Megasonic cleaning with controlled boundary layer thickness and associated systems and methods

Abstract

Megasonic cleaning systems and methods of using megasonic pressure waves to impart cavitation energy proximate a surface of a microelectronic substrate are disclosed herein. In one embodiment, a megasonic cleaning system includes a process tank for containing a liquid, a support element for carrying a substrate submerged in the liquid, and first and second transducers positioned in the tank. The first transducer is further positioned and/or operated to initiate cavitation events in a bulk portion of the liquid proximate a surface of the substrate. The second transducer is further positioned and/or operated to control an interface of fluid friction between the substrate and the bulk portion of the liquid.

1 . A megasonic cleaning system for cleaning a microelectronic substrate, the system comprising:
 a process tank for containing a liquid;   a support element for carrying a microelectronic substrate at least partially submerged in the liquid;   a first transducer positioned within the process tank configured to produce a first sonic field in the liquid, the first sonic field having a first wave approach angle relative to a surface of the substrate; and   a second transducer positioned within the process tank configured to produce a second sonic field in the liquid, the second sonic field having a second wave approach angle relative to the surface of the substrate, the second wave approach angle being different than the first wave approach angle;   wherein the first transducer is configured to initiate cavitation events in a bulk portion of the liquid proximate the surface of the substrate via the first sonic field, and wherein the second transducer is configured to control an interface of fluid friction between the substrate and the bulk portion of the liquid via the second sonic field.   
   
   
       2 . The system of  claim 1  wherein the first and second wave approach angles have a difference of about 90 degrees or −90 degrees. 
   
   
       3 . The system of  claim 1  wherein the first and second wave approach angles have a difference of about 180 degrees. 
   
   
       4 . The system of  claim 1  wherein the substrate includes a localized zone at the surface of the substrate and the first and second sonic fields define a cleaning zone that is smaller than the localized zone of the substrate, and wherein the system further comprises a support arm carrying the first and second transducers, the support arm being moveable to scan the cleaning zone through the localized zone of the substrate. 
   
   
       5 . The system of  claim 1 , further comprising a controller operably coupled with the first and second transducers, the controller including a processor and a memory storing processing instructions for at least (1) operating the first transducer at a first frequency that initiates the cavitation events and (2) operating the second transducer at a second frequency that controls the interface of fluid friction. 
   
   
       6 . A system for cleaning a microelectronic substrate, the system comprising:
 a vessel for containing a liquid;   a first transducer disposed within the vessel;   a second transducer disposed within the vessel; and   a signal delivery device operably coupled with the first and second transducers, the signal delivery device, in operation, outputting a first time-varying electrical signal that oscillates the first transducer and a second time-varying electrical signal that oscillates the second transducer;   wherein the first electrical signal has a first frequency that induces cavitation phenomena in the liquid via the first transducer and the second electrical signal has a second frequency that regulates a thickness of a boundary layer in the liquid via the second transducer, and wherein the second frequency of the second electrical signal is greater than the first frequency of the first electrical signal.   
   
   
       7 . The system of  claim 6  wherein the first frequency of the first electrical signal is in a first range of about 1 MHz to 3 MHz and the second frequency of the second electrical signal is in a second range of about 2 MHz to 5 MHz. 
   
   
       8 . The system of  claim 6 , further comprising a support element for carrying a single microelectronic substrate, the support element positioning the substrate proximate the first and second transducers. 
   
   
       9 . The system of  claim 6 , further comprising a support element for carrying a batch of two or more microelectronic substrates, the support element positioning a cassette or boat carrying the substrates proximate the first and second transducers. 
   
   
       10 . The system of  claim 6  wherein the first transducer is generally perpendicular with the second transducer. 
   
   
       11 . The system of  claim 6  wherein the first transducer is generally parallel with the second transducer but not coplanar with second transducer. 
   
   
       12 . The system of  claim 6  wherein the first and second transducers are adjacent one another but orientated to deliver sonic energy at different wave approach angles. 
   
   
       13 . A cleaning method, comprising:
 communicating first sonic waves to a surface portion of a substrate that is immersed in a liquid, the first sonic waves having a first frequency and producing a boundary layer region in the liquid adjacent the surface portion of the substrate; and   adjusting a width of the boundary layer by communicating second sonic waves to the surface portion of the substrate, the second sonic waves being concurrently communicated with the first sonic waves and having a second frequency that is greater than the first frequency of the first sonic waves.   
   
   
       14 . The method of  claim 13  wherein the first sonic waves impinge on the surface portion at a first approach angle and the second sonic waves impinge on the surface portion at a second approach angle, the first approach angle being different than the second approach angle. 
   
   
       15 . The method of  claim 13  wherein the second sonic waves are communicated through the substrate en route to the surface portion of the substrate. 
   
   
       16 . The method of  claim 13  wherein the surface portion comprises a first localized surface portion, and wherein the method further comprises communicating the first and second sonic waves to a second localized surface portion of the substrate. 
   
   
       17 . The method of  claim 13  wherein adjusting the width of the boundary layer further comprises decreasing the width of the boundary layer by increasing the second frequency of the second sonic waves. 
   
   
       18 . The method of  claim 13  wherein adjusting the width of the boundary layer further comprises increasing the width of the boundary layer by decreasing the second frequency of the second sonic waves. 
   
   
       19 . A method for cleaning a microelectronic substrate, the method comprising:
 at least partially submerging the substrate in a liquid-phase fluid, the substrate including a surface having particles attached to the surface;   imparting a first waveform to the substrate via a first transducer in fluid communication with the substrate; and   removing the particles from the substrate by imparting a second waveform to the substrate via a second transducer in fluid communication with the substrate, the second transducer being askew or non-coplanar with the first transducer, and the second waveform being concurrently imparted with the first waveform.   
   
   
       20 . The method of  claim 19  wherein the first waveform induces cavitation events of a first energy in the fluid and the second waveform induces cavitation events of a second energy in the fluid, the first energy being greater than the second energy. 
   
   
       21 . The method of  claim 19  wherein the first waveform imparts cavitation energy to the particles, but the second waveform does not impart cavitation energy to the particles. 
   
   
       22 . The method of  claim 19  wherein the first and second waveforms cannot remove the particles unless they are imparted concurrently to the substrate. 
   
   
       23 . A method for cleaning microelectronic surfaces, the method comprising:
 controlling a cavitation boundary layer in a liquid using a first sonic wave at a first frequency, the cavitation boundary layer being adjacent a microelectronic surface that includes particles attached to the surface; and   inducing cavitation events in the liquid by impinging a second sonic wave on the microelectronic surface, the second sonic wave having a second frequency that is less than the first frequency of the first sonic wave, and the second sonic wave not imparting cavitation energy to individual particles without the first sonic wave.   
   
   
       24 . The method of  claim 23  wherein the cavitation boundary layer has a first thickness when the first sonic wave is imparted to the surface, and the cavitation boundary layer has a second thickness when the first sonic wave is not imparted to the surface, the first thickness being less than the second thickness. 
   
   
       25 . The method of  claim 23  wherein the second frequency of the second sonic wave is greater than 1 MHz and the first frequency of the first sonic wave is greater than the second frequency.