed magnets.
`As may be appreciated, those in the field of communica-
`tions systems and components continue to search for new
`methods for increasing system performance and integration.
`In particular,
`it would be advantageous to provide new
`methods for improving the quality of piezoelectric films. A
`high—quality AlN piezoelectric film deposited on a reflecting
`multi-layer acoustic mirror stack is a method to produce
`high—quality, RF front—end filters for GHz applications.
`These objectives and further advantages of this invention
`may appear more fully upon considering the detailed
`description given below.
`SUMMARY OF THE INVENTION
`
`Summarily described, the invention embraces a quality-
`assurance method for use in the fabrication of piezoelectric
`
`

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`US 6,342,134 B1
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`3
`films for electronic devices, particularly resonators. The
`method comprises determining the surface roughness of an
`insulating layer on which the piezoelectric film is to be
`deposited and achieving a surface roughness for the insu-
`lating layer that
`is sufficiently low to achieve the high-
`quality piezoelectric film. According to one aspect of the
`invention, the low surface roughness for the insulating layer
`is achieved with use of a rotating magnet magnetron system
`for improving the uniformity of the deposited layer. Accord-
`ing to other aspects of the invention, the high—quality piezo—
`electric film is assured by optimizing deposition parameters
`or monitoring and correcting for the surface roughness of the
`insulating layer pre-fabrication of the piezoelectric film.
`BRIEF DESCRIPTION OF THE DRAWINGS
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`10
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`15
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`For a better understanding of the invention, an exemplary
`embodiment is described below, considered together with
`the accompanying drawings, in which:
`FIG. 1 is a perspective schematic illustration of an acous-
`tic resonator;
`FIG. 2 is a schematic diagram of a reactive sputtering
`arrangement with rotating magnetrons for use in performing
`the inventive method;
`FIG. 3 is a representative graph illustrating determination .
`of the cross-over point for AlN; and
`FIG. 4 is a representative graph illustrating determination
`of the cross-over point for SiOz; and
`FIG. 5 is a block diagram showing steps for performing
`an exemplary inventive method.
`It
`is to be understood that these drawings are for the
`purposes of illustrating the concepts of the invention and are
`not to scale.
`
`30
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Although specific features and configurations are dis-
`cussed below, it should be understood that these examples
`are for purposes of illustration only. One skilled in the
`relevant art will recognize that other steps, configurations
`and arrangements may be used without departing from the
`spirit and scope of the invention.
`The invention pertains to a method for obtaining high-
`quality piezoelectric films. FIG. 1 is a perspective schematic
`illustration of an acoustic resonator, which may be fabri-
`cated using the inventive method. The resonator 100 com-
`prises a substrate 110, a film or layer 120 of piezoelectric
`material, and a means for retaining acoustic energy in the
`piezoelectric film, such as a Bragg reflecting region 125,
`between the substrate 110 and film 120. Alternative to the
`
`reflecting region 125, a layer of air (not shown) may be used
`to suspend the film 120 above the substrate 110. Abottom
`electrode 135 and top electrode 130 are disposed on opposite
`surfaces of the piezoelectric film 120.
`'lhe layer of piezoelectric material advantageously com-
`prises AlN, but may be made of any suitable material that
`has piezoelectric qualities sufficient for the particular reso-
`nator application. Typical piezoelectric materials include,
`for example, quartz, zinc oxide (ZnO), and ceramic mate—
`rials such as lithium niobate (LiNbOB), lithium tantalate
`(LiTaOB), paratellurite (TeOE), and lead titanate zirconate
`(PZT). The substrate typically is comprised of silicon but
`may be fabricated with other materials such as quartz,
`sapphire, polysilicon, aerogel, and aluminum oxide (A1203).
`Advantageously, with the invention,
`the electrodes and
`particularly the bottom electrode 135 may comprise alumi-
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`4
`num (Al) or a metal stack using titanium and Al. Besides use
`of Al and/or Ti/Al, other metals having a low sheet resis-
`tance and low surface roughness may be used for fabricating
`the electrodes 130, 135. Previous stacked metal electrodes
`often have comprised Ti/TiN/Al as the composition of
`choice.
`
`it would be advantageous to non—destructively
`Notably,
`predict and assure the quality of the piezoelectric film 120
`before the film is deposited. Traditionally, methods for
`determining the quality of the piezoelectric films have been
`applied after the piezoelectric films 120 are deposited (e.g.,
`X-ray diffraction) but these methods may destroy or damage
`the devices. Additionally, it would be advantageous to avoid
`use of collimated metals (i.e., metals deposited with a
`collimator)
`in order to eliminate film thickness non-
`uniformity associated with collimated depositions. To avoid
`such non-uniformities, this invention provides an inventive
`process to produce a high-quality piezoelectric film without
`use of the collimator. The inventive method is advantageous
`in that high-quality piezoelectric films can be fabricated
`when there is a decrease in the process window for depos-
`iting textured Ti—Al, and the quality of the piezoelectric
`film can be assured pre—fabrication, i.e., before the piezo—
`electric film is deposited. This is accomplished by fabricat-
`ing insulating layers 125 having a surface roughness that is
`sufficiently low to assure the high-quality piezoelectric film
`(e.g. <10 A), non-destructively evaluating these layers, and
`further smoothing them, if necessary. This method also
`reliably predicts the quality of the piezoelectric film before
`it is deposited.
`According to one aspect of the invention, a process
`involving use of rotating magnet magnetrons and pulsed DC
`power supplies is applied to deposit the insulating layers
`and/or piezoelectric film. Rotating magnet magnetrons are
`used to provide film thickness uniformity, and such rotating
`magiet magnetrons are made possible in that process param-
`eters are optimized to achieve a high deposition rate and
`correct index of refraction. According to another aspect of
`
`the invention,
`the process comprises determination of a
`
`
`
`“cross—over point” such that the target will remain e ective
`in emitting atoms and resisting the deposition of materials
`thereon. The “cross-over point" is defined as the point at
`which a pressure increase in the chamber 210 (see FIG. 2)
`becomes non-linear with the flow of reactive gasses, and is
`further defined below. Additionally, the process makes use
`of the recognition that the quality of the piezoelectric film
`can be improved by addressing the surface roughness of the
`layers on which the piezoelectric film is deposited,
`i.e.,
`providing an underlying insulating layer having a relatively
`smooth surface produces a higher quality piezoelectric film.
`Thus, according to another aspect of the invention,
`the
`surface roughness of the insulating layers are monitored and
`smoothed,
`if necessary, before the piezoelectric film is
`deposited.
`Notably, the contemporaneously—filed Bower application
`referenced above (which is assigned to the present assignee
`and incorporated herein), describes the recognition that the
`surface roughness of the electrode underlying the piezoelec-
`tric film (FIG. 1, 135) alfects the quality of the piezoelectric
`film 120. The Bower application thus describes a method of
`making a device having a piezoelectric film comprising
`controlling the surface roughness of the metal layer, which
`may include controlling the surface roughness of the insu-
`lating layers 125 underlying the metal layer 135. The Bower
`method comprises use of textured titanium (and a collimator
`in the deposition process), which increases the system
`tolerance or process window for surface roughness. In other
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`US 6,342,134 B1
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`5
`words, when the collimator is used, the insulating layers 125
`do not have to be as smooth as when the collimator is not
`used in order to achieve a high-quality piezoelectric film,
`The collimator also is known, however, for decreasing the
`thickness uniformity of the titanium layer.
`With this invention, a process is described for minimizing
`the surface roughness of the underlying insulating layers on
`which the electrode and piezoelectric films are deposited,
`and with the instant method, the use of a collimator is not
`required. The surface roughness of the underlying layers 125
`may be reduced to achieve a maximum texture for the
`piezoelectric films and optimal operation. of the resonator.
`For example, the insulating. layers should be fabricated so
`they have root mean square (RMS) morphology reflecting a
`surface roughness of about
`less than 10 Angstroms. The
`RMS value reflects a true average, absolute value for the
`deviation or difference in the surface morphology from a
`mean value of zero, the value of zero reflecting a completely
`smooth surface. The RMS value is defined by the square root
`of the difference between the mean square and the square of
`the mean, or in other words, it is the normalized average
`value of the roughness relative to the median of the mea-
`sured roughness.
`A “high-quality” piezoelectric film is defined herein as a
`film having a texture reflecting a good crystalline orientation
`of atoms,
`low stress (less than about 50 megaPascals
`{MPa}), and appropriate index of refraction (approximately
`2.0, more preferably 2.0710005, and more preferably
`207810.005). Fihn thickness uniformity advantageously is
`0.4% one Sigma, with a more preferred thickness uniformity
`of 0.2% one Sigma, with an even more preferred thickness
`uniformity of 0.1% three Sigma. Thus, the “quality” of a
`piezoelectric film as that term is used herein relates to at
`least one of the texture, stress, uniformity, and/or index of
`refraction of the piezoelectric film. The term “texture” as
`used herein is intended to describe the crystallographic
`alignment of grains in a polycrystalline film wherein “maxi-
`mum texture” denotes a film having an alignment
`(orientation) of grains centered about a single direction at an
`angle subtended from (relative to) the growth direction. The
`texture and thus quality of the piezoelectric film can be
`defined by reference to its “rocking curve." More
`specifically, ideally crystallographic directions of the grains
`are centered about a single direction; as mentioned above,
`the performance of the piezoelectric film (e.g. in a piezo—
`electric resonator), is dependent upon the crystalline orien-
`tation of the atoms comprising the film. Typically, however,
`there will be a gaussian distribution of directions about
`which the grains are centered. The smaller the distribution,
`the closer the film is to maximum texture. The distribution
`of grain directions may be plotted to define a peak, and the
`width of the peak at half its maximum height (full-width at
`half maximum) (FWHM), i.e., the “rocking curve” width,
`reflects a value for defining the quality of the film texture.
`The “rocking curve” width is thus the figure of merit for the
`film texture, i.e., the smaller the distribution, the smaller the
`rocking curve, and the closer the film is to maximum texture,
`The piezoelectric film advantageously is formed with a
`l’WlIM rocking curve of less than 35°, with a more
`preferred low rocking curve being less than about 25°, and
`more preferably less than 15° (FWHM).
`According to one aspect of the invention, a high—quality
`piezoelectric film may be fabricated by applying a rotating
`magetron and pulse DC reactive sputtering process to fab-
`ricate one or more layers of the piezoelectric device, includ-
`ing the insulating layers (FIG. 1, 125), and/or the piezoelec-
`tric film (FIG. 1, 120). FIG. 2 is a simplified, schematic
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`6
`representation of a rotating magnetron sputtering apparatus
`for use in depositing the piezoelectric 120, the insulating
`layers 125, and the electrodes 130, 135 on the substrate 110.
`The apparatus includes a chamber 210 (e.g., a plasma
`chamber), and a pair of electrodes (the target 260 and anode
`ring 225) within the chamber. The electric potential applied
`to the electrodes may be controlled by a pulsed DC power
`source 230 or other suitable source. Various sources are
`provided for injecting gases into the chamber. A first flow
`control source 240 injects noble gases into the chamber (e.g.,
`Ar, Ze, and Kr), and a second flow control source 250
`supplies a reactive gas (02, N2, etc.) into the chamber 210.
`The gases are supplied via gas delivery port 245.
`A target material
`is positioned within the sputtering
`chamber. The substrate 110 is also positioned therein, and
`disposed such that it is in communication with the target and
`gasses within the chamber. The target material 260 is
`mounted adjacent a rotating magnet assembly 280 which
`includes a magnet array to produce a magnetic field to
`penetrate the target material 260 and form an arc over its
`surface facing the substrate 110. Arotation motor 300 causes
`the rotating magnet assembly to rotate about an axis of
`rotation with respect to the target 260. The magnetic field
`generated across target 260 helps to confine free electrons
`near he surface of the target. The increased concentration of
`
`ions excited by these electrons at the target surface increases
`
`he e iciency of the sputtering process. The pressure within
`he chamber can be regulated with pressure regulators 240,
`250 and throttle valve 270. An RF power supply 235 applies
`a bias voltage to the substrate platen 115 to control tensile
`ilm stress.
`
`
`
`
`
`A high-quality film may be achieved with this apparatus,
`and various parameters can be applied to optimize the
`arocessing conditions. Applicants have discovered a method
`comprising optimization of the reaction parameters to
`achieve a high-deposition rate and correct index of refrac-
`ion. The method includes a step of determining the “cross-
`over point” in the process. The “cross—over point” is defined
`as the point at which a pressure increase in the chamber 210
`Jecomes non—linear with the flow of reactive gases, and this
`joint reflects a reactive gas flow rate that strongly effects a
`reaction with the target material. For example, FIG. 3 is a
`representative graph illustrating determination of the cross-
`over point in a process involving fabrication of an AlN
`3iezoelectric film,
`in which the reactive gas comprises
`nitrogen and the target comprises aluminum. FIG. 4 is a
`representative graph illustrating determination of the cross—
`over point in a process involving fabrication of an SiO2
`ayer, in which the reactive gas comprises oxygen and the
`arget comprises silicon.
`Referring to FIG. 3, the characteristic pressure-flow curve
`shows two asymptotic regimes. At low flow of reactive gas
`(to the left of the graph), all the nitrogen is being reacted
`with metallic aluminum so introduction of small amounts of
`nitrogen does not increase the chamber pressure. As the gas
`flow is increased, gas is present in the chamber that cannot
`react with metal, and the unreacted gas causes an increase in
`pressure in the chamber. At a very high flow rate (to the right
`of the graph), the target material becomes filly “poisoned” or
`coated with the reaction product (AlN) such that any further
`gas admitted into the chamber will not react with target
`material and will directly contribute to a pressure increase.
`The plot of pressure versus reactive gas flow (as in FIGS. 3
`and 4) shows a “cross-over curve” from the behavior of a
`metallic target fully consuming reactive gas to the behavior
`of a fully poisoned target showing increasing pressure with
`further admission of reactive gas.
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`

`US 6,342,134 B1
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`FIG. 5 is a block diagram showing illustrative steps of one
`aspect of the invention. The invention comprises use of the
`cross-over curve in optimizing the reactive sputtering depo-
`sition process with use of rotating magnet magnetrons and
`pulsed DC POWCL The method includes a step 0f providing 5
`the target material within a sputtering chamber, with the
`target material being oriented on a rotating magnet assembly
`for producing a magnetic field across the target material. A
`substrate within the sputtering chamber in open communi-
`cation with the target, and a pair of electrodes (target and
`anode ring) are positioned within the sputtering chamber. As
`the process begins, the magnet rotation begins and is stabi-
`lized at a level sufficient to achieve uniform target erosion.
`A suitable rotation speed is 282 rpms. The cross-over point
`for the flow of a reactive gas into the sputtering chamber is
`determined with appropriate settings to the pulse frequency
`and pulse width. The noble gas is introduced into the
`sputtering chamber, and then the reactive gas is added so that
`it reacts with a portion of the target material. The reactive
`gas is introduced incrementally into the sputtering chamber
`so that the flow rate of the reactive gas is maintained at a rate
`corresponding substantially to, but greater than, the flow rate
`at
`the cross-over point. A film is thus deposited on the
`substrate as the chamber is maintained just above the
`cross-over point. Immediately below (less than 3 seem) the
`cross—over point will result in a metallic film. Above the
`cross-over point, the deposition rate decreases for at least 20
`secm and until the target is totally poisoned, becomes an
`insulator, and a plasma can no longer be maintained.
`As the deposition process is carried out, a pulsed DC
`voltage is applied across the pair of electrodes (the target and
`anode ring) that are positioned within the sputtering cham—
`ber so that ions from the noble gas impinge upon the target
`material and eject atoms therefrom. Freed atoms of the target
`material react mostly at the target with the reactive gas to
`form a coating on the substrate. Maintaining the flow rate
`near the cross-over point
`is effective in optimizing the
`deposition conditions; the target will continue to be conduc-
`tive while depositing a coating on the substrate, and poi—
`soning of the target itself is controlled. Additionally, various
`other aspects of the process may be managed to improve the
`deposition rate and overall quality of the deposited films, For
`example,
`the relevant parameters of the process may be
`monitored and adjusted, considering that the deposition rate
`is inversely proportional to the amount of reactive gas, the
`power supply pulse width, and the deposition pressure, and
`
`is directly proportional to the power. The deposition rate is
`
`
`
`una ected by the power supply frequency. The index of
`refraction is inversely proportional to both the amount of
`reactive gas and the power supply pulse width.
`According to one aspect of the invention, these deposition
`parameters may be adjusted to achieve an optimal deposition
`rate for producing substantially stress-free piezoelectric
`films having the desired index of refraction. Using monitor
`wafers, the index of refraction of the deposited coating can
`be monitored and adjusted to achieve a desired index of
`refraction for the coating. This may comprise a coarse
`adjustment with reactive gas, followed by a fine tuning with
`pulse width. 'l'he current and/or voltage can be monitored
`and frequency adjusted to achieve a stable waveform. The
`pulse width of the DC voltage can be adjusted to improve the
`homogeneity of the film or coating formed on the substrate.
`The DC power can be increased or the pressure or flow rate
`reduced to improve the surface roughness for the film.
`The inventive process is advantageous for depositing
`piezoelectric films and also for optimizing the processing
`conditions for depositing the insulating layers, which will
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`impact upon the texture of the piezoelectric films deposited
`hereon. As applied to the insulating layers, additional
`aarameters to consider are that a reduction in the surface
`roughness of the resultant Si()2 films is achieved by increas-
`ing the power and/or decreasing the pressure. According to
`he invention, these parameters may be adjusted and opti-
`mized to achieve the highest deposition rate that produces
`ilms having the desired surface roughness and index of
`refraction. Applying this evaluative process, films may be
`consistently produced that have the desired index of refrac—
`ion and a surface textureoof less than 25 A (RMS), and more
`areferably less than 10 A.
`Applying the realization that the piezoelectric film quality
`can be improved by providing a good starting platform for
`he piezoelectric film (i.e., by providing an underlying
`insulating layer having a relatively smooth surface texture),
`he surface roughness of the insulating layer may be mea-
`sured and if necessary, further processed to achieve optimal
`quality piezoelectric films. The surface roughness of an SiO2
`ilm may be non—destructively measured by an Acoustic
`Force Microscope (AFM) located within the silicon labora-
`ory in which the piezoelectric device is produced, e.g., a
`class 10 clean room where all silicon fabrication takes place
`or other Silicon Fabrication Research Laboratory (SFRL). In
`other words, the surface roughness of the insulating layer is
`measured in-situ during the fabrication. Further smoothing
`can be achieved by chemical mechanical polishing and to a
`lesser extent with a hydrogen hot sputter etch process (HSE)
`(a patent on this process was filed in the United Kingdom by
`Trikon Technologies Inc. The Trikon process teaches a flow
`rate of hydrogen at 50 sccm. However, applicants have
`discovered that a reduction in the flow rate of hydrogen gas
`provides significant advantages in producing films having
`the desired texture. For example, a reduction in hydrogen
`flow rate to 25 sccm (from the Trikon 50 sccm flow rate) has
`resulted in an improved surface smoothing capability that
`does not impact upon the overall thickness of the SiO2 film.
`The following examples will serve to further typify the
`nature of the invention but should not be construed as a
`limitation on the scope thereof, which is defined by the
`appended claims.
`
`EXAMPLE 1
`Deposition of an AlN Film
`In operation, a first step comprises determining the cross-
`over curve on a monitor wafer. This can be accomplished by
`arbitrarily setting the pulse frequency at 100 kllz and the
`pulse width at 25 n8.
`is introduced into the
`Next, argon (the noble gas)
`chamber, preferably at the lowest value at which a plasma
`can be maintained. Asuitable flow rate forAr is 20 sccm and
`a resultant chamber pressure of 1.5 mTorr. Initially,
`the
`nitrogen is introduced incrementally (e.g., in 1 seem steps),
`as the voltage, current and chamber pressure are monitored
`to determine the flow rate that corresponds to the cross—over
`point. (Any one of the voltage, current, or chamber pressure
`plotted against flow rate will indicate the cross-over point;
`any one of these factors may be analyzed, or all three may
`be analyzed for confirmation.) Nitrogen (the reactive gas) is
`introduced into the chamber at a flow rate that corresponds
`substantially to (e.g., about 3 sccm above) the flow rate at the
`cross—over point, i.e., the point at which the target becomes
`nitrided.
`The film is then deposited at a gas ratio approximately 3
`seem greater than the cross-over point. The stress and index
`of refraction of the deposited material are measured. The
`voltage-current of the magnetron should be monitored and if
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`US 6,342,134 B1
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`9
`necessary, the frequency should be adjusted to achieve a
`stable waveform. The frequency does not affect the film
`quality and may be freely adjusted.
`The amount of reactive gas and power supply pulse width
`are adjusted to achieve the desired index of refraction. The
`gas ratios may be varied to make coarse corrections. The
`reverse-bias pulse width may be adjusted for fine-tuning
`correction of the indexithe more the pulse width is
`increased (that
`is,
`the larger percentage of the selected
`frequency it covers),
`the higher the index will be, but
`increasing the pulse width also lowers the deposition rate.
`RF bias is applied to the substrate to control tensile stress.
`Monitor wafers are used to determine the bias that results in
`zerotfifty MPa stress when the index is fine tuned. The
`system should be monitored and corrected for zero stress
`when the index is fine-tuned.
`The pulsed DC power may be adjusted as described in the
`above-referenced Miller application, incorporated herein, to
`enhance the desired film constituency, stress, and texture
`through more efficient reaction by the reactive gasses. Each
`power change requires re-optimization of the index of
`refraction and stress.
`
`EXAMPLE 2
`Deposition of a Silicon Dioxide Insulating Film
`A silicon dioxide (SiOZ) film may be fabricated using a
`silicon target doped with §2% boron. Oxygen is