Vacuum chuck with venturi jet for converting positive pressure to a vacuum

A vacuum chuck (10) holds a semiconductor wafer (56) securely in place during manufacturing processes. An external chuck (12) has a hollow center portion receiving a spindle support (14) and shaft (16). A positive pressure is applied through the shaft to a nozzle assembly (26) that rests on the spindle support. The nozzle assembly is further housed within a cavity in an internal chuck (28) that rests within a cup in the external chuck. The nozzle assembly use a venturi jet (44) to convert the positive pressure to a vacuum. A plurality of vacuum ports (34 and 36) from the cavity of the internal chuck transfer the vacuum to an upper surface (40) of the internal chuck to hold the semiconductor wafer in place. A plurality of exhaust ports (30 and 32) from the cavity of the internal chuck exhaust gases radially across the upper surface (13) of the external chuck toward its perimeter to prevent undesired chemicals from reaching the underside of the semiconductor wafer.

BACKGROUND OF THE INVENTION 
The present invention relates in general to semiconductor device 
fabrication and, more particularly, to a vacuum chuck with a venturi jet 
for converting a positive pressure into a vacuum that securely holds a 
semiconductor wafer during manufacturing processes. 
A common process in semiconductor device fabrication is chemical etching 
where material is removed from specific areas of the semiconductor wafer 
by a chemical process. Areas on one surface the semiconductor wafer not 
intended to be etched are first masked. A liquid etchant, for example 
hydrochloric acid, is applied to the reactive surface of the semiconductor 
wafer. The etchant chemically removes material from the unmasked areas. 
The wafer is typically spun at a high rate to evenly distribute the 
etchant during the process. Etching is also used for bulk removal of 
material in thinning processes to achieve specific electrical 
characteristics. 
The semiconductor wafers are commonly processed one at a time to maintain 
strict processing standards. One technique involves holding the 
semiconductor wafer in place by grasping the edges of the wafer with pins 
during the etching process. It is undesirable to allow the etchant to 
reach the bottom unreactive surface of the semiconductor wafer. A nitrogen 
gas is blown onto the bottom of the semiconductor wafer to remove any 
etchant. As the etching process proceeds, the diameter of the 
semiconductor wafer reduces as material at the edge etches away. The 
grasping of the semiconductor wafer by the edge pins becomes less secure 
allowing movement of the wafer and possible damage. 
To solve the pin grasping problems, vacuum chucks have been used to 
securely hold the semiconductor wafer in place. The semiconductor wafer is 
positioned on the vacuum chuck and a negative pressure (vacuum) is drawn 
through the vacuum chuck to create a suction that secures the wafer in 
place. Unfortunately, the negative pressure also tends to draw liquid 
etchant around the edge to the bottom side of the semiconductor wafer. The 
liquid etchant removes semiconductor material on the bottom side of the 
wafer which is undesirable. Attempts have been made to block the migration 
of etchant from the upper surface to the bottom side of the semiconductor 
wafer including polymer coating and taping, but most if not all techniques 
involve additional processing steps and complexity that add to the overall 
cost of manufacture. 
Hence, a need exists for an apparatus that holds the wafer securely 
throughout the etching process and further prevent the etchant from 
reaching the bottom side of the semiconductor wafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a top view of vacuum chuck 10 is shown with an upper 
surface upon which a six-inch semiconductor wafer is placed. Vacuum chuck 
10 draws a vacuum on the semiconductor wafer to hold it securely in place 
during manufacturing processes such as chemical etching on the topside of 
the semiconductor wafer. The vacuum chuck also exhausts gases radially 
across its upper surface toward the perimeter to prevent chemical etchant 
from reaching the underside of the semiconductor wafer. Two 
cross-sectional views are illustrated in FIGS. 2 and 3 to show various 
ports and gas flow patterns. The dashed lines in FIG. 1 show the relative 
cross-sectional views of FIGS. 2 and 3. 
Turning to FIG. 2, external chuck 12 is fabricated from PTFE or PVDF 
teflon, or metal in the shape as shown. The figures are approximately full 
scale. A protective cover 13 is attached to the upper surface of external 
chuck 12 with screws, adhesive, or other retaining mechanism. Protective 
cover 13 is made of polyethylene, PTFE teflon, or other polymer material 
compatible with the etching processes. A slotted spindle support 14 
including metal shaft 16 protrudes through the hollow center portion of 
external chuck 12. A metal key lock 18 with slot 20 is placed in the 
bottom inlet of external chuck 12. Pin 22 extending radially from shaft 16 
slides into slot 20 of key lock 18 to lock the spindle support assembly 
into the proper position such that the top of the slotted spindle support 
reaches approximately the level of the base of cup 24 in external chuck 
12. Nozzle assembly 26 rests on top of slotted spindle support 14 with its 
stem extending down between the slots. The slots in spindle support 14 are 
deeper than the bottom stem from nozzle assembly 26 to allow purge gas to 
exhaust and join the purge gas exhausting from ports 30 and 32 which 
provides a barrier gas flow to the back side of the semiconductor wafer. 
Internal chuck 28 is fabricated from PTFE or PVDF teflon, or metal in the 
shape as shown in FIG. 2. Internal chuck 28 includes a bottom cavity of 
proper size to receive nozzle assembly 26. Gas exhaust ports 30 and 32 
extend from the nozzle assembly cavity radially to the cavity of cup 24. 
Two additional gas exhaust ports (not shown) are positioned 90 degrees 
with respect to exhaust ports 30 and 32. In the cross-sectional view of 
FIG. 3, gas vacuum ports 34 and 36 extend from the nozzle assembly cavity 
to upper surface 40 of internal chuck 28 as shown. Two additional gas 
vacuum ports (not shown) are positioned 90 degrees with respect to vacuum 
ports 34 and 36. A porous media insert 38 is placed on upper surface 40 of 
internal chuck 28 to evenly distribute the vacuum across the surface area 
of the porous media insert. Porous media insert 38 provides a uniform 
surface to support semiconductor wafer 56 while preventing contaminants 
from entering the vacuum chamber. Porous media insert 38 is preferably 
made of teflon, PVDF, polypropylene, sintered metal powders, or ceramic. 
Nozzle assembly 26 is inserted into the lower cavity of internal chuck 28 
and the resulting assembly is placed into external chuck 12 to rest on the 
base of cup 24. A spacing remains between internal chuck 28 and protective 
cover 13 such that gas exiting from the four gas exhaust ports (e.g. ports 
30 and 32) blows radially across protective cover 13 toward its perimeter. 
Nozzle assembly 26 is shown in further detail in FIG. 4 representing one 
feature of the present invention. The lower stem of nozzle assembly 26 
houses a venturi jet 44. Four slide tubes 46 (three are shown) extend 
radially in 90 degree increments from the exit of the venturi jet nozzle 
into the cavity of cup 24. Slide tubes are positioned with one opening 
adjacent to the exit of the nozzle assembly to create the vacuum pressure 
exerted against the semiconductor wafer. Slide tubes 46 are horizontally 
adjustable in position relative to the exit of the nozzle assembly for 
regulating the vacuum pressure. O-rings 48 and 50 provide vacuum seals for 
slide tubes 46. 
The operation and use of vacuum chuck 10 proceeds as follows. The backside 
of semiconductor wafer 56 is masked to isolate areas not intended for 
etching. Semiconductor wafer 56 is placed upside down on protective cover 
13. A liquid etchant, for example hydrochloric acid, is applied to the 
backside (facing up) of semiconductor wafer 56. In order to ensure 
uniformity of etchant across the reactive surface to semiconductor wafer 
56, vacuum chuck 10 rotates at say 3000 revolutions per minute. 
Vacuum chuck 10 draws a vacuum on the center region of semiconductor wafer 
56 above porous media insert 38 to hold it securely in place. A gas that 
is inert to semiconductor wafer 56 and the chemical etching processes, for 
example nitrogen, is forced upward through shaft 16 at three to eighty 
PSIG into gas entrance chamber 52 of FIG. 4. The gas creates a positive 
pressure in shaft 16 and chamber 52. Venturi jet 44 reduces the surface 
area of chamber 52 and increases the gas velocity as it flows through the 
jet nozzle into gas exit chamber 54. The high velocity gas flowing 
adjacent to one open end of slide tubes 46 creates a negative pressure and 
draws gas from surface 40 through the vacuum ports into the nozzle 
assembly cavity of internal chuck 28, which is sealed by O-rings 48 and 
50. The gas flow continues into slide tubes 46 to the low pressure nozzle 
of venturi jet 44 and out gas exit chamber 54. The pressure drop at the 
exit of venturi jet 44 is a function of its orifice size and the positive 
pressure of gas flowing up through shaft 16 feeding entrance chamber 52. 
The pressure changes according to the conservation of mechanical energy 
through conversion of fluid velocity to pressure, neglecting friction, 
expansion, and turbulence effects. The pressure change is negative, i.e. a 
vacuum, and proportional to the square of the flow rate. 
The gas drawn from the nozzle assembly cavity creates a vacuum that is 
transferred through gas vacuum ports 34 and 36 to upper surface 40 of 
internal chuck 28 where it is evenly distributed by porous media insert 38 
across the center region of semiconductor wafer 56. Porous media insert 38 
also provides a uniform surface for semiconductor wafer 56 to rest upon 
which prevents distortion or deflection of the wafer under the force of 
the vacuum. Porous media insert 38 further prevents contaminants from 
entering the vacuum chamber. The vacuum acts on the center region of 
semiconductor wafer 56 and holds it securely in place during the etchant 
spinning without causing damage to any part of the semiconductor wafer. 
The combination of the gas flowing into venturi jet 44 and the gas drawn 
through slide tubes 46 to create the vacuum exits from chamber 54 and 
flows through the four gas exhaust ports (e.g. ports 30 and 32) into cup 
24. The gas flows through the spacing between external chuck 12 and 
internal chuck 28 and exits vacuum chuck 10 at the top surface perimeter 
of internal chuck 28. The gas continues radially along the upper surface 
of external chuck 12 between protective cover 13 and semiconductor wafer 
56. The positive pressure exerted by the exhausting gas serves to form a 
gas barrier around the perimeter of vacuum chuck 10 and keeps the liquid 
etchant on the top surface of semiconductor wafer 56 from migrating to and 
chemically reacting with the underside of semiconductor wafer 56. 
By now it should be appreciated that the vacuum chuck uses a venturi jet to 
convert a positive pressure to a vacuum. The vacuum is transferred by 
ports to the center region of the semiconductor wafer holds it securely in 
place during manufacturing processes. The vacuum chuck also exhausts gases 
radially across its upper surface toward its perimeter to create a gas 
barrier and prevent undesired chemicals from reaching the underside of the 
semiconductor wafer. 
While specific embodiments of the present invention have been shown and 
described, further modifications and improvements will occur to those 
skilled in the art. It is understood that the invention is not limited to 
the particular forms shown and it is intended for the appended claims to 
cover all modifications which do not depart from the spirit and scope of 
this invention.