Determination of residual particles on surfaces. An updated method for particle extraction using ultrasonics

Theuseofultrasoniccleaningfortheremovalofsmallparticles[1,2]fromsurfacesisacommonpracticeinmicroelectronics,optics,discdrive,andsemiconductorindustries.Theprimaryparticleremovalmechanismsinvolvepowerful burstsofenergyfrommillionsoftinycavitationimplosions,whichhelpdislodge particlesbyimpactingthesurfacewithshockwaves.Thispowerfulimpactisaccompaniedbymicro-streamingcurrentswhichproducesahighlyacceleratedliquidthatrinsesawaytheloosenedparticles.Therelativeeffectsofthesetwo mechanismsvarywithvaryingtheappliedultrasonicfrequency.Atlowerfrequencies(e.g.,40kHz),cavitationisintense,whileitbecomesmildwithincrease oftheappliedfrequency[2].Hence,recommendationsforproperharnessingof theultrasonicfieldtoachievemaximumsurfacecleanlinesswithminimumero-

sivedamagemustbebasedonevaluationandtesting.Theenergyreleasedfrom cavitationimplosionsisdirectlyproportionaltotheradiusofthegeneratedcavitiesr,whichintum,isinverselyproportionaltotheultrasonicfrequencyfused:

EimplosionocrEimplosionoc1/f Surfaceerosionofdelicatesurfacesbyultrasoniccavitationsiswellknown

[3,4]. Itisobviousthatifinadequatecleaningprocessparametersareemployed,the

desiredcleanlinessmaynotbeachieved.Residualparticlecontaminationon surfacescandegradetheperformance,productionyieldandlifeexpectancyofa product.Greaterimplicationsareexpressedfortheintegrityofcriticalsurfacesin nano-technologyapplicationssuchaswaferfabrication,integratedcircuitsand harddiskdriveindustries.Inthemedicalfield,contaminatedimplantsordevices canhavegravehealthconsequences.