Self evacuating micro environment system

Details Self evacuating micro environment system Self evacuating micro environment system

2001-10-01

2003-10-28

Bratlie, Steven A. (Department: 3652)

Material or article handling

Apparatus for moving material between zones having different...

C414S222040, C414S222070, C414S222130, C414S939000, C414S940000

Reexamination Certificate

active

06637998

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention is directed to processing of substrates or wafers used in the manufacture of semiconductor devices. In particular, the present invention is directed to a mobile cart-based self-evacuating micro-environment system designed to transport a group of substrates in a vacuum-sealed container between processing chambers during the manufacture of semiconductor devices.
Silicon wafers having diameters up to 300 mm, and gallium arsenide wafers are used in the manufacture of semiconductor devices. Large substrates are also used in the manufacture of flat panel display devices. Many processing steps are required to fabricate devices on the surfaces of these wafers and panels (herein referred to as substrates). The steps are performed inside various tools within a fabrication building. These tools perform specialized functions, for example, layering, patterning, doping and heat treating. The partially completed devices are highly sensitive to contamination during the fabrication process. Therefore the substrates must remain in controlled environments within the tools. However, the substrates must also be transported between the various tools during fabrication. Consequently, the substrate surfaces must be protected from ambient contamination during transport. In some cases, groups of substrates are transported between tools in closed containers, or micro-environments, often referred to as standard mechanical interface (SMIF) pods. Typically, 300 mm wafers are transported in Front Opening Universal Pods (FOUPs). These containers are typically filled with clean ambient air or filtered inert gas, such as nitrogen.
The internal pressure in these transport containers is typically near the atmospheric value. Atmospheric pressure containers are convenient when interfacing with atmospheric operations such as wet processing and photolithography. However, many processing steps are conducted at reduced pressures. For example, sputter deposition is performed at pressures as low as 10
−6
Torr. Substrates received from SMIF pods must therefore be placed in intermediate loadlock chambers designed to evacuate the atmosphere around the substrates prior to processing, and to return the substrates to atmospheric pressure after processing. Such cyclic evacuation and venting of loadlock chambers consumes significant quantities of energy, thereby increasing substrate processing cost. These additional steps also reduce the productivity of the tool, since no processing can occur in an individual loadlock during evacuation or venting, although tools are typically used with multiple loadlocks, wherein while one loads, the other can be processed. The present invention can eliminate the need for these multiple loadlocks.
The above productivity problem can be lessened by evacuating and venting the loadlock chamber as quickly as possible. However, rapid evacuation, accomplished through high pumping speeds, can cause excessive adiabatic cooling of the gas, leading to condensation of trace moisture in the loadlock chamber. The condensed moisture consists partially of aerosol droplets suspended in the loadlock chamber atmosphere. The resulting water droplets scavenge and react with trace contaminants in the loadlock chamber environment, thereby producing reaction products in the form of suspended residue particles. These particles can rapidly deposit on the substrate surfaces by turbulent and convective motion, or by gravitational settling. As the pressure continues to drop in the loadlock chamber, the settling speed of the particles increases, resulting in an increased rate of particle deposition on the substrates.
The above described adiabatic cooling is opposed by natural warming provided by the loadlock chamber walls. Thus, the condensation process can be prevented by pumping-down at a sufficiently low rate that heat transfer from the loadlock chamber walls prevents excessive gas cooling. B. Y. H. Liu, T. H. Kuehn and J. Zhao in “Particle Generation During Vacuum Pump Down”,
Proceedings of the
37
th Annual Technical Meeting of the Institute of Environmental Sciences
, San Diego, Calif., May 6-10, 1991, pp. 737-740, show that the suspended particle concentration in pumped chambers is directly related to a Z number given as:
Z=&tgr;&ohgr;/&xgr;,
where &tgr; is the pumping time constant,
&tgr;=
V/S
(sec),
V is the chamber volume, S is the pumping speed, and
&xgr;=
V/A
(cm)
is the chamber volume to surface area ratio. The rate of heat penetration &ohgr; from the chamber walls to the gas is given by:
&ohgr;=[
g&agr;/Pr]
1/3
(cm/sec),
where g is the gravitational constant, the Prandtl number Pr is given by:
Pr=&ngr;/&agr;,
&ngr; is the kinematic viscosity, and &agr; is the thermal diffusivity of the gas.
Experimental tests by Liu et al. (see B. Y. H. Liu, T. H. Kuehn and J. Zhao in “Particle Generation During Vacuum Pump Down”,
Proceedings of the
37
th Annual Technical Meeting of the Institute of Environmental Sciences
, San Diego, Calif., May 6-10, 1991) showed that higher values for Z, as produced by lower pumping speeds, resulted in lower concentrations of suspended residue particles in the gas. For example, at Z=4.17, the measured particle concentration reached ~
10
4
per cm
3
, while at Z=18.5, the suspended particle concentration reached only ~1 per cm
3
. However, as stated above, low pumping speeds significantly increase processing time and thereby increase the costs associated with use of the tool. Alternatively, more rapid pumping speeds tend to produce higher concentrations of deposited residue particles on substrate surfaces, thereby significantly reducing semiconductor device yield, and increasing processing cost.
An additional significant problem encountered during the storage and transport of substrates between tools is exposure to molecular contamination released (or outgassed) particularly from the internal surfaces of plastic SMIF pods and the like. It is well known in the field of semiconductor fabrication that such molecular contaminants can produce deleterious effects on sensitive device surfaces. Such molecular contaminants tend to accumulate and increase in concentration in the pod's internal atmosphere. D. Hou, P. Sun, M. Adams, T. Hedges, and S. Govan in “Comparative Outgassing Studies on Existing 300 mm Wafer Shipping Boxes and Pods”,
Proceedings of the ICCCS
14
th International Symposium on Contamination Control
, Phoenix Ariz., Apr. 26-May 1, 1998, pp. 419-428, show that wafer pods can outgas significant quantities of volatile organic contamination, and that such contaminants can deposit on wafer surfaces. Test results showed that commonly used polymer additives with high boiling points were absorbed on wafer surfaces. Such contaminants tend to cause a further reduction in device yield.
Additional molecular contaminants, such as atmospheric moisture or oxygen, can cause undesired native oxide growth on substrate surfaces. Additionally, atmospheric contaminants, such as organics and metallics, reduce device performance and limit production yields. Such molecular and ionic contaminants can enter substrate containers during exposure to the atmosphere, or through minor leaks in non-hermetically sealed containers.
An additional problem encountered during the storage and transport of substrates between tools is exposure to particulate contamination generated internally by the substrates, transport mechanisms and containers. When substrates and loading/unloading machinery rub against other surfaces, microscopic particles are produced through abrasion. It is well known in the field of semiconductor fabrication that particles as small as 0.01 micrometer can produce substantial defects on modern semiconductor devices. Particles of this size can remain suspended for prolonged periods inside substrate containers.
FIG. 1
shows that the settling time of such microscopic particles under atmospheric pressure (760 Torr) is very long. Only under reduced container pressure can a rapid

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