Drying and gas or vapor contact with solids – Process – Gas or vapor pressure is subatmospheric
Reexamination Certificate
2000-05-30
2001-02-27
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor pressure is subatmospheric
C034S412000, C034S467000, C034S497000, C034S092000, C034S558000, C134S019000
Reexamination Certificate
active
06192601
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the reduction of particle contamination on wafers, and more specifically, to the use of thermophoresis to reduce particle contamination on wafers during wafer transport within a multi-chamber or multi-station substrate processing tool.
Integrated circuit (IC) processing technology is continuously concerned with reducing the feature size of components to increase the amount of circuitry that can be packed onto an integrated circuit of a given size and to increase the speed of operation by reducing the distance that signals need to travel within such circuits. Particulates with a diameter even several times smaller than the feature size of a component can cause failure of the IC if the particulate was present at a critical location during an important process step. This problem is particularly acute for large area ICs such as microprocessors and greater than four (4) megabit memories because such ICs have an increased area over which a critical defect can occur.
The multilayer structure of some ICs also increases the effect of particulates on yield because a particulate incorporated into one level of an IC can affect not only the circuitry of that level, but also circuitry on other levels. A defect in an embedded level of an IC can propagate through overlying areas, distorting the topography and thus disrupting operation of those circuit elements. For at least these reasons, it is important to minimize the amount of particulates that come into contact with a wafer immediately before and during processing.
FIG. 1
illustrates some common particulates and particulate sizes that are present in the ambient atmosphere. Expensive, intricate clean rooms and clean room procedures are used to significantly reduce the amount of airborne particulates to which a wafer is exposed during IC fabrication. Unfortunately, clean rooms cannot prevent particulates from being produced within an integrated circuit fabrication system, such as a multi-chamber or multi-station substrate processing tool.
Several techniques have been used within individual processing chambers within such tools to help reduce the likelihood of substrate contamination, including chamber cleaning procedures. For some processing procedures, the substrate is heated within the processing chamber which helps produce thermophoretic forces capable of repelling some contaminants away from the substrate.
However, contamination of wafers can be particularly acute during wafer transport, that is, during transfer of a wafer to the processing chamber or between chambers within a multi-chamber wafer processing system. During transport, the wafer may be exposed to different gases, possibly existing at different pressures and temperatures. These gases and the chambers possibly contain different particulate matter capable of contaminating the wafers. For example, a processing chamber may have a buildup of aluminum fluoride particles on the walls which can dislodge and contaminate a wafer being transferred into the processing chamber. Further, processing system components, such as robots, valves and the like, may generate particles, notwithstanding efforts to keep these components clean.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for reducing particle contamination during wafer or substrate processing, and more specifically, reduces particle contamination during the transport of a wafer within a substrate processing system. As indicated, it is not uncommon for some substrate contamination to occur during transport of the substrate from one station to another within the substrate processing system. The present invention involves the use of temperatures and pressures within the substrate processing system in a manner which facilitates thermophoretic forces to reduce the amount of particles which may impact a substrate surface.
Thermophoresis occurs in a gas having a temperature gradient. Gas molecules located in the hotter region of the gas have greater momentum than gas molecules in the cooler region. As a result, particulate matter suspended within the gas will be impacted on one side by molecules that are more energetic than the molecules impacting the particle on the other side. The more energetic molecules in the hotter gas region have a greater momentum and, therefore, transfer more momentum to the particle than less energetic molecules in the cooler gas region. This difference in momentum transferred by the gas molecules to the particle is capable of driving some particles toward the cooler gas region.
If, for example, an object such as a substrate were heated to a temperature that exceeded the temperature of the surrounding gas environment, the heated substrate would transfer a portion of that heat to gas molecules located near the heated substrate. Gas molecules located further from the substrate surface would be heated to a lesser degree, or not at all, thereby establishing a temperature gradient in the gas. The resulting thermophoretic forces within the gas are capable of propelling at least some particles towards the cooler gas, and hence, away from the heated substrate. Depending in part on the temperature gradient established, particles of some size may experience thermophoretic effects that overcome the effects of gravity or other forces otherwise causing the particles to propagate towards the substrate.
At least a part of the present invention is the recognition that controlling a substrate's temperature, and the temperature and pressure of the substrate's environment, during substrate transfer can cause at least some particles to be repelled away from the substrate.
The present invention provides methods of reducing particulate contamination using thermophoresis during the transfer of a substrate. In one embodiment, the method includes the step of preheating a substrate in a preheater to a desired temperature. The preheated substrate is transferred from the preheater to a buffer region having a pressure therein that is between about two (2) Torr and about seven hundred and sixty (760) Torr. The preheated substrate is transferred from the buffer region to a reaction chamber. In this manner, by heating the substrate to a desired temperature and introducing the substrate into a buffer region having a desired pressure, preferably between about two (2) Torr and about seven hundred and sixty (760) Torr, gas molecules within the buffer region experience a localized heating close to the substrate and, as a result, produce thermophoretic forces which help repel particles away from the substrate surface. While some prior art systems preheat the substrate prior to transferring the substrate to a processing chamber, the transferring step typically occurs in near vacuum conditions (i.e., pressures less than about one (1) Torr and typically less than about 500 milli-Torr). As indicated the present invention uses higher pressures to help produce thermophoretic effects during substrate transferring steps.
In one aspect of the method, during the transferring steps, the substrate desired temperature and buffer region pressure are at levels sufficient to create thermophoretic effects capable of repelling at least some particles away from the substrate. In one particular aspect, particles repelled away from the substrate by thermophoresis have a diameter that is between about 0.01 microns and about 1.00 microns. Such particles can be a wide range of contaminants, for example, particles of aluminum fluoride.
In one aspect of the method, the preheater comprises a preheat chamber. Alternatively, the preheater may comprise a load lock, whereby the load lock contains multiple substrates. In another aspect, the preheating step includes preheating the substrate to a temperature that is between about 200 degrees Celsius and about 700 degrees Celsius. Heating the substrate to such a desired temperature and transferring the preheated substrate into the buffer region having a specified buffer region pressure provides for thermophoretic effects capabl
Chandrachood Madhavi
Ghanayem Steve G.
Applied Materials Inc.
Townsend and Townsend and Crew
Wilson Pamela
LandOfFree
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