(12) United States Patent
`Barber et al.
`
`I IIIII IIIIIIII Ill lllll lllll lllll lllll lllll lllll lllll lllll 111111111111111111
`US006342134Bl
`US 6,342,134 Bl
`Jan.29,2002
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) METHOD FOR PRODUCING
`PIEZOELECTRIC FILMS WITH ROTATING
`MAGNETRON SPUTTERING SYSTEM
`
`(75)
`
`Inventors: Bradley Paul Barber, Chatham, NJ
`(US); Ronald Eugene Miller,
`Riegelsville, PA (US)
`
`(73) Assignee: Agere Systems Guardian Corp.,
`Orlando, FL (US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by O days.
`
`(21) Appl. No.: 09/503,225
`
`(22) Filed:
`
`Feb. 11, 2000
`
`Int. Cl.7 ................................................ C23C 14/34
`(51)
`(52) U.S. Cl. ............................ 204/192.18; 204/192.13;
`204/192.22; 204/192.3; 427 /8; 216/38;
`29/25.35
`(58) Field of Search ....................... 204/192.12, 192.13,
`204/192.18, 192.22, 192.3; 427 /8; 216/38;
`29/25.35
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`4,642,163 A * 2/1987 Greschner et al. .......... 156/643
`5,378,341 A
`1/1995 Drehman et al.
`...... 204/298.18
`5,651,865 A * 7/1997 Sellers .................. 204/192.13
`5,683,558 A * 11/1997 Sieck et al. ............ 204/192.12
`5,693,197 A
`12/1997 Lal et al. ................. 204/192.2
`
`5,702,573 A
`5,830,327 A
`5,935,641 A *
`6,001,227 A
`
`* 12/1997
`11/1998
`8/1999
`12/1999
`
`Biberger et al. . ... ... 204/192.12
`Kolenkow ............. 204/192.12
`Beam, III et al.
`.......... 427/100
`Pavate et al. . . . . . . . . . . 204/298.12
`
`OTHER PUBLICATIONS
`Vossen et al., "Thin Film Processes", pp. 48, Dec. 1978. *
`* cited by examiner
`
`Primary Examiner-Nam Nguyen
`Assistant Examiner-Steven H. VerSteeg
`(74) Attorney, Agent, or Firm-Lowenstein Sandler PC
`
`(57)
`
`ABSTRACT
`
`A quality-assurance method is described that is useful in the
`fabrication of piezoelectric films of electronic devices, par(cid:173)
`ticularly resonators for use in RF filters. For example, 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(cid:173)
`lating layer that is sufficiently low to achieve the high(cid:173)
`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(cid:173)
`ing to other aspects of the invention, the high-quality piezo(cid:173)
`electric film is assured by optimizing deposition parameters
`including determination of a "cross-over point" for reactive
`gas flow and/or monitoring and correcting for the surface
`roughness of the insulating layer pre-fabrication of the
`piezoelectric film.
`
`7 Claims, 4 Drawing Sheets
`
`ROTATING MAGNET
`MAGNETRON
`
`CONDUCTIVE
`TARGET
`
`POSITION SUBSTRATE
`ONTO PLATEN
`
`FLOW NOBLE GAS
`
`BEGIN MAGNET ROTATION
`
`INCREMENT REACTIVE
`GAS 1 SCCM
`
`All AIN CROSSOVER POINT
`
`INCREASE REACTIVE GAS
`FLOW RATE 3 SCCM
`
`DEPOSIT FILM ONTO
`MONITOR WAFER
`
`FINE ADJ.
`PULSE WIDTH
`
`COARSE ADJ.
`REACTIVE FLOW GAS
`
`All AIN DESIRED INDEX
`Of REFRACTION
`
`- - - - - - - - - - ; INCREASE RF BIAS
`
`ADJUST P/S FREQUENCY
`FOR STABILITY
`
`ATTAIN DESIRED
`TENSILE STRESS
`
`REDUCE NOBLE
`GAS FLOW
`
`INCREASE
`POWER
`
`DECREASE
`PRESSURE
`
`ATTAIN DESIRED
`SURFACE ROUGHNESS
`
`DEPOSIT HIGH QUALITY
`INSULATING FILM
`
`Page 1 of 10
`
`APPLIED MATERIALS EXHIBIT 1005
`
`

`

`U.S. Patent
`US. Patent
`
`Jan.29,2002
`Jan. 29, 2002
`
`Sheet 1 0f4
`Sheet 1 of 4
`
`US 6,342,134 Bl
`US 6,342,134 B1
`
`FIG. 1
`
`135
`
`W/Ill/mAw‘
`
`
`
`
`l “‘
`'9“\‘\\\\\\\\““\\\“\\\\\\\\\\\\\\\\\Vm
`
`—
`
`
`
`110
`110
`
`125
`
`Page 2 of 10
`
`Page 2 of 10
`
`

`

`U.S. Patent
`
`Jan.29,2002
`
`Sheet 2 of 4
`
`US 6,342,134 Bl
`
`FIG. 2
`
`300
`
`280
`
`I
`I
`I
`I
`I
`
`I
`
`I
`
`110
`/
`
`115
`
`'--- le
`
`v24S
`
`I -
`
`+ I
`
`\
`230
`
`I
`
`I
`
`RF
`\
`235
`
`y2so
`I
`~
`~225
`I
`
`I
`
`L,,-210
`
`n
`
`CRYO PUMP
`
`v270
`
`'--
`
`I
`I
`l
`
`I
`I
`I
`
`240
`
`250
`
`Page 3 of 10
`
`

`

`U.S. Patent
`
`Jan.29,2002
`
`Sheet 3 of 4
`
`US 6,342,134 Bl
`
`FIG. 3
`
`1. 8
`
`CHAMBER
`PRESSURE
`(mT)
`
`1.3-+--~~~~~~~~~~~~~~~~~
`
`15
`
`17
`
`21
`19
`NITROGEN GAS FLOW (seeml
`
`23
`
`25
`
`FIG. 4
`
`CHAMBER
`PRESSURE
`(mT)
`
`2.4
`2.3
`2.2
`2.1
`2
`1. 9
`1. 8
`1. 7
`1. 6
`1.5
`1. 4
`0
`
`5
`
`20
`15
`10
`OXYGEN GAS FLOW (seeml
`
`25
`
`30
`
`Page 4 of 10
`
`

`

`U.S. Patent
`
`Jan.29,2002
`
`Sheet 4 of 4
`
`US 6,342,134 Bl
`
`ROTATING MAGNET
`MAGNETRON
`
`FIG. 5
`
`CONDUCTIVE
`TARGET
`
`I
`COARSE ADJ.
`REACTIVE FLOW GAS
`
`-
`
`INCREASE RF BIAS -
`
`DECREASE
`PRESSURE
`
`,---
`
`FINE ADJ.
`PULSE WIDTH
`
`'
`
`REDUCE NOBLE
`GAS FLOW
`
`INCREASE
`POWER
`
`POSITION SUBSTRATE
`ONTO PLATEN
`
`FLOW NOBLE GAS
`
`BEGIN MAGNET ROTATION
`
`INCREMENT REACTIVE
`GAS 1 SCCM
`
`'
`'
`'
`'
`'
`
`ATTAIN CROSSOVER POINT
`
`INCREASE REACTIVE GAS
`FLOW RATE 3 SCCM
`I
`DEPOSIT FILM ONTO
`MONITOR WAFER
`I
`ATTAIN DESIRED INDEX
`OF REFRACTION
`I
`ADJUST P/S FREQUENCY
`FOR STABILITY
`I
`ATTAIN DESIRED
`TENSILE STRESS
`I
`ATTAIN DESIRED
`SURFACE ROUGHNESS
`
`'
`
`DEPOSIT HIGH QUALITY
`INSULATING FILM
`
`Page 5 of 10
`
`

`

`US 6,342,134 Bl
`
`1
`METHOD FOR PRODUCING
`PIEZOELECTRIC FILMS WITH ROTATING
`MAGNETRON SPUTTERING SYSTEM
`
`RELATED APPLICATIONS
`
`This application is related to U.S. patent application Ser.
`No. 09/502,868, now abandoned titled "Method for Produc(cid:173)
`ing Devices Having Piezoelectric Films," filed concomi(cid:173)
`tantly herewith by inventors Bower, Pastalan, and Ritten(cid:173)
`house and assigned to the present assignee (hereinafter the
`"Bower application"), which is incorporated herein by ref(cid:173)
`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(cid:173)
`ticularly useful in fabricating acoustic resonators and semi(cid:173)
`conductor devices.
`
`BACKGROUND OF THE INVENTION
`
`5
`
`2
`reactive sputter deposition techniques have been used. Sput(cid:173)
`ter 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
`the 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
`that is sputtered onto the substrate, either directly or upon
`further reaction with freed target material. In DC reactive
`sputtering, a direct current electrical potential is applied
`10 within the sputtering chamber in which a reactive sputtering
`process is carried out. However, typical sputtering and
`reactive sputtering techniques, including DC reactive
`sputtering, often do not provide adequate deposition rates. A
`pulse DC sputtering method for efficiently depositing thin
`15 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 U.S. patent
`application Ser. No. 09/145,323, to Miller et al., "Pulse DC
`Reactive Sputtering Method for Fabricating Piezoelectric
`20 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
`techniques used to deposit the films, i.e., the pulse width of
`the positive portion of the applied voltage is adjusted based
`25 on its effect on the desired film constituency, stress, and
`texture.
`Magnetron sputtering systems are known in which
`magnetically-enhanced targets are used to confine the
`plasma discharge along a particular path and enhance the
`flow of target material. See, e.g., U.S. Pat. No. 5,830,327 to
`Kolenkow, "Methods and Apparatus for Sputtering with
`RotatingMagnet Sputter Sources"; U.S. Pat. No. 5,693,197
`to Lal et al., "DC Magnetron Sputtering Method and Appa(cid:173)
`ratus": and U.S. Pat. No. 5,378,341 to Drehman et at,
`"Conical Magnetron Sputter Source," all of which are incor-
`35 porated herein. Use of magnetron sputtering has been
`problematic, however, for depositing silicon dioxide films.
`Because silicon dioxide is a good insulator, a film suffi(cid:173)
`ciently thick to cause arcing problems is rapidly formed at
`certain areas of the target, i.e., splats or regions of silicon
`40 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
`45 dioxide films. See, e.g., U.S. 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
`50 dioxide films"). Additionally, previous methods of deposit(cid:173)
`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(cid:173)
`tions systems and components continue to search for new
`55 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
`60 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.
`
`Communications systems typically include a variety of
`devices ( e.g., filters, mixers, amplifiers, integrated circuits,
`and so forth). Communications systems are useful for trans(cid:173)
`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(cid:173)
`cies (e.g., PCS, ISM, etc). As systems are continually 30
`developed in response to market pressures, the demand for
`increased performance and reduced size intensifies. Market
`forces demand increased integration and reduction of com(cid:173)
`ponent size.
`Resonators such as Bulk Acoustic Wave (BAW) resona(cid:173)
`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(cid:173)
`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 µm). 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(cid:173)
`tageous alignment of the crystalline axis within the piezo(cid:173)
`electric film. An example of an advantageous film orienta(cid:173)
`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
`
`65
`
`SUMMARY OF THE INVENTION
`Summarily described, the invention embraces a quality(cid:173)
`assurance method for use in the fabrication of piezoelectric
`
`Page 6 of 10
`
`

`

`US 6,342,134 Bl
`
`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(cid:173)
`lating layer that is sufficiently low to achieve the high(cid:173)
`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(cid:173)
`ing to other aspects of the invention, the high-quality piezo(cid:173)
`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
`
`20
`
`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 aeons-
`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 25
`of the cross-over point for AlN; and
`FIG. 4 is a representative graph illustrating determination
`of the cross-over point for Si02 ; 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.
`
`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(cid:173)
`tance and low surface roughness may be used for fabricating
`the electrodes 130, 135. Previous stacked metal electrodes
`5 often have comprised Ti/TiN/Al as the composition of
`choice.
`Notably, it would be advantageous to non-destructively
`predict and assure the quality of the piezoelectric film 120
`before the film is deposited. Traditionally, methods for
`10 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
`15 collimator) in order to eliminate film thickness non(cid:173)
`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(cid:173)
`electric film is deposited. This is accomplished by fabricat(cid:173)
`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
`30 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
`35 used to provide film thickness uniformity, and such rotating
`magnet magnetrons are made possible in that process param(cid:173)
`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
`40 "cross-over point" such that the target will remain effective
`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
`45 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
`50 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(cid:173)
`tric film (FIG. 1, 135) affects the quality of the piezoelectric
`60 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(cid:173)
`lating layers 125 underlying the metal layer 135. The Bower
`65 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
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Although specific features and configurations are dis(cid:173)
`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(cid:173)
`quality piezoelectric films. FIG. 1 is a perspective schematic
`illustration of an acoustic resonator, which may be fabri(cid:173)
`cated using the inventive method. The resonator 100 com(cid:173)
`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. A bottom
`electrode 135 and top electrode 130 are disposed on opposite
`surfaces of the piezoelectric film 120.
`The layer of piezoelectric material advantageously com(cid:173)
`prises AlN, but may be made of any suitable material that
`has piezoelectric qualities sufficient for the particular reso(cid:173)
`nator application. Typical piezoelectric materials include,
`for example, quartz, zinc oxide (ZnO), and ceramic mate(cid:173)
`rials such as lithium niobate (LiNb0 3 ), lithium tantalate
`(LiTa03 ), paratellurite (Te0 2), 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 (A12 0 3 ).
`Advantageously, with the invention, the electrodes and
`particularly the bottom electrode 135 may comprise alumi-
`
`55
`
`Page 7 of 10
`
`

`

`US 6,342,134 Bl
`
`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(cid:173)
`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 2.07±0.005, and more preferably
`2.078±0.005). Film 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(cid:173)
`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(cid:173)
`electric resonator), is dependent upon the crystalline orien(cid:173)
`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 55
`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
`FWHM rocking curve of less than 3.5°, with a more
`preferred low rocking curve being less than about 2.5°, and 60
`more preferably less than 1.5° (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(cid:173)
`ricate one or more layers of the piezoelectric device, includ- 65
`ing the insulating layers (FIG. 1, 125), and/or the piezoelec(cid:173)
`tric film (FIG. 1, 120). FIG. 2 is a simplified, schematic
`
`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
`5 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
`10 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 (0 2 , N2 , etc.) into the chamber 210.
`The gases are supplied via gas delivery port 245.
`A target material is positioned within the sputtering
`15 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
`20 penetrate the target material 260 and form an arc over its
`surface facing the substrate 110. A rotation 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
`25 near the surface of the target. The increased concentration of
`ions excited by these electrons at the target surface increases
`the efficiency of the sputtering process. The pressure within
`the chamber can be regulated with pressure regulators 240,
`250 and throttle valve 270. An RF power supply 235 applies
`30 a bias voltage to the substrate platen 115 to control tensile
`film stress.
`A high-quality film may be achieved with this apparatus,
`and various parameters can be applied to optimize the
`processing conditions. Applicants have discovered a method
`35 comprising optimization of the reaction parameters to
`achieve a high-deposition rate and correct index of refrac(cid:173)
`tion. The method includes a step of determining the "cross(cid:173)
`over point" in the process. The "cross-over point" is defined
`as the point at which a pressure increase in the chamber 210
`40 becomes non-linear with the flow of reactive gases, and this
`point 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(cid:173)
`over point in a process involving fabrication of an AlN
`45 piezoelectric film, in which the reactive gas comprises
`nitrogen and the target comprises aluminum. FIG. 4 is a
`representative graph illustrating determination of the cross(cid:173)
`over point in a process involving fabrication of an Si02
`layer, in which the reactive gas comprises oxygen and the
`50 target 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.
`
`Page 8 of 10
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`US 6,342,134 Bl
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`5
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`7
`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(cid:173)
`sition process with use of rotating magnet magnetrons and
`pulsed DC power. The method includes a step of providing
`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(cid:173)
`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(cid:173)
`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
`seem 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(cid:173)
`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(cid:173)
`tive while depositing a coating on the substrate, and poi(cid:173)
`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 45
`power supply pulse width, and the deposition pressure, and
`is directly proportional to the power. The deposition rate is
`unaffected 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 55
`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. The current and/or voltage can be monitored
`and frequency adjusted to achieve a stable waveform. The 60
`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 65
`piezoelectric films and also for optimizing the processing
`conditions for depositing the insulating layers, which will
`
`8
`impact upon the texture of the piezoelectric films deposited
`thereon. As applied to the insulating layers, additional
`parameters to consider are that a reduction in the surface
`roughness of the resultant Si02 films is achieved by increas-
`ing the power and/or decreasing the pressure. According to
`the invention, these parameters may be adjusted and opt