Opto-acoustic methods and apparatus for perfoming high resolution acoustic imaging and other sample probing and modification operations

Abstract

One aspect of the present invention concerns scanning acoustic microscopes in which sound waves used for imaging purposes are generated by an opto-acoustical process. A scanning acoustic microscope of the present invention includes an opto-acoustic transducer assembly having a substrate. Formed in the substrate of the opto-acoustic transducer assembly is a layer of opto-acoustic material. When pulsed light waves impinge the layer of opto-acoustic material, pulsed sound waves are created. An acoustic lens also formed in the substrate focuses the pulsed sound waves which are then used to probe the physical and mechanical properties of a sample object. Pulsed sound waves reflecting off the sample object return to the opto-acoustic transducer where the pulsed sound waves impinge the layer of opto-acoustic material. The impinging sound waves change at least one optical property of the layer of opto-acoustic material. This change, which is dependent on changes to the pulsed sound waves caused by the interaction of the pulsed sound waves and the sample object, is then sensed using pulsed light waves. In one possible embodiment of the present invention, the layer of opto-acoustic material is deposited on the substrate in a plurality of non-contiguous concentric rings. The plurality of non-contiguous concentric rings operates as an acoustic analogue of a Fresnel lens.

1 . An opto-acoustic transducer assembly comprising: 
 a substrate comprised of material which is transparent to light used with the opto-acoustic transducer assembly;    a layer of opto-acoustic material formed on a surface of the substrate, where the layer of opto-acoustic material generates sound waves when struck by light; and    an acoustic lens to focus sound waves generated by the layer of opto-acoustic material.    
   
   
       2 . The opto-acoustic transducer assembly of  claim 1  where the acoustic lens comprises a concave cavity formed in the substrate.  
   
   
       3 . The opto-acoustic transducer assembly of  claim 2  where the layer of opto-acoustic material is deposited on a surface of the substrate bounding the concave cavity.  
   
   
       4 . The opto-acoustic transducer assembly of  claim 2  where the concave cavity is semi-hemispherical, whereby sound waves generated by the layer of opto-acoustic material when struck by light are focused substantially to a point focus by the acoustic lens.  
   
   
       5 . The opto-acoustic transducer assembly of  claim 2  where the concave cavity is partially cylindrical, whereby sound waves generated by the layer of opto-acoustic material when struck by light are focused substantially to a line focus by the acoustic lens.  
   
   
       6 . The opto-acoustic transducer assembly of  claim 2  where the layer of opto-acoustic material and the acoustic lens are formed on opposite sides of the substrate.  
   
   
       7 . The opto-acoustic transducer assembly of  claim 1  where the opto-acoustic material comprises As 2 Te 3 .  
   
   
       8 . The opto-acoustic transducer assembly of  claim 1  where the layer of opto-acoustic material is deposited on the surface of the substrate in a plurality of non-contiguous concentric rings, and whereby the non-contiguous concentric rings further comprise the acoustic lens which focuses the sound waves generated by the layer of opto-acoustic material substantially to a point focus.  
   
   
       9 . The opto-acoustic transducer assembly of  claim 1  where the layer of opto-acoustic material vibrates after being struck by a single pulse of light of predetermined characteristics for a predetermined number of cycles at a predetermined frequency before damping out, where the predetermined number of cycles is dependent upon respective acoustic impedances of the layer of opto-acoustic material and the substrate.  
   
   
       10 . The opto-acoustic transducer assembly of  claim 9  where the predetermined frequency is at least 15 GHz.  
   
   
       11 . The opto-acoustic transducer assembly of  claim 9  where the predetermined number of cycles is between 2 and 10, inclusive.  
   
   
       12 . The opto-acoustic transducer assembly of  claim 1  where the layer of opto-acoustic material is deposited on the surface of the substrate in a plurality of non-contiguous parallel strips, and whereby the non-contiguous parallel strips further comprise the acoustic lens which focuses the sound waves generated by the layer of opto-acoustic material substantially to a line focus.  
   
   
       13 . The opto-acoustic transducer assembly of  claim 8  where the layer of opto-acoustic material is formed from at least one of GaN; InGaN; Cr.  
   
   
       14 . The opto-acoustic transducer assembly of  claim 8  where a central region defined by an innermost ring of opto-acoustic material is transparent and functions as a sub-wavelength optical aperture for a near-field optical microscope.  
   
   
       15 . The opto-acoustic transducer assembly of  claim 8  where the plurality of non-contiguous concentric rings is formed from multiple layers deposited on the surface of the substrate.  
   
   
       16 . The opto-acoustic transducer assembly of  claim 15  where one of the multiple layers comprises GaN.  
   
   
       17 . The opto-acoustic transducer assembly of  claim 15  where one of the multiple layers comprises InGaN.  
   
   
       18 . The opto-acoustic transducer assembly of  claim 15  where one of the multiple layers comprises Cr.  
   
   
       19 . The opto-acoustic transducer assembly of  claim 1  where the substrate comprises sapphire.  
   
   
       20 . The opto-acoustic transducer assembly of  claim 1  whereby the acoustic lens of the opto-acoustic transducer assembly is operative to collect sound waves originally generated by the layer of opto-acoustic material after the sound waves have interacted with a sample object, and where a property of the layer of opto-acoustic material changes when the collected sound waves impinge the layer of opto-acoustic material.  
   
   
       21 . The opto-acoustic transducer assembly of  claim 20  where the property is the reflectivity of the opto-acoustic material.  
   
   
       22 . The opto-acoustic transducer assembly of  claim 20  whereby the change in a property of the layer of opto-acoustic material caused by sound waves impinging the layer of opto-acoustic material in turn changes at least one of a following property of light of predetermined characteristics impinging the layer of opto-acoustic material: an intensity of the light; a phase of the light; a direction of the light; a polarization of the light.  
   
   
       23 . The opto-acoustic transducer assembly of  claim 1  where the opto-acoustic transducer assembly comprising, in part, a layer of opto-acoustic material which generates sound waves when struck by light comprises a first opto-acoustic transducer assembly, the first opto-acoustic transducer assembly operating in combination with: 
 a second opto-acoustic transducer assembly, where the second opto-acoustic transducer assembly is operative to collect sound waves originally generated by the first opto-acoustic transducer assembly after the sound waves have interacted with a sample object, the second opto-acoustic transducer assembly comprising: 
 a second substrate;  
 a second layer of opto-acoustic material formed on a surface of the second substrate whereby when sound waves impinge the second layer of opto-acoustic material a property of the opto-acoustic material changes; and  
 a second acoustic lens.  
   
   
   
       24 . The opto-acoustic transducer assembly of  claim 23  where the property of the second layer of opto-acoustic material which changes is a reflectivity of the second layer of opto-acoustic material.  
   
   
       25 . The opto-acoustic transducer assembly of  claim 23  whereby the change in a property of the second layer of opto-acoustic material caused by sound waves impinging the second layer of opto-acoustic material in turn changes at least one of a following property of light of predetermined characteristics impinging the second layer of opto-acoustic material: an intensity of the light; a phase of the light; a direction of the light; a polarization of the light.  
   
   
       26 . A transducer assembly for use in an instrument which is in turn used to probe a sample object, the transducer assembly having dual piezo-electric and opto-acoustic modes of operation, the transducer assembly comprising: 
 a substrate;    a layer of opto-acoustic material deposited on a surface of the substrate, the layer of opto-acoustic material being electrically conductive;    a layer of piezo-electric material deposited on the layer of opto-acoustic material;    a layer of electrically conductive material deposited on the layer of piezo-electric material;    an acoustic lens; and    whereby when a pulsed electric potential is applied between the layer of opto-acoustic material and the layer of conductive material, a pulsed electric field is created which causes the layer of piezo-electric material to deform and thereby create pulsed sound waves which are focused by the acoustic lens and then used to probe the sample object, and whereby after the pulsed sound waves interact with the sample object and are collected by the acoustic lens, the collected pulsed sound waves impinge the layer of opto-acoustic material thereby changing at least one optical property of the layer of opto-acoustic material.    
   
   
       27 . A scanning acoustic microscope comprising: 
 at least one light source for generating pulsed light, where the pulsed light will be used at least in a pump mode to generate pulsed sound waves to interact with a sample object to be probed using the pulsed sound waves;    at least one opto-acoustic transducer assembly comprising: 
 a substrate comprised of material which is transparent to the pulsed light generated by the at least one light source;  
 a layer of opto-acoustic material formed on a surface of the substrate, where the layer of opto-acoustic material generates pulsed sound waves when struck by the pulsed light generated by the at least one light source;  
 an acoustic lens to focus pulsed sound waves generated by the layer of opto-acoustic material;  
   a pump mode optical assembly for coupling the pulsed light generated by the at least one light source to the substrate of the opto-acoustic transducer assembly, where the pulsed light coupled to the opto-acoustic transducer assembly by the pump mode optical assembly is used to generate sound waves to interact with the sample object;    a table for mounting the sample object to be probed; and    a computer control for controlling the operation of the scanning acoustic microscope.    
   
   
       28 . The scanning acoustic microscope of  claim 27  further comprising: 
 a probe mode optical assembly for coupling pulsed light generated by the at least one light source to the at least one opto-acoustic transducer assembly, where the pulsed light generated by the at least one light source coupled to the at least one opto-acoustic transducer assembly by the probe mode optical assembly is used to sense a change in an optical property of the layer of opto-acoustic material caused by sound waves after they have interacted with the sample object being probed by the scanning acoustic microscope.    
   
   
       29 . The scanning acoustic microscope of  claim 28  where the optical property is a reflectivity of the layer of opto-acoustic material.  
   
   
       30 . The scanning acoustic microscope of  claim 28  whereby the change in a property of the layer of opto-acoustic material caused by sound waves impinging the layer of opto-acoustic material in turn changes at least one of a following property of light of predetermined characteristics impinging the layer of opto-acoustic material: an intensity of the light; a phase of the light; a direction of the light; a polarization of the light.  
   
   
       31 . The scanning acoustic microscope of  claim 28  where the scanning acoustic microscope operates in a reflection mode, whereby pulsed sound waves are reflected by the sample object back to the at least one opto-acoustic transducer assembly, and where the scanning acoustic microscope further comprises: 
 a beamsplitter for splitting pulsed light generated by the at least one light source, where resulting component pulsed light beams are coupled to the pump mode optical assembly and the probe mode optical assembly, respectively.    
   
   
       32 . The scanning acoustic microscope of  claim 28  whereby the scanning acoustic microscope operates in a reflection mode, and where: 
 the at least one light source further comprises a first and second light source, where the first light source generates pulsed light for coupling to the pump mode optical assembly and where the second light source generates pulsed light for coupling to the probe mode optical assembly.    
   
   
       33 . The scanning acoustic microscope of  claim 32  where the first light source coupled to the pump mode optical assembly is optimized for operation with the layer of opto-acoustic material to generate pulsed sound waves.  
   
   
       34 . The scanning acoustic microscope of  claim 32  where the second light source coupled to the probe mode optical assembly is optimized for operation with the layer of opto-acoustic material to measure changes in an optical property of the layer of opto-acoustic material caused by sound waves impinging the layer of opto-acoustic material.  
   
   
       35 . The scanning acoustic microscope of  claim 28  whereby the scanning acoustic microscope operates in a transmission mode, and where: 
 the at least one opto-acoustic transducer assembly further comprises a first and second opto-acoustic transducer assembly, where the first opto-acoustic transducer assembly is coupled to the pump mode optical assembly and generates pulsed sound waves for interacting with the sample object to be probed by the scanning acoustic microscope and where the second opto-acoustic transducer assembly is coupled to the probe mode optical assembly, where the pulsed light coupled to the second opto-acoustic transducer assembly by the probe mode optical assembly is used to sense the change of an optical property of the opto-acoustic material when pulsed sound waves transmitted by the sample object impinge the opto-acoustic material.    
   
   
       36 . The scanning acoustic microscope of  claim 35  where: 
 the at least one light source further comprises a first and second light source, where the first light source generates pulsed light for coupling to the pump mode optical assembly and where the second light source generates pulsed light for coupling to the probe mode optical assembly.    
   
   
       37 . The scanning acoustic microscope of  claim 36  where the first light source coupled to the pump mode optical assembly is optimized for operation with the layer of opto-acoustic material to generate pulsed sound waves.  
   
   
       38 . The scanning acoustic microscope of  claim 36  where the second light source coupled to the probe mode optical assembly is optimized for operation with the layer of opto-acoustic material to measure changes in an optical property of the layer of opto-acoustic material caused by sound waves impinging the layer of opto-acoustic material.  
   
   
       39 . The scanning acoustic microscope of  claim 27  further comprising a coupling fluid for coupling the sound waves generated by the opto-acoustic transducer assembly to the sample object.  
   
   
       40 . The scanning acoustic microscope of  claim 28  further comprising: 
 a photodetector for detecting the probe mode pulsed light waves after the probe mode pulsed light waves have reflected off of the layer of opto-acoustic material of the opto-acoustic transducer assembly.    
   
   
       41 . The scanning acoustic microscope of  claim 40  further comprising: 
 a polarizer for preventing scattered pump-mode light pulses from reaching the photodetector.    
   
   
       42 . The scanning acoustic microscope of  claim 28  further comprising: 
 a pump mode modulation means for modulating an amplitude of the pump mode pulsed light prior to the pump mode pulsed light impinging the layer of opto-acoustic material of the opto-acoustic transducer assembly.    
   
   
       43 . The scanning acoustic microscope of  claim 42  where the pump mode modulation means comprises an electro-optic modulator.  
   
   
       44 . The scanning acoustic microscope of  claim 42  where the pump mode modulation means comprises an acousto-optic modulator.  
   
   
       45 . The scanning acoustic microscope of  claim 42  further comprising: 
 a lock-in amplifier coupled to an output of the photodetector and an input of the pump mode modulation means for controlling the pump mode modulation means in dependence upon a signal received from the photodetector.    
   
   
       46 . The scanning acoustic microscope of  claim 28  where the pump mode optical assembly further comprises a reflector mounted on a movable stage, where the movable stage is coupled to the computer control and is operative to control a difference in arrival times between pump mode pulsed light and probe mode pulsed light at the opto-acoustic transducer assembly.  
   
   
       47 . The scanning acoustic microscope of  claim 28  where the probe mode optical assembly further comprises a reflector mounted on a movable stage, where the movable stage is coupled to the computer control and is operative to control a difference in arrival times between pump mode pulsed light and probe mode pulsed light at the opto-acoustic transducer assembly.  
   
   
       48 . The scanning acoustic microscope of  claim 42  where the pump mode modulation means operates at a first frequency, the probe mode optical assembly further comprising: 
 a probe mode modulation means for modulating the amplitude of the probe mode pulsed light at a second frequency different from the first frequency used to modulate the pump mode pulsed light, the modulation of the probe mode pulsed light occurring prior to the probe mode pulsed light impinging the layer of opto-acoustic material of the opto-acoustic transducer assembly.    
   
   
       49 . The scanning acoustic microscope of  claim 48  where the probe mode modulation means comprises an electro-optic modulator.  
   
   
       50 . The scanning acoustic microscope of  claim 48  where the pump mode modulation means comprises an acousto-optic modulator.  
   
   
       51 . The scanning acoustic microscope of  claim 28  further comprising: 
 a frequency-doubling crystal for creating a frequency component at twice a nominal frequency of the of the light source prior to the coupling of the pulsed light to the pump and probe mode optical assemblies.    
   
   
       52 . The scanning acoustic microscope of  claim 51  further comprising: 
 a dichroic mirror that transmits a frequency component of pulsed light at the nominal frequency of the at least one light source to the pump mode optical assembly and reflects the component at twice the nominal frequency to the probe mode optical assembly.    
   
   
       53 . The scanning acoustic microscope of  claim 51  further comprising: 
 a dichroic mirror that transmits a frequency component of pulsed light at the nominal frequency of the at least one light source to the probe mode optical assembly and reflects the component at twice the nominal frequency to the pump mode optical assembly.    
   
   
       54 . The scanning acoustic microscope of  claim 52  further comprising: 
 a half-wave plate for rotating a polarization of the probe mode pulsed light coupled to the probe mode optical assembly by the dichroic mirror.    
   
   
       55 . The scanning acoustic microscope of  claim 27  further comprising: 
 a near-field optical microscope, whereby the scanning acoustic microscope and near-field optical microscope provide dual, optic and acoustic, modes of operation.    
   
   
       56 . The scanning acoustic microscope of  claim 27  where the layer of opto-acoustic material is deposited on the surface of the substrate in a plurality of non-contiguous concentric rings, and whereby the non-contiguous concentric rings further comprise the acoustic lens, and where a central region formed by an innermost ring of opto-acoustic material is transparent and functions as a sub-wavelength optical aperture for a near-field optical microscope, the scanning acoustic microscope further comprising: 
 the near-field optical microscope, whereby the scanning acoustic microscope and near-field optical microscope provide dual, optic and acoustic, modes of operation.    
   
   
       57 . The scanning acoustic microscope of  claim 27  further comprising: 
 a piezo-electric transducer assembly, where the piezo-electric transducer assembly is operative to collect pulsed sound waves originally generated by the opto-acoustic transducer assembly after the pulsed sound waves have interacted with the sample object, the piezo-electric transducer assembly comprising: 
 a substrate;  
 a layer of piezo-electric material formed on a surface of the substrate, at least one electrical property of the layer of piezo-electric material changing when pulsed sound waves impinge the layer of piezo-electric material;  
 an acoustic lens formed in the substrate to collect the pulsed sound waves originally generated by the opto-acoustic transducer assembly after the pulsed sound waves have interacted with the sample object; and  
 means to measure the change in the at least one electrical property of the layer of piezo-electric material.  
   
   
   
       58 . A scanning acoustic microscope comprising: 
 a piezo-electric transducer assembly comprising: 
 a substrate;  
 a layer of piezo-electric material formed on a surface of the substrate which generates pulsed sound waves to interact with a sample object to be probed with the pulsed sound waves when a voltage is applied to the layer of piezo-electric material;  
 an acoustic lens formed in the substrate to focus the pulsed sound waves;  
 a voltage source coupled to the layer of piezo-electric material;  
   at least one opto-acoustic transducer assembly, where the opto-acoustic transducer assembly is operative to collect pulsed sound waves originally generated by the piezo-electric transducer assembly after the pulsed sound waves have interacted with the sample object, the opto-acoustic transducer assembly comprising: 
 a substrate;  
 a layer of opto-acoustic material formed on a surface of the substrate, where a property of the layer of opto-acoustic material changes when pulsed sound waves collected by the opto-acoustic transducer assembly impinge the layer of opto-acoustic material;  
 an acoustic lens to collect pulsed sound waves generated by the piezo-electric transducer assembly after the pulsed sound waves have interacted with the sample object;  
   at least one light source for generating pulsed light, where the pulsed light will be used at least in a probe mode to measure the change in a property of the layer of opto-acoustic material caused by the pulsed sound waves collected by the opto-acoustic transducer assembly impinging on the layer of opto-acoustic material after the pulsed sound waves have interacted with the sample object;    a probe mode optical assembly for coupling the pulsed light generated by the at least one light source to the opto-acoustic transducer assembly;    a table for mounting the sample object to be probed; and    a computer control for controlling the operation of the scanning acoustic microscope.    
   
   
       59 . A method for sensing physical properties of a sample object using an instrument having at least one opto-acoustic transducer assembly, the method comprising: 
 generating pulsed light waves with a light source;    directing the pulsed light waves to a layer of opto-acoustic material incorporated in the at least one opto-acoustic transducer assembly using a pump mode optical assembly;    generating pulsed sound waves through the interaction of the layer of opto-acoustic material and the pulsed light waves directed to the layer of opto-acoustic material by the pump mode optical assembly;    focusing the pulsed sound waves using at least one acoustic lens;    coupling the pulsed sound waves to a coupling medium, whereby the coupled pulsed sound waves interact with the sample object and are changed in such a way that at least one physical property of the sample object can be sensed through the change brought about by the interaction;    collecting the pulsed sound waves with the at least one acoustic lens after the pulsed sound waves have interacted with the sample object;    coupling the collected pulsed sound waves onto a detector;    sensing the properties of the sound waves using the detector; and    deriving information concerning the sample object from the properties of the sound waves sensed by the detector.    
   
   
       60 . The method of  claim 59  whereby the detector comprises the at least one opto-acoustic transducer assembly and where sensing the properties of the sound waves further comprises: 
 directing probe-mode pulsed light waves to the layer of opto-acoustic material of the at least one opto-acoustic transducer assembly using a probe mode optical assembly, where the collected pulsed sound waves impinging the layer of opto-acoustic material change at least one optical property of the layer of opto-acoustic material, and whereby the probe-mode pulsed light waves are used to measure the change in the at least one optical property of the layer of opto-acoustic material caused by the impinging pulsed sound waves.    
   
   
       61 . The method of  claim 59  whereby the detector comprises a piezo-electric device.  
   
   
       62 . The method of  claim 59 , where the instrument is used in a scanning mode.  
   
   
       63 . The method of  claim 62 , where the information comprises an image of the sample object.  
   
   
       64 . The method of  claim 62 , where the information concerns the topography of the sample object.  
   
   
       65 . The method of  claim 64 , where the method further comprises: 
 scanning the opto-acoustic transducer assembly in two dimensions across a surface of the sample object to collect information sample points, where each of the sample points concern height information about the surface of the sample object at particular (x,y) co-ordinates on the surface of the sample object.    
   
   
       66 . The method of  claim 65  where the height information is determined by adjusting the height of the opto-acoustic transducer assembly.  
   
   
       67 . The method of  claim 59 , where the information concerns the mechanical properties of the sample object.  
   
   
       68 . The method of  claim 59 , where the information concerns cracks present in the sample object.  
   
   
       69 . A method for performing a physical operation on a sample object using an instrument having at least one opto-acoustic transducer assembly, the method comprising: 
 generating pulsed light waves with a light source;    directing the pulsed light waves to a layer of opto-acoustic material incorporated in the at least one opto-acoustic transducer assembly using a pump mode optical assembly;    generating pulsed sound waves through the interaction of the layer of opto-acoustic material and the pulsed light waves directed to the layer of opto-acoustic material by the pump mode optical assembly;    focusing the pulsed sound waves using at least one acoustic lens;    coupling the pulsed sound waves to a coupling medium, whereby the focused pulsed sound waves perform a physical operation on the sample object.    
   
   
       70 . The method of  claim 69  where the physical operation is an acoustic tweezers operation.  
   
   
       71 . The method of  claim 69  where the sample object is a mask used in semiconductor fabrication and where the physical operation comprises a repair of the mask.  
   
   
       72 . The method of  claim 69  where the sample object is a mask used in semiconductor fabrication and where the physical operation comprises forming a pattern on the mask.  
   
   
       73 . The method of  claim 69  where the sample object is a semiconductor wafer and the physical operation comprises a repair of the semiconductor wafer.  
   
   
       74 . The method of  claim 69  where the sample object is a semiconductor wafer and the physical operation comprises forming a pattern on the semiconductor wafer.  
   
   
       75 . The method of  claim 69  where the sample object is a semiconductor chip and the physical operation comprises a repair of the semiconductor chip.  
   
   
       76 . The method of  claim 69  where the sample object is a semiconductor chip and the physical operation comprises forming a pattern on the semiconductor chip.