Abstract:
Provided is a method of manufacturing a multi-frequency surface acoustic wave (SAW) device on a common piezoelectric substrate. The method features varying the resonant frequency of waveguide elements of the SAW device using a single etch step. The etch step removes a sub-portion of multiple layers of conductive film disposed on the substrate.

Description:
The present invention relates to integrated circuits. Specifically, the present invention is directed to a method for forming a surface acoustic wave (SAW) device. 
   A SAW device consists of coupled gratings that confine acoustic energy and leads to resonances, and of a coupled transducer that permits the excitation and detection of the acoustic waves. A typical structure of a SAW device  10  is shown in  FIG. 1 . 
   Referring to  FIG. 1 , SAW device  10  includes reflector elements  12  and  14  and a filter element  16  disposed on a substrate  18  and connected together with one or more I/O pads  20   a  and  20   b  employing interconnects  22   a ,  22   b  and  22   c . Reflector elements  12  and  14  and filter element  16  produce a waveguiding effect on acoustic waves propagating therethrough, defining a plurality of waveguide elements. 
   To that end, reflector element  12  includes a plurality of spaced-apart conductive elements  12   a  extending parallel to each other. A first busbar  12   b  is connected in common with one end of each of conductive elements  12   a , and a second busbar  12   c  is connected in common with the ends of conductive elements  12   a  disposed opposite to busbar  12   b . Similarly, reflector element  14  includes a plurality of spaced-apart conductive elements  14   a , with a first busbar  14   b  connected in common with one end thereof. A second busbar  14   c  is connected in common with the ends of conductive elements  14   a  disposed opposite to busbar  14   b.    
   Filter element  16  includes first  16   a  and second  16   b  sets of conductive and spaced-apart interdigitated transducers (IDTs). An end of IDTs  16   a  disposed opposite to IDTs  16   b  is connected in common to busbar  16   c , and an end of TDTs  16   b  disposed opposite to IDTs  16   a  are connected in common to a busbar  16   d . Each of IDTs  16   a  extends from busbar  16   c  toward IDTs  16   b  and terminates spaced-apart from busbar  16   d . Each of IDTs  16   b  extends from busbar  16   d  toward IDTs  16   a  and terminates spaced-apart from busbar  16   c . The arrangement of IDTs  16   a  and  16   b  results in an IDT  16   a  extending between adjacent IDTs  16   b  and an IDT  16   b  extending between adjacent IDTs  16   a.    
   The frequency of operation of SAW device  10  may be controlled, inter alia, as a function of the thickness of conductive elements  12   a  and  14   a , as well as IDTs  16   a  and  16   b . SAW device  10  is also easily manufactured in a cost-effective manner employing standard semiconductor processing techniques. These factors combine to make SAW device  10  desirable for use as an RF band-pass filter in portable communication devices, such as mobile phones and portable computers. As a result, there are many prior art techniques directed toward forming SAW devices. 
   U.S. Pat. No. 6,367,133 to Ikada et al. discloses, in pertinent part, a method of manufacturing a dual frequency surface acoustic wave apparatus. The apparatus includes first and second surface acoustic wave devices having different electrode film thicknesses on a common piezoelectric substrate. The method includes providing a piezoelectric substrate and forming a first conductive film on an entire surface of the piezoelectric substrate. A first resist layer is deposited over the entire surface of the first conductive film. The first resist layer is then patterned followed by a dry etch to form IDT electrodes of a first surface acoustic wave device, a short-circuit wiring electrode for establishing electrical connection between comb-shaped electrodes of the IDT electrodes, and a conductive film provided in a region including the entire area in which the second surface acoustic wave device is constructed. Thereafter, a wet etch is employed to remove the conductive film provided in the region including the entire area in which the second surface acoustic wave device is constructed. A second resist layer is then deposited on the entire surface of the piezoelectric substrate, and the substrate is then heated. In this manner, removed are portions of the second resist layer in superimposition with electrodes of the second surface acoustic wave device. This results in the formation of a second conductive film having the same film thickness as the electrode film thickness of the second-surface acoustic wave device. Thereafter, a lift process is employed to remove the second resist layer and the second conductive film deposited on the second resist. This forms the electrodes of the second surface acoustic wave device, exposes the electrodes of the first surface acoustic wave device and disconnects the short-circuit wiring electrode in the first surface acoustic wave device. 
   Japanese Unexamined Patent Application 
   Publication No. 10-190390, discloses a method of manufacturing a surface acoustic wave apparatus in which a plurality of surface acoustic wave filter devices are disposed on a common piezoelectric substrate. To that end, a conductive film is formed on the piezoelectric substrate, and a resist is formed along the entire surface of the conductive film. Patterning of the resist is performed, forming a patterned resist layer. A dry etch process is employed to form electrodes of a first surface acoustic wave device. Thereafter, deposition of a second resist layer occurs, with a portion thereof not in superimposition with the first surface acoustic wave device being patterned, forming a patterned region in the second resist layer. A conductive film is deposited over the second resist layer, with a second acoustic wave device being formed in the patterned region thereof. Then a lift-off is performed leaving two sets of electrodes having differing thickness. A drawback with these techniques for producing SAW devices is the lengthy and complex steps required to provide the same with multiple frequency operation. 
   A need exists, therefore, to provide improved techniques for producing multiple frequency surface acoustic wave devices. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method of manufacturing a surface acoustic wave device on a piezoelectric substrate. The method features varying the resonant frequency of waveguide elements, such as interdigitated transducer electrodes (IDTs), by a single etch step that removes a sub-portion of multiple layers of conductive film disposed on the substrate. To that end, one embodiment of the method includes forming a patterned resist layer, having gaps therein, on the piezoelectric substrate. The gaps in the resist layer expose portions of the piezoelectric substrate. Multiple layers of conductive material are formed with the gaps to produce a plurality of waveguide elements, such as IDTs. The waveguide elements have a resonant frequency associated therewith. The resonant frequency of a subset of the plurality of waveguide elements is varied by removing one of the multiple layers associated with the subset. To that end, an additional resist layer is disposed atop of the multiple layers of conductive films and the resist layer. A sub-section of the additional resist layer is removed to expose a sub-section of the multiple layers of conductive film and one of the layers of the same is removed. Thereafter, a lift off technique is employed to remove all material present on the substrate, excepting material associated with the waveguide elements. These and other embodiments of the invention are described more fully below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified plan view of a prior art surface acoustic wave (SAW) device; 
       FIG. 2  is a simplified plan view of a dual frequency SAW device that may be formed in accordance with the present invention; 
       FIG. 3  is a cross-sectional view showing a piezoelectric substrate with a first patterned resist layer disposed thereon to form a SAW device in accordance with a first method of the present invention; 
       FIG. 4  is a cross-sectional view of the structure shown in  FIG. 3  with an adhesion film disposed thereon, in accordance with the first method of the present invention; 
       FIG. 5  is a cross-sectional view of the structure shown in  FIG. 4  with a first conductive film disposed on the adhesion film, in accordance with the first method of the present invention; 
       FIG. 6  is a cross-sectional view of the structure shown in  FIG. 5  with a second conductive film disposed on the first conductive film, in accordance with the first method of the present invention; 
       FIG. 7  is a cross-sectional view of the structure shown in  FIG. 6  with a second resist layer disposed on the second conductive film, in accordance with the first method of the present invention; 
       FIG. 8  is a cross-sectional view of the structure shown in  FIG. 7  with a sub-section of the second resist layer being removed to expose the sub-portion of the second conductive film in superimposition therewith, in accordance with the first method of the present invention; 
       FIG. 9  is a cross-sectional view of the structure shown in  FIG. 8  with the sub-portion of the second conductive film being removed, exposing the first conductive film in superimposition with the sub-portion; 
       FIG. 10  is a cross-sectional view of the structure shown in  FIG. 9  after a lift-off technique to form a dual frequency SAW device, in accordance with the first method of the present invention; 
       FIG. 11  is a cross-sectional view showing a piezoelectric substrate with a second patterned resist layer disposed thereon to form a SAW device in accordance with a second method of the present invention; 
       FIG. 12  is a cross-sectional view of the structure shown in  FIG. 11  with an adhesion film disposed thereon, in accordance with the second method of the present invention; 
       FIG. 13  is a cross-sectional view of the structure shown in  FIG. 12  with a first conductive film disposed on the adhesion film, in accordance with the second method of the present invention; 
       FIG. 14  is a cross-sectional view of the structure shown in  FIG. 13  with a cap film disposed on the conductive film, in accordance with the second method of the present invention; 
       FIG. 15  is a cross-sectional view of the structure shown in  FIG. 14  with a second conductive film disposed on the cap film, in accordance with the second method of the present invention; 
       FIG. 16  is a cross-sectional view of the structure shown in  FIG. 15  with a second resist layer disposed on the second conductive film, in accordance with the second method of the present invention; 
       FIG. 17  is a cross-sectional view of the structure shown in  FIG. 16  with a sub-section of the second resist layer being removed to expose the sub-portion of the second conductive film in superimposition therewith, in accordance with the second method of the present invention; 
       FIG. 18  is a cross-sectional view of the structure shown in  FIG. 17  with the sub-portion of the second conductive film being removed, exposing the cap film in superimposition therewith; and 
       FIG. 19  is a cross-sectional view of the structure shown in  FIG. 18  after a lift-off technique to form a dual frequency SAW device, in accordance with the second method of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 2 , a multi-frequency SAW device  30  in accordance with the present invention includes first  32  and second  34  SAW circuits. SAW circuit  32  includes a plurality of waveguide elements, such as a pair of reflector elements  32   a  and  32   b  with a filter element  32   c  connected therebetween. Reflector elements  32   a  and  32   b  and filter element  32   c  are electrically coupled as discussed above with respect to reflector elements  12  and  14  and filter element  16 , of  FIG. 1 . Referring again to  FIG. 2 , SAW circuit  34  includes reflector elements  34   a  and  34   b  with a filter element  34   c  connected therebetween that are electrically coupled in a similar fashion. 
   Referring to both  FIGS. 2 and 3 , in accordance with one embodiment of the present invention, the method to form multi-frequency SAW device  30  commences with depositing a resist layer  40  atop of a piezoelectric substrate  42 . Substrate  42  may be formed from any known piezoelectric material, such as piezoelectric single crystal material, piezoelectric ceramic material and the like. Exemplary materials from which substrate  42  may be formed include LiTaO 3 , LiNbO 3 , Li 2 B 4 O 7 , La 3 Ga 5 SiO 14  and Pb(Zr x Ti 1−x )O 3 . Alternatively, substrate  42  may be a standard semiconductor substrate formed, for example, from silicon and coated with a layer of piezoelectric material. 
   Resist layer  40  is patterned so as to have gaps  44 ,  46 ,  48 ,  50  and  52  present therein, exposing portions  44   a ,  46   a ,  48   a ,  50   a  and  52   a  of substrate  42 . Gaps  44 ,  46 ,  48 ,  50  and  52  may have identical or differing dimensions provide portions  44   a ,  46   a ,  48   a ,  50   a  and  52   a  with identical or differing surface areas. Any type of resist material may be employed such as positive photoresist, negative photoresist and the like. Resist layer  40  may be deposited using known techniques, e.g., spin-on techniques. 
   Referring to  FIGS. 3 ,  4  and  5 , an adhesion layer  54  is deposited to cover resist layer  40  and portions  44   a ,  46   a ,  48   a ,  50   a  and  52   a  of substrate  42 . The material from which adhesion layer  54  is formed facilitates desired adhesion between substrate  42  and a conductive film  56  formed adjacent thereto. Considering that conductive film  56  may be formed from any type of conductive material, adhesion layer  54  may be formed from any material suitable to facilitate adhesion between conductive film  56  and substrate  42 . For example, conductive film  56  may be formed from titanium (Ti), aluminum (Al), nickel (Ni), tungsten (W) and copper (Cu), and adhesion layer  54  may be formed from any of the aforementioned materials, as well as titanium nitride (TiN) and/or chromium (Cr). In the present example, adhesion layer  54  is formed from titanium and conductive film  56  is formed from aluminum. The thickness of adhesion layer  54  is selected to have minimal effect on the desired operational frequencies of the resulting SAW device. In the present example, adhesion layer  54  has a thickness in a range of 20–50 angstroms. The thickness of conductive film  56  is chosen to define the operational frequency of SAW circuit  32 , shown in  FIG. 2 . Referring again to  FIG. 5 , in the present example conductive film  56  has a thickness in a range of 1,500 to 2,500 angstroms. Both adhesion layer  54  and conductive film  56  may be deposited using any known deposition technique, such as, chemical vapor deposition, physical vapor deposition, electroplating and the like. 
   Referring to  FIG. 6 , a second conductive film  58  is formed atop of conductive film  56 . Conductive film  58  may be formed from material that is the same or different from the material from which conductive film  56  is formed. The thickness of conductive film  58  is selected to define the frequency of operation for SAW circuit  34 , shown in  FIG. 2 , based upon the material employed to form second conductive film  58  and the presence of conductive film  56 . In the present example, second conductive film  58  is formed from titanium material and has a thickness in a range of 500–1,100 angstroms. 
   Referring to  FIGS. 2 ,  7  and  8 , formed atop of second conductive layer  58  is a second resist layer  60 . Resist layer  60  is deposited to facilitate subsequent patterning and removal thereof so that a portion in superimposition with SAW circuit  34  remains while the portion in superimposition with SAW circuit  32  is removed. To that end, resist layer  60  may be deposited using any known process and to achieve any known topography. In the present example, resist layer  60  is formed employing a spin-on process to completely cover second conductive film  58  and define a planar surface  62 . Thereafter, surface  62  is patterned so that a sub-section of resist layer  60  is developed away to expose a sub-portion of second conductive layer  58  in superimposition with a region  64  of substrate  42  in which SAW circuit  32  is formed. 
   Referring to  FIGS. 8 and 9 , region  64  is exposed to an etch process, e.g., wet or dry etch processes, to remove the area of second conductive film  58  in superimposition with region  64 . In this manner, the etch process varies the frequency associated with SAW circuit  32 , shown in  FIG. 2 , so that the frequency is defined by the thickness of conductive film  56  in gap  46 , shown in  FIG. 9 . Thereafter, a lift off technique is employed to remove all material from substrate  42  in superimposition with region  64 , excepting conductive film  56  and adhesion layer  54  disposed in gaps  44 ,  46  and  48 . The lift off technique also removes all material from substrate  42  in superimposition with region  66 , excepting adhesion layer  54 , as well as first  56  and second  58  conductive films disposed in gaps  50  and  52 . In this manner, SAW circuits  32  and  34  are formed on substrate  42 , shown in  FIG. 10 . 
   In accordance with another embodiment of the present invention, the method to form multi-frequency SAW device  30 , shown in  FIG. 2 , commences with deposition of a resist layer  140  atop of a piezoelectric substrate  142 , shown in  FIG. 11 . In this manner, portions of resist layer  140  are provided with gaps  144 ,  146 ,  148 ,  150  and  152  present therein to expose portions  144   a ,  146   a ,  148   a ,  150   a  and  152   a  of substrate  142 . 
   Referring to both  FIGS. 11 and 12 , an adhesion layer  154  is deposited to cover resist layer  140  and portions  144   a ,  146   a ,  148   a ,  150   a  and  152   a . A first conductive film  156  is deposited to cover adhesion layer  154 , shown in  FIG. 13 . Adhesion layer  154  and first conductive film  156  may be formed from any suitable material. In the present example adhesion layer  154  is formed from titanium and first conductive film  156  is formed from aluminum having a thickness in a range of 1,500 to 2,500 angstroms. 
   Referring to  FIG. 14 , a cap layer  157 , or etch stop layer, is deposited over first conductive film  156 . Cap layer  157  functions to prevent compromise of the structural integrity of first conductive film  156  when subjected to etchants. As a result, any suitable material may be employed to form cap layer  157 , dependent upon the etching chemistries. In the present example, cap layer  157  is formed from titanium or chromium and has a thickness associated therewith that has minimal influence on the desired operational frequency of SAW circuits  32  and  34 , shown in  FIG. 2 . 
   Referring to  FIG. 15 , following formation of cap layer  157 , a second conductive film  158  is formed atop of cap layer  157 . Conductive film  158  may be formed from material that is the same or different from the material from which conductive film  156  is formed. The thickness of conductive film  158  is selected to define the frequency of operation for SAW circuit  34 , shown in  FIG. 2 , based upon the material employed to form second conductive film  158 , shown in  FIG. 15 . In the present example, second conductive film  158  is formed from aluminum (Al) and has a thickness in a range of 2,500 to 4,500 angstroms. 
   Referring to  FIGS. 16 and 17 , formed atop of second conductive film  158  is a second resist layer  160 . Resist layer  160  is deposited to facilitate subsequent patterning and removal thereof so that a portion thereof remains in superimposition with SAW circuit  34 , shown in  FIG. 2 , while the portion in superimposition with SAW circuit  32 , shown in  FIG. 2 , is removed. Resist layer  160  may be deposited using any known process to achieve any known topography. In the present example, resist layer  160  is formed employing a spin-on process to completely cover second conductive film  158  and define a planar surface  162 . Thereafter, surface  162  is patterned so that a sub-section of resist layer  160  is developed away to expose a sub-portion of second conductive layer  158  in superimposition with a region  164  of substrate  142  in which SAW circuit  32 , shown in  FIG. 2 , is formed. 
   Referring to  FIGS. 17 and 18 , a sub-portion of second conductive film  158  in superimposition with region  164  is exposed to an etch process, e.g., wet or dry etch processes. This removes the sub-portion of second conductive film  158  in superimposition with region  164 . The presence of cap layer  157  maintains the structural and electrical integrity of conductive film  156  in the presence of the etching chemistry. In this manner, the etch process varies the frequency associated with SAW circuit  32 , shown in  FIG. 2 , so that the frequency is defined by the thickness of conductive film  156  in gap  146 , shown in  FIG. 18 . Thereafter, a lift off technique is employed to remove all material from substrate  142  in superimposition with region  164 , excepting cap layer  157 , conductive film  156  and adhesion layer  154  disposed in gaps  144 ,  146  and  148 . The lift off technique also removes all material from substrate.  142  in superimposition with region  166 , excepting adhesion layer  154 , as well as first conductive film  156 , cap layer  157  and second conductive film  158  disposed in gaps  150  and  152 . In this manner, SAW circuits  32  and  34  are formed on substrate  142 , shown in  FIG. 19 . 
   The embodiments of the present invention described above are exemplary and the scope of the invention should, therefore, not be determined with reference to the above description. Rather, the scope of the invention should be determined with reference to the appended claims along with their full scope of equivalents.