Electrostatic discharge protection of static sensitive devices cleaned with carbon dioxide spray

An apparatus and method that enhances removal of contaminating particles from surfaces of a static-sensitive components that are cleaned using a carbon dioxide cleaning spray produced by a jet spray gun. The apparatus has a programmable power supply that is connected to ground, to the static-sensitive component, and to the jet spray gun. The static-sensitive component is cleaned using the cleaning spray and the surface charge generated on the surface of the component or substrate is simultaneously monitored to determine the amount and polarity of the charge that is generated thereon. The programmable power supply then applies a reverse bias to the jet spray gun that is equal to and has the opposite polarity of the charge that is generated on the surface of the static-sensitive component or substrate, which neutralizes the charge generated on the surface of the component.

BACKGROUND 
The present invention relates generally to cryogenic aerosol spray cleaning 
systems, and more particularly, to an apparatus and method for protecting 
static-sensitive devices from damage due to electrostatic discharge when 
they are cleaned using a carbon dioxide spray cleaning system. 
The assignee of the present invention manufactures and sells carbon dioxide 
(CO.sub.2) jet spray cleaning equipment under the ECO-SNOW.TM. brand. The 
carbon dioxide jet spray cleaning equipment uses a jet spray nozzle and 
orifice combination fed from a pressurized liquid carbon dioxide tank to 
generate a spray of CO.sub.2 snow containing solid aerosol particles and 
gas. Selection of the particular nozzle and orifice combination and tank 
pressure determines the aggressiveness of the snow when it is used to 
clean surfaces contaminated with particulates. 
It is known that cryogenic aerosol spray cleaners generate static charge on 
surfaces of components during cleaning. Unfortunately, the static charge 
buildup hinders removal of the contaminating particles from the surface of 
the component by the cryogenic aerosol spray. This is because the static 
charge buildup increases the attraction between the surface of the 
component and the contaminating particles that the cryogenic aerosol spray 
intends to remove. Furthermore, it is not desirable to increase the 
surface charge on static sensitive components, because they may be damaged 
by such charge. Typical static sensitive components include complementary 
metal oxide semiconductor (CMOS) devices and magnetoresistive read-write 
heads, for example. The CMOS devices have about a 50 volt sensitivity 
level, and the magnetoresistive read-write heads have about a 5 volt 
sensitivity level, and are thus very sensitive to electrostatic charge. 
It is therefore desirable to eliminate the charge on static sensitive 
components during cryogenic aerosol spray cleaning. This is currently done 
during cryogenic aerosol spray cleaning with a shower of ions generated by 
a corona discharge system. The ability of the corona discharge system to 
remove static charge from the static sensitive component dictates how long 
the component may be sprayed before it must be allowed to "de-stat" in the 
shower of ions produced by the corona discharge system. This is not a very 
effective way to clean static sensitive components. 
Accordingly, it is an objective of the present invention to provide an 
apparatus and method for protecting static-sensitive devices from damage 
due to electrostatic discharge when they are cleaned using a carbon 
dioxide spray cleaning system. 
SUMMARY OF THE INVENTION 
To meet the above and other objectives, the present invention provides for 
an apparatus and method that removes contaminating particles from a 
surface of a static-sensitive component or substrate that is cleaned using 
a carbon dioxide cleaning spray. The apparatus comprises a computer that 
is coupled to a programmable power supply that has one output coupled to 
ground, a second output coupled to a static-sensitive component that is to 
be cleaned using the carbon dioxide cleaning spray, and a third output 
coupled to a carbon dioxide spray gun used to clean the static-sensitive 
component. The present invention generates electrostatic charge that is 
used to balance the charge produced by the carbon dioxide spray during 
cleaning of the contaminated surface of the static-sensitive component. 
The present invention biases the cleaning spray to compensate for the 
charging of the surface of the static-sensitive component by the carbon 
dioxide cleaning spray. This is achieved using a closed loop system 
wherein a computer monitors the surface of the static-sensitive component 
and controls charge supplied by a programmable power supply to the carbon 
dioxide spray gun. As the surface of the static-sensitive component starts 
to charge with respect to earth ground, the power supply is controlled to 
add opposite polarity charge to the carbon dioxide cleaning spray. This 
continuously compensates for any charge build-up and protects the 
static-sensitive component during cleaning. 
It is necessary for the programmable power supply to be able to bias the 
surface of the static-sensitive component and the carbon dioxide cleaning 
spray both positively and negatively, because components charge in 
accordance with their relative positions on the Triboelectric scale 
relative to the position of the aerosol spray on the Triboelectric scale. 
Materials such as Teflon, for example, can have thousands of volts of 
static charge build-up after cleaning, while metals tend to have much less 
static charge build-up. 
The present method comprises the following steps. A static-sensitive 
component that is to be cleaned using a carbon dioxide cleaning spray is 
connected to a programmable power supply. The carbon dioxide spray gun 
used to spray the carbon dioxide cleaning spray is also connected to the 
programmable power supply. The programmable power supply is connected to a 
computer that is used to monitor the charge build-up on the surface of the 
static-sensitive component caused by the cleaning spray when it impacts 
the surface. As the surface charge build-up on the static-sensitive 
component increases or decreases, the computer causes the programmable 
power supply to oppositely bias the spray gun, which induces an opposite 
charge on the spray gun, in response to the increase or decrease in 
surface charge build-up of the static-sensitive component. The relative 
amount of charge on the static-sensitive component is continuously 
monitored and the charge on the spray gun is reversed biased in an amount 
equal to the charge build-up on the static-sensitive component which 
compensates for the charge build-up and protects the static-sensitive 
component during cleaning.

DETAILED DESCRIPTION 
Referring to the drawing figures, FIG. 1 illustrates apparatus 10 in 
accordance with the principles of the present invention that removes 
contaminating particles from a surface 11 of a static-sensitive component 
12 or substrate 12 that is cleaned using a carbon dioxide cleaning spray 
13 produced by a jet spray gun 18. The carbon dioxide jet spray 13 
generated by the jet spray gun 18 (or nozzle and orifice combination 18) 
fed from a pressurized liquid carbon dioxide tank 19 to generate a spray 
13 of CO.sub.2 snow containing solid aerosol particles and gas. 
The cleaning spray 13 generates a charge on the surface 11 of the 
static-sensitive component 12 or substrate 12 during cleaning, which can 
adversely affect or damage the static-sensitive component 12 or substrate 
12. The present invention minimizes or eliminates this charge build-up to 
permit complete cleaning of the surface 11 of the static-sensitive 
component 12 or substrate 12 without producing potentially harmful static 
charge thereon. 
The apparatus 10 comprises a computer 14 that is coupled to a programmable 
power supply 15 that has its outputs 16, 17a, 17b respectively coupled to 
ground, to the static-sensitive component 12 or substrate 12 that is to be 
cleaned, and to the jet spray gun 18. The present invention monitors the 
static charge build-up on the static-sensitive component 12 and generates 
a reverse-biased electrostatic charge that is applied to the jet spray gun 
18 that neutralizes the charge generated on the surface 11 of the 
contaminated component 12 or substrate 12 during spray cleaning. 
The present invention must be able to bias the surface 11 of the component 
12 or substrate 12 and the jet spray gun 18 both positively and 
negatively, because materials that make up the component 12 or substrate 
12 charge according to their relative positions on the Triboelectric scale 
relative to the position of the carbon dioxide spray 13 on the 
Triboelectric scale. Materials such as Teflon, for example, may exhibit 
thousands of volts of static charge build-up after cleaning. In contrast, 
metals typically have much less static charge build-up. 
FIG. 2 illustrates one method 20 of removing contaminating particles from a 
surface 11 of a static-sensitive component 12 or substrate 12 that is 
cleaned using a carbon dioxide cleaning spray 13. The present method 20 
comprises the following steps. 
A static-sensitive component 12 or substrate 12 that is to be cleaned is 
connected 21 to a programmable power supply 15. A jet spray gun 18 used to 
spray the carbon dioxide cleaning spray 13 is also connected 22 to the 
programmable power supply 15. The static-sensitive sensitive component 12 
or substrate 12 is then cleaned 23 using the cleaning spray 13 and the 
surface charge generated on the surface 11 of the component 12 or 
substrate 12 is simultaneously monitored 24 to determine the amount and 
polarity of the charge that is generated thereon. 
The programmable power supply 15 then caused to apply 25 a reverse bias to 
the jet spray gun 18 that is equal to and has the opposite polarity of the 
charge that is generated on the surface 11 of the static-sensitive 
sensitive component 12 or substrate 12. This application of reverse bias 
to the jet spray gun 18 neutralizes the charge generated on the surface 11 
of the component 12 or substrate 12. The monitoring of the charge on the 
surface 11 of the static-sensitive component 12 or substrate 12 may be 
easily achieved in a routine manner by appropriately programming 25 of the 
computer 14. The amount of voltage or charge applied to the jet spray gun 
18 depends upon the material from which the component 12 or substrate 12 
is made. 
Therefore, by monitoring the static charge build-up on the static-sensitive 
component 12 and generating a reverse-polarity electrostatic charge that 
is equal to the charge build-up that is applied to the jet spray gun 18, 
the charge generated on the surface 11 of the contaminated component 12 or 
substrate 12 during spray cleaning is neutralized. This allows cleaning of 
the component 12 or substrate 12 without causing damage thereto resulting 
from electrostatic charge build-up. This protects the static-sensitive 
component 12 or substrate 12 during cleaning. 
Thus, an apparatus and method of enhancing the removal of contaminating 
particles on surfaces of an electrostatically sensitive components or 
substrates when they are cleaned using a carbon dioxide cleaning spray 
have been disclosed. It is to be understood that the described embodiments 
are merely illustrative of some of the many specific embodiments which 
represent applications of the principles of the present invention. For 
example, additional cryogenic aerosols such as nitrous oxide, argon and 
xenon may be used in certain applications instead of a carbon dioxide 
spray. Clearly, numerous and other arrangements can be readily devised by 
those skilled in the art without departing from the scope of the 
invention.