Method and apparatus for reducing particulate contamination in microchip processing are disclosed. The method and apparatus comprise means to reduce particle velocity toward the wafer before the particles can be deposited on the wafer surface. A reactor using electric fields to reduce particle velocity and prevent particulate contamination is disclosed. A reactor using a porous showerhead to reduce particle velocities and prevent particulate contamination is disclosed.

BACKGROUND OF THE INVENTION 
Microchip wafers can be processed by flowing processing gases over the 
surface of a wafer. Many gas flow processing steps are required to 
accomplish the required etches and depositions, and uniform processing 
conditions must be maintained over the wafer surface for the processing to 
yield functional microchips. The processing equipment also must be 
carefully designed to limit the number of impurities that are introduced. 
High yields of good microchips are only achievable with careful process 
control and minimal impurities. 
Chemical vapor deposition (CVD) is one example of such a gas flow process. 
A susceptor holds the wafer in a processing chamber. The processing gases 
are directed evenly over the surface of the wafer by forcing the gases to 
flow through one or more perforated plates, commonly called showerheads 
and typically oriented parallel to the wafer. Due to the flow restriction 
offered by the showerhead, the gas accelerates while passing through the 
showerhead and flows evenly over the wafer surface. The processing gases 
are continually exhausted from the chamber after they pass the wafer. 
A primary factor reducing the yield of good microchips from such a reactor 
is the unintended deposition of particles. Processing gases can contain 
small numbers of particle contaminants. Particle contaminants can also be 
generated from the various parts of the reactor and suspended in the gas. 
As the gas accelerates while passing through the showerhead, the entrained 
particles are also accelerated. Particles can be accelerated through 
conventional showerheads to high enough velocities to be inertially 
deposited on the wafer. Even if this acceleration is insufficient to cause 
inertial deposition, particles can still be deposited on the wafer by 
forces such as gravity and thermophoretic effects. There is a need for 
improvements to the design of processing chambers to reduce the 
contamination by particles and therefore increase the yield of good 
microchips. 
SUMMARY OF THE INVENTION 
An object of the present invention is to reduce particle contamination in 
microchip processing. 
Another object of the invention is to reduce particle acceleration in the 
showerhead. 
Another object of the invention is to decelerate particles so that reduced 
numbers are inertially deposited on the wafer. 
Another object of the invention is to use electrostatic force to reduce 
particle deposition by balancing other forces acting on contaminant 
particles, such as gravity. 
Additional objects, advantages, and novel features will become apparent to 
those skilled in the art upon examination of the following description or 
may be learned by practice of the invention. The objects and advantages of 
the invention may be realized and attained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
One embodiment of the invention uses a porous showerhead. The many tortuous 
paths through the porous showerhead serve to evenly distribute the 
processing gas flow. The tortuous paths can also reduce particle 
contamination by reducing the number of particles having high velocities 
toward the wafer. 
In another embodiment of the invention an electric field is used to repel 
charged particles from the surface of the wafer. The electric field is 
generated between a showerhead and the wafer surface. The processing gas 
flows freely and evenly in the presence of the electric field while 
contaminant particles are electrostatically inhibited from being deposited 
on the wafer surface. 
Another embodiment of the invention uses two electric fields to reduce 
particulate contamination. The processing gas flows through a first 
electric field that can prevent particles of one charge from proceeding 
toward the wafer. The processing gas then flows through a second electric 
field that can prevent oppositely charged particles from being deposited 
on the wafer surface.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a cross sectional view of a prior art CVD reactor. Processing 
gas can be introduced to the processing chamber 5 from an external source 
connected at gas inlet 10. Gas can be exhausted from the chamber 5 via 
exhaust ports 15. The wafer 25 being processed is held on a susceptor 20. 
The gas is directed evenly onto the surface of the wafer by a perforated 
first showerhead 30. Internal baffles 35 in the chamber 5 can help direct 
the gas evenly across the wafer 25 and to the exhaust ports 15. The gas 
flow above the wafer 25 is represented in the figure by streamlines 40. 
Contaminant particles 45 in the gas flow can be deposited on the wafer 25 
by their inertia or by other forces such as gravity. 
The wafer is typically 8 inches in diameter. The showerhead is typically 8 
inches in diameter, and 0.1 inch to 1.0 inch thick. The processing chamber 
can be approximately 14 inches across. The processing gases can be 
introduced to the chamber at rates from 50 SCCM to 4000 SCCM. Operating 
pressures can range from 10 mTorr to one atmosphere. 
FIG. 2 shows the CVD reactor of FIG. 1 improved by one embodiment on the 
invention. In a CVD process, contaminant particles are often charged to 
one polarity. As disclosed herein, charged contaminant particles are 
repelled from the wafer 25 by an electric field 50 generated between the 
showerhead 30 and the wafer 25. These, particles are then carded out of 
the chamber 5 with the processing gas exhaust or deposited on the lower 
surface of the showerhead 30 rather than deposited on the wafer 25. 
The electric field can be generated in various ways familiar to those of 
ordinary skill in the art. For example, if the wafer 25 is electrically 
connected to the susceptor 20, a voltage source 55 can be connected 
between the susceptor 20 and the showerhead 30 to generate the desired 
electric field. Suitable values of electric field can be on the order of 
50 to 500 volts per centimeter. In operation, particles of the same 
polarity as the wafer are repelled by the electric field. The uncharged 
gas flow is unaffected, and thus the desired processing proceeds as it 
would without the electric field. The charged contaminant particles,, 
however, are accelerated away from the wafer by the electric field, 
reducing the deposition of contaminants and thus improving the yield of 
functional microchips. 
Another embodiment of the invention is shown in FIG. 3. An second, upstream 
electric field 51 is introduced between the first showerhead 30 and an 
upstream showerhead 31. Contaminant particles of a first polarity 46 are 
repelled by the upstream electric field 51 and prevented from reaching the 
first showerhead 30. Contaminant particles with opposing polarity 45 are 
repelled by the first electric field 50 and deposited onto the surface of 
the first showerhead 30 or carried out with the processing gas exhaust, as 
described above. 
The electric fields can be generated in various ways familiar to those of 
ordinary skill in the art. Suitable values of electric fields may be on 
the order of 50 to 500 volts per centimeter. The combination of two 
electric fields can decelerate particles of either polarity, reducing 
particle contamination of the microchip. The electric fields can be chosen 
so that the upstream electric field repels particles present in the input 
gas flow and the first electric field repels particles generated from the 
showerhead. 
While the upstream electric field will trap particles of one polarity, any 
particles of opposing polarity will be accelerated toward the first 
showerhead. The value of the first electric field is chosen in view of the 
rest of the reactor design to repel particles accelerated by the upstream 
electric field. In the simplest example, the first electric field is 
greater in magnitude than the upstream electric field so that particles 
accelerated by the upstream electric field are still repelled from the 
wafer by the first electric field. 
FIG. 4 shows a prior art showerhead design. The perforations 105 comprise a 
number of various sized holes drilled through the plate 110. The plate is 
typically of a constant thickness of from 0.1 inch to 1.0 inch. The 
perforations in the showerhead distribute the processing gas flow evenly 
across the wafer. Flowing the gas through the showerhead holes creates 
local high speed gas jets. These high speed gas jets can result in high 
contaminant particle velocities toward the wafer, increasing the 
likelihood of inertial particle deposition on the wafer surface. In 
addition, the showerhead may be an additional source of contamination as 
particles sloughed off the showerhead are carried to the wafer by high 
speed gas jets. 
A further embodiment of the invention is shown in FIG. 5. The straight 
holes in the prior art showerhead have been eliminated. In their stead, 
the showerhead can be made of a porous material. The many tortuous paths 
120 through the pores can distribute the gas evenly without developing the 
high speed jets that lead to particle contamination. Suitable porous 
showerheads can be made from sintered metal, stacks of screens, or 
filters. The processing environment usually indicates that the showerhead 
be made of a metal such as aluminum. All of these are commercially 
available, and can be used according to the specific design requirements 
for chamber configuration, gas flow conditions, and processing performance 
desired. The important consideration for reducing particle contamination 
due to the showerhead is that the showerhead provide sufficient flow 
restriction to result in uniform flow distribution while the number of 
pores and the tortuosity of the paths reduce the gas jetting phenomenon 
and discourage the high speed transport of contaminant particles. 
Other embodiments of this principle will also be apparent to those skilled 
in the art. The number, magnitude, and orientation of the electric fields 
can be changed to better reduce particle contamination. For example, the 
electric fields could be generated transversely to the gas flow, 
accelerating particles toward the sides of the gas flow and away from the 
wafer surface. Combinations of the embodiments described here will also be 
apparent to those skilled in the art. For example, the use of a porous 
showerhead to decelerate particles can be combined with electric fields to 
further decelerate particles and discourage their deposition on the wafer 
surface. 
The particular sizes and equipment discussed above are cited merely to 
illustrate a particular embodiment of the invention. It is contemplated 
that the use of the invention may involve components having different 
sizes and shapes as long as the principle, the reduction of contaminant 
particle velocity toward the wafer, is followed. It is intended that the 
scope of the invention be defined by the claims appended hereto.