Abstract:
A method and apparatus for decomposing a layer of silicon oxide on a silicon wafer is described which employs the application of a heated mist of aqueous HF to the cooled wafer surface. The technique is applied to the analysis of silicon wafers for trace impurities using a scanning fluid drop to collect the residue containing the impurities after the silicon oxide has been decomposed. The novel method offers an order of magnitude increase in the rate of silicon oxide decomposition over the prior art which uses a vapor phase decomposition technique. In addition the novel method provides better control and safer disposition of the corrosive vapors over the prior art. The apparatus comprises a movable dome fitted with a carrier gas supply and a means for injecting a heated aqueous HF mist generated by a specially designed mist generator into the carrier gas flow. The flow mist droplets are drawn from the flow onto the cooled wafer surface providing a thin layer of liquid aqueous HF which reacts with the oxide layer at a faster rate than previously used HF vapor.

Description:
This is a division of patent application Ser. No. 08/968,151, filing date Nov. 17, 1997, now U.S. Pat. No. 6,053,984 A Method And Apparatus For Decomposition Of Silicon Oxide Layers For Impurity Analysis Of Silicon Wafers, assigned to the same assignee as the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes related to the analysis of impurities in silicon wafers. 
     (2) Description of Prior Art 
     The manufacture of very large scale integrated (VLSI) circuits involves hundreds of discrete processing steps beginning with the introduction of blank silicon wafers. The quality an purity of the starting silicon wafers is, without question, one of the most crucial factors in the performance of the semiconductor devices in the finished product. The current high density, high performance, low cost technology makes widespread use of the metal oxide silicon field effect transistor which depends upon a thin silicon oxide gate insulator. This gate oxide is grown by thermal oxidation of the surface of the silicon wafer. 
     Trace metallic Impurities within the wafer surface and in the chemicals used to grow layers thereon or to clean or treat oxide layers thereon have a deleterious effect on the performance of the gate oxide as well as on it&#39;s reliability. Because of these serious consequences great strides have been taken to provide the highest quality control of the starting material. Additionally, the processing of defective wafers can result in enormous yield losses. 
     Fortunately, great strides have been by taken by silicon wafer manufacturers to provide reliable substrates. Analytical methods have been found widespread use to properly qualify and characterize silicon wafers. Among these are atomic absorption spectroscopy, emission spectroscopy, inductively coupled mass spectrometry, and X-ray fluorescence. 
     A well known sampling method which has been developed and cited by Maeda, et. al., U.S. Pat. No. 4,990,459 is a vapor phase decomposition (VPD) technique. The VPD technique extracts and concentrates trace levels of metallic contaminants from the surface of a test wafer by decomposing a layer of silicon oxide with HF vapors. The residue, which contains the non volatile impurities is then collected in a small droplet of a suitable acid such as hydrofluoric acid. The droplet is systematically moved across the entire wafer surface so that all the residue is collected. The recovered droplet is then analyzed by the well known analytical methods mentioned hereinbefore. 
     Referring to FIG. 1 there is shown a cross section of a prior art sampling technique using VPD, as cited by Maeda et.al. a test wafer  10  having a silicon oxide layer  11  on its surface is placed into a closed chamber  12 . A pool of aqueous HF  13 , located elsewhere within the chamber  12 , emits HF vapors  14  which fill the chamber and, in time, decompose the silicon oxide layer  11 . The wafer is then removed and any residue on the polished side of the wafer  10  is collected by a manual method involving the passage of a collection droplet across the wafer surface by tilting the wafer, thereby rolling the drop over the entire surface. 
     In an earlier patent by the present inventors, Petvai, et.al. U.S. Pat. No. 5,569,328, the sample collection technique was greatly improved by providing automating the movement of the collection droplet. An inert carrier is used to contain the droplet as well as increase the contact area of the droplet. Not only is the reliability and reproducibility of sample collection improved by this apparatus, but the cycle time and the risk of external contamination are greatly reduced. The wafer is mounted on a table having a programmable rotation. The apparatus provides a robotic arm which transports the wafers from a cassette to a VPD chamber where HF vapors decompose the silicon oxide layer. The wafer then passes to the droplet collection station where the sample is collected by a droplet on a pre-loaded sample carrier delivered from a carousel. The entire apparatus operates in an internal class  1  environment. 
     The long time required to decompose the silicon oxide layer by the use of vapor etching technique illustrated by FIG. 1 affects the cycle time and thereby limits the production capability of the apparatus. This is especially true when thicker, thermally grown, silicon oxide layers are examined. The flash mist method provided by the current invention greatly increases the decomposition rate of the oxide layer. 
     In order to place the embodiments of this invention into a proper perspective, a brief review of the prominent details of the acid droplet fluid scanner apparatus cited by Petvai, et.al. is now given utilizing FIG. 2 which corresponds to FIG. 4 of that patent. 
     Wafers, loaded in a cassette, are introduced into the system  41  which encloses a class  1  particle environment, through a small systems interface  42  and placed on cassette stand  44 . A pickup fork  46 , under robotic control  50  transports a test wafer (not shown) from the cassette stand  44 , first to a bar code reader  51 , where the wafer is identified, and thence to a VPD etching chamber  52  wherein the silicon oxide layer is decomposed. The robotic arm  50  then delivers the wafer to a rotatable table  54  which is fitted with a vacuum chuck. A translating arm mechanism  58  retrieves a droplet carrier from a carrousel (not shown) and positions the carrier on the fork  57  near the edge of the mounted wafer. A precision liquid handler  61  on the robotic arm  50  retrieves a pre-measured volume of liquid and delivers it to the droplet carrier. The wafer table  54  is rotated in a prescribed sequence as the translational arm  58  moves the captured droplet toward the center of the wafer, thereby traversing the entire wafer surface and collecting any residue for the analysis. At the completion of this cycle, the liquid handler  61  retrieves the droplet from the droplet carrier and deposits it back either into a cup in the carousel  60  of an auto sampler, where it is retained for analysis, or onto an inert membrane fitted onto a carrier fixture designed for any of the analytical equipment of choice. The wafer is delivered to the receiving cassette  62 . The computer system  47  with accompanying keyboard  48  and mouse  49  are also shown. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an improved method for decomposition of a silicon oxide layer on a silicon wafer which is much faster than prior art methods thus enabling it&#39;s usage for real-time production lines. 
     It is another object of this invention to provide an improved method for decomposition of a silicon oxide layer on a silicon wafer which can be incorporated in a fast cycle time, production capacity, fully automated multi-wafer testing apparatus. 
     It is yet another object of this invention to provide a method and apparatus for producing ultra clean wafer surfaces as a final step in blank wafer production, and as a surface preparation step for most manufacturing processes in semiconductor manufacturing lines. 
     It is another object of this invention to describe an efficient and reliable processing station for decomposing a silicon oxide layer on a silicon wafer and collecting an analysis sample of residues by fluid scanning which can be applied as a fast cycle time, production capacity automated wafer testing apparatus. 
     It is yet another object of this invention to provide a method and apparatus for depositing ultra thin uniform liquid films on flat substrates. 
     It is yet another object of this invention to provide a method and apparatus for depositing ultra clean thin liquid films, said films being free of all detectable metallic impurities on flat substrates. 
     These objects and others which will become apparent are accomplished by an apparatus which creates an ultra clean flowing mist of liquid droplets. The mist flow is directed towards the center of a cooled substrate, whereupon it spreads radially over the surface of the substrate. The larger droplets in the stream are drawn to the surface of the cooled substrate by Bernoulli action of the flowing gas. 
     The deposited liquid layer of aqueous HF reacts with the silicon oxide layer at a much faster rate than the HF vapors used in prior art. As the liquid reacts, the oxide layer is volatilized consuming the aqueous reactant which is continuously being refreshed from the mist stream. 
     The conditions of the mist application can be controlled by the temperature of the liquid reservoir of the mist generator and the flow rate of the carrier gas. In addition, and unlike the VPD prior art, the mist application can be quickly switched on and off, thereby improving control and reducing purge time. Safety is also improved by improved containment and by way that the flow of corrosive materials may be started and stopped by the operation of a solenoid valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a prior art method of VPD. 
     FIG. 2 is a plan view of a prior art automatic fluid scanning apparatus. 
     FIG. 3 is a plan view of an automatic fluid scanning apparatus which utilizes the embodiments of the current invention. 
     FIG. 4 is a cross sectional view of a first embodiment of the flash mist chamber dome assembly of the current invention. 
     FIG. 5 is a cross sectional view of a mist generator which produces and delivers a heated mist flow the flash mist chamber of this invention. 
     FIG. 6 is a cross sectional view of a portion dome assembly of the current invention showing critical dimensions for maintaining mist flow 
     FIG. 7 is a cross sectional view of the dome assembly of the current invention including the dome lifting mechanism in the lowered position during the mist application step. 
     FIG. 8 is a cross sectional view of the dome assembly of the current invention including the dome lifting mechanism in the raised position during the droplet scanning step. 
     FIG. 9 is a cross sectional view of a second embodiment of the flash mist chamber dome assembly of the current invention. 
     FIG. 10 is a flow chart illustrating the steps used in processing a wafer with the apparatus described by this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiments of the current invention the construction of a flash mist chamber and a method for its use in decomposing a layer of silicon oxide on a silicon wafer will be described. Although the apparatus is designed to be used for the preparation of a silicon wafer for sample collection by a fluid scanner, it is not limited to that application. The flash mist chamber is also capable of depositing ultra thin and very clean uniform films of liquids on other substrates. However, in order to provide a proper understanding of the apparatus and of its capabilities it will be described in the context of silicon oxide decomposition for chemical analysis. 
     The flash mist chamber replaces the VPD sub-station  62  (In FIG.  2 ). Unlike the prior art apparatus, the SiO 2  decomposition step and the fluid scanning operation are performed at the same station, thus eliminating the need to transport the wafer from one station to another. In addition, various other improvements to the overall design and function of the fluid scanner will also become apparent. 
     FIG. 3 is a plan view of a fully automatic fluid scanner  90  with a wafer processing station  18 . Wafer processing station  18  comprises a rotatable table  22  which is serviced by a flash mist chamber  24  which will be hereinafter described, and a translational arm  97  which performs the fluid scan. The various components of the apparatus are housed in an enclosure  91  which provides a class  1  particle environment. The components will be described in the order in which they are employed in the processing of a wafer within the fluid scanner  90 . The sequence of steps, performed as a wafer is processed in the fluid scanner, are given in the flow chart of FIG.  10  and are appropriately referenced in the description of the embodiments. 
     A silicon wafer having a silicon oxide layer over its surface is selected  300  (FIG. 10) from input cassette  92  by a robotic wafer handler  94 , for example, the Brooks Magnatran 6F wafer transfer robot manufactured by Brooks Automation, Inc., Chelmsford, Mass. (USA). The wafer is presented by the handler  94  to a bar code reader or equivalent  95  where the wafer identification is read and recorded  310  (FIG.  10 ), and thereafter delivered  320  (FIG. 10) by the wafer handler  94  to the rotatable table  22 . The dome of the flash mist chamber  24 , which is a key component of the current invention, is lowered over the wafer  330  (FIG. 10) by a pneumatic mechanism (not shown). Decomposition of the SiO 2  is then accomplished by the controlled application of a mist containing HF. The operation produces a thin “flash” coating of the liquid etchant on the wafer surface. The mist delivery is maintained until the silicon oxide layer is decomposed and the wafer surface becomes hydrophobic. The chamber  24  is then raised and the surface of the wafer is subjected to droplet scanning by the translational arm  97  which holds a droplet carrier  97 A. 
     The liquid handler  61  on robot arm  50  (FIG. 2) is now replaced by a computer interfaced random access auto sampler  96 , for example, the Model ASX-510 manufactured by Cetac Technologies, 5600 South 42nd. Street, Omaha, Nebr. (68107), USA. An integrated auto sampler carousel  60 , for example, the Model ASX-100 manufactured by Cetac Technologies is provided to manage the droplet samples. 
     Following the fluid scanning procedure  380  (FIG. 10) the wafer is retrieved from table  22  and delivered  400  (FIG. 10) to the receiving cassette  93  by the robot wafer handler  94 . The collected fluid droplet is delivered  390  (FIG. 10) to the carousel  60  for subsequent chemical analysis. 
     Computer data control is provided through an I/O port  112  and through a keyboard  110 . Supply and drain lines  102  provide DI water, vacuum, controlled gas flow, gas exhaust, chemical supply, and drain. A video camera  114  provides remote observation of the droplet collection procedure after the dome assembly  24  is raised, subsequent to the oxide decomposition process. Conventional power supplies, pumps, exhaust gas scrubbers, temperature and gas flow controllers, are located externally to the enclosure  91 . 
     Referring now to FIG. 4 there is shown a cross sectional view of the essential features of a flash mist chamber  24  engendered in a first embodiment of this invention. A wafer processing station  18  is provided with a rotatable table  22  which is fitted with a vacuum chuck to hold a test wafer  20 . The table is fabricated from a chemically resistant material which must also be capable of being machined to a flatness of about 10 microns RMS. A preferred material, which has superior chemical resistance and excellent machinability is “Ultem 2300™”, a chemically inert polymer with embedded glass fibers, manufactured by the General Electric Company. The rotation of the table  22  is controlled by a programmable stepping motor (not shown). 
     A cylindrical dome assembly  24  having a cup shaped portion  25  and an axial tube  26  is located over the wafer  20 . The axial tube  26 , through which a mist flow is discharged over the wafer  20  has an inside diameter of about ¾ inch and passes through the cup shaped portion  25  of the dome assembly  24  forming an extension  26 A within the cup shaped portion  25 . The wafer dome assembly  24 , table  22 , and other components exposed to the corrosive chemicals used in this invention are constructed of a chemically inert material, for example, polypropylene. An infrared temperature sensor  36  is mounted on the dome  25  to monitor the wafer surface temperature. For large diameter wafers, multiple temperature sensors could be fitted on the dome to monitor wafer surface temperature. 
     A mechanical means, which will be hereinafter described, is provided to raise and lower the dome assembly  24  over the wafer  20 . The dome assembly  24  is set in a raised position during wafer mount and dismount and during the droplet scanning procedure. FIG. 4 shows the dome assembly in the lowered position wherein the station  18  is configured for the application of a flowing mist which decomposes the silicon oxide layer  21  on the surface of the wafer. 
     At the upper end of the axial tube portion  26 , a side tube  31  connects to a mist generator  33 . The mist generator  33  produces a fine mist of liquid droplets of aqueous HF in a nitrogen carrier gas, the size and distribution of which is determined by the operation of the mist generator. A suitable etchant composition contained in the mist generator  33  is a 10:1 dilution of commercially available semiconductor grade 49%HF. The side tube  31  is preferably fitted with a directional microwave source  82  which re-heats the mist entering tube  26  to the proper temperature and also oxidizes trace impurities. Alternatively a resistance heating collar or an rf source may be used to re-heat the mist. 
     A second mist generator  43  is connected to the column  26  via side tube  41 . Mist generator  43  provides a mist of water droplets in a nitrogen carrier gas which is applied prior to the HF decomposition step to cool the wafer surface. Details of the mist generators  33  and  43  will be hereinafter described. Additional mist generators may be added to the dome assembly  18  to permit successive and/or concurrent application of other mist compositions, for example an organic solvent. 
     The mist flow  35  from either mist generator travels radially across the wafer at a speed determined by carrier gas flow controllers  32 , 42 . The gas mist flow  35 C across the wafer  20  creates a low pressure region above the wafer  20  surface due to the Bernoulli effect. This draws the heavier liquid droplets from the flow and onto the surface of the wafer where they adsorb and coalesce over the silicon oxide layer  21 . 
     In operation the wafer surface is initially cooled  340  (FIG. 10) by the application of a water mist from mist generator  43 . This mist flow  35 A cools the wafer surface, largely by evaporative cooling, to a temperature of about 10° C. When this temperature is achieved as indicated by the IR sensor  36 , the cooling mist flow  35 A is halted and the HF mist flow  35 B is begun  350  (FIG. 10) from mist generator  33 . The deposited liquid layer, now containing HF, reacts with the silicon oxide layer  21 , decomposing it by the general reaction: 
     
       
         6HF+SiO 2 →SiF 6 +2H 2 O+H 2    
       
     
     The dome  25  is designed to distribute the gas flow to provide uniform coverage of the wafer by the depositing liquid film. The products of the reaction are volatile and are taken into the flow, exiting along with undeposited mist, at the edge of the wafer where they are exhausted through receiving channels  38 . The receiving channels  38  direct the flow to conventional scrubbers located elsewhere. As can be seen in the figure, the flow stream of the mist mixture is well contained within the system. Thus, under normal operation, the surrounding components of the apparatus are not subjected to the corrosion by escaping fumes. The walls of the dome assembly may also be heated to further prevent contamination of interior dome walls. 
     The decomposition proceeds until the entire silicon oxide layer  21  has been volatilized, leaving behind a residue containing any non-volatile contaminants which were initially present in the oxide layer. The endpoint of the operation is determined by the onset of hydrophobicity of the wafer surface. The use of a transparent material for the dome assembly  24  or the provision of a window in the dome assembly  24  permits the observation of the wafer surface. However, operation of the station in a production mode usually permits the use of a specified time period to assure completion of the oxide decomposition. 
     The mist flow for the decomposition process, as determined by the flow controller  32 , is experimentally optimized to the chemical exchange rate between the HF and the silicon oxide  21 . The deposition rate of the HF mist is between about 1.5 and 2.5 μl/cm 2  min. and preferably about 2 μl/cm 2 /min. 
     Upon completion of the oxide decomposition, the mist flow  35 B is halted by stopping the flow of nitrogen through the mist generator  33  by the flow controller  32 . The remaining mist within the dome area is purged  360  (FIG. 10) by re-starting nitrogen flow through mist generator  43 . Alternatively and preferably, an separate nitrogen line to the dome assembly  24 , independent of the mist generators, may be used to purge the system. The dome assembly  24  is then raised out of the way to allow access of the translational arm of the fluid scanner enabling the fluid droplet sample collection procedure. 
     The mist generator  33  which is an essential part of this embodiment is shown in a detailed cross section in FIG.  5 . The body of the generator consists of a lower portion  72  which forms a reservoir for the chemical etchant  73  and an upper portion  71  which contains a system of baffles  74  which return larger liquid droplets to the reservoir and stabilize the mist. The body of the generator  70  is constructed of a chemically inert material such as polypropylene or Teflon™. A gas inlet tube  76  is provided to admit a mist generating and carrier gas, typically nitrogen, into reservoir  72  discharging it though a bubbler  78 . Mist particles are formed during the passage of the carrier gas through the liquid chemical etchant  73  which is heated by a resistance heater  80  located within the reservoir  72 . The temperature of the chemical etchant in the reservoir is sensed by a thermocouple  84  within the liquid  73 , which controls the operation of the resistance heater  80 . 
     Optionally, a fluid level sensor  86  may be provided to signal an external etchant supply to dispense etchant through inlet  88 , whereby a constant level of liquid may be maintained within the generator. The droplet size distribution and mist concentration in the output stream are controlled by the temperature of the liquid  73  in the reservoir, the nitrogen flow rate, and by the system of baffles  74 . Larger droplets in the mist stream are returned to the reservoir by the baffles  74 . The arrangement of the baffles  74  must be determined experimentally in order to accomplish the desired droplet size of the mist stream exiting the bubbler. A preferred droplet diameter in the current application is 10 microns or thereabout. 
     A directional microwave source  82  at the mist generator discharge tube  31  is used to re-heat the mist droplets which have cooled in their travel through the baffles of the mist generator. A commercially available microwave source having a capacity of about 10 watts is suitable for this application. The application of microwave energy to re-heat the mist droplets has the added advantage of elimination of trace metallic impurities from the mist. Re-heating of the mist ensures the proper temperature differential between the mist and the wafer surface upon which deposition takes place. This temperature differential is critical to the speed of the process, but not to the process itself. The preferred mist temperature is 40° C. or thereabout and a temperature differential of 30° C. or thereabout. 
     Alternatively, re-heating may be accomplished with a resistance heater wrapped around the output tube  31  of the mist generator in place of the microwave source  82 , however, the benefits of microwave mist purification are sacrificed. An rf coil surrounding the discharge tube  31  may also be used. 
     Mist generator  43  (FIG. 4) is basically identical to mist generator  33  with the exception that water is contained in its reservoir and mist heating is not required. 
     FIG. 6 shows is a cross section of the lower portion of the dome assembly in the lowered position showing the dimensions which are critical to maintaining proper laminar flow and a uniform steady state liquid deposition over the wafer. The opening of the axial tube  26 A from which the mist flow is discharged over the wafer  20  is between about 1 and 4 inches above the wafer surface  201 . The lower lip of the dome assembly  25  is between about 0.5 and 1.0 cm above the plane of the wafer surface  202 . The base of the mist laminar flow region is less than about 1 mm above the wafer surface  203 . 
     FIG. 7 is a cross section showing the dome assembly in the lowered position over the mounted wafer  20  and illustrating the pivoted raising/lowering mechanism  210  which is operated by a computer controlled pneumatic cylinder  212  and anchored on the base plate  214  of the system enclosure. two pneumatically operated guide pins  216  located below the lower lip of the dome  25  are in a raised position as the dome assembly  24  is lowered into position over the wafer. They serve to bring the dome to rest at the proper spacings as illustrated in FIG.  6  and concentric with the wafer edge. After the dome has come to rest, the pins  216  are retracted so as not to disrupt the mist flow during the flash mist application. 
     FIG. 8 is a cross section showing the dome assembly  24  in the raised position as it appears during the fluid scanning operation  380  (FIG.  10 ). The translational arm  97  with the fluid droplet carrier  97 A is shown extended over the wafer  20  as it performs a surface scan. 
     In a second embodiment of the invention a flash mist assembly is described which uses a vortex tube instead of a water droplet mist to provide cooling of the wafer surface prior to the application of the silicon oxide decomposing HF mist. The vortex tube was developed in 1930 by the French physicist George Ranque. A compressed gas is cause to flow in a spiral pattern in a tube forming a vortex. Upon reaching the end of the tube, some of the flow exits the tube and some is forced back, passing through the center of the vortex at a lower speed. Heat exchange takes place between the slower and faster moving streams, thereby causing cooler gas to emit from one end of the tube. By directing a flow of nitrogen cooled by a vortex tube at the wafer, the surface may be cooled to proper temperature in less than 10 seconds. 
     Referring to FIG. 9, there is shown a flash mist dome assembly which has a vortex tube  250  fitted to the end of tube  26 . Nitrogen is admitted by flow controller  252  and cooled by the action of the vortex tube  250  emitting a cooled nitrogen flow  254  towards the wafer  20 . Other components of the dome assembly are identical to those of the first embodiment. 
     Vortex tubes are commercially available in many capacities from the Vortec Corporation, 3770 Ridge Pike, Collegeville, Pa. 19426. A unit having a rating of 8 SCFM or thereabout is suitable for cooling a wafer for flash mist application of the current invention. 
     The operation of flash mist oxide decomposition station will now be described with reference to FIGS. 5,  7 ,  9 , and  10 . With the dome assembly  24  in the raised position, the test wafer is placed on the rotatable table  22  by the handler  94  Vacuum is applied to the chuck and a slow table rotation is begun. The dome assembly  24  is lowered by the pneumatic cylinder  212  over the wafer to the specified dome-to-wafer spacing, and the stop pins  216  are retracted. 
     A nitrogen flow  254  is begun though the vortex tube  250  by activating the flow controller  252 . The flow is maintained for about 15 seconds thereby cooling the wafer surface  340  (FIG. 10) to a temperature of between about 5° C. and 20° C. and preferably at 10° C. or thereabout whereupon the nitrogen flow through the vortex tube  250  is shut off. Meanwhile the fluid reservoir of the mist generator  33  is filled to a working level with an aqueous HF solution, for example a 10% HF/H2O solution. The heater  80  is activated and the temperature of the HF solution is raise to a temperature of 40° C. or thereabout. 
     The solenoid valve  32  next opened causing nitrogen to flow through the bubbler  78 . of mist generator  33 . A mist flow begins above the reservoir and is stabilized by the baffles  74 . The stabilized mist flow is then introduced into the axial tube  26  passing through the field of the microwave source  82  wherein the mist is heated to a temperature of between about 30° C. and 40° C. The microwave unit also eliminates trace metallic impurities from the mist flow. The mist flow is maintained for a period of time sufficient to decompose the entire silicon oxide layer  350  (FIG.  10 ). Under the conditions cited in this embodiment, the rate of removal of the silicon oxide layer is approximately 100 Å/minute. In the prior art VPD technique a 360 Angstrom silicon oxide layer requires 5 minutes for complete decomposition. Using the application of acid mist according to the current invention the same layer can be decomposed in approximately 15 seconds. 
     When the silicon oxide decomposition has been completed, the mist flow is stopped by the closure of solenoid valve  32 . Nitrogen is then flowed through the dome assembly  24  for a period of  5  seconds or thereabout to purge the assembly of etchant vapors  360  (FIG.  10 ). The dome assembly  24  is then raised  370  (FIG. 10) to provide access to the wafer by the translational arm  97  which collects the residue sample from the wafer  20  by scanning the wafer surface with a liquid acid droplet supported in the carrier  97 A. The subsequent steps including the scanning droplet sample collection step  380  (FIG. 10) and the return of the wafer to a receiving cassette for further processing  400  (FIG. 10) are well known and are discussed in detail in the references. 
     The dome assembly with the attached mist generator with wafer cooling by either water mist as in the first embodiment or by vortex tube as in the second embodiment may be used separately from the fluid scanner apparatus for the deposition of other liquids onto substrate surfaces. The apparatus may be used to deposit thin uniform liquid layers onto substrates where such layers may be required. The extraordinary control of the mist/gas mixture attainable by the current apparatus is due in large part by the capability of instantaneous re-heating of the mist at the generator discharge. Re-heating at this point provides excellent control of the temperature differential between the mist and the substrate upon which deposition takes place. The temperature differential is critical to the speed of the process, but not the to the process itself. 
     The flash mist process of this invention permits the rapid decomposition and volatilization of silicon oxide films having thicknesses several microns thick. The depth of penetration of a thick oxide layer is operator controlled. 
     Whereas the embodiments of this invention illustrate the use of HF for the decomposition of silicon oxide films, the apparatus and method taught is not limited to this application. Other materials may similarly be controllably volatilized by other etchants where such volatilization is possible. 
     While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.