JP2577162B2 - Method and apparatus for controlling a temperature difference occurring in a heated silicon substrate in a load lock chamber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method and an apparatus for controlling a temperature difference (thermal gradient) generated in a heated silicon substrate supported inside a load lock chamber.

[0002]

2. Description of the Related Art Consider, for example, a workstation in a wafer processing system for the production of memory devices. Such workstations often include a load lock chamber through which silicon wafers, one at a time, are removed from the chemical vapor deposition chamber, for example, before being sent out by a storage elevator. Wafers removed from the chemical vapor deposition chamber can be at about 360 ° C. and are not cooled explicitly on the storage elevator when held in a vacuum. Since the wafer is brittle and easily damaged, it is only supported by its outer peripheral surface area. The transfer of heat in a vacuum to the storage elevator is determined by the close contact between the wafer and the elevator. If the contour of the wafer surface does not perfectly match the planar contour of the storage elevator, the contact between the surfaces will be poor and the conductivity will be low. Even when the contours match, the roughness and light weight of the wafer surface minimizes its contact with the elevator surface.

[0003] As the load lock chamber vents, the wafer begins to cool off rapidly. This venting gas conducts heat from the wafer to the storage elevator and cools the portion of the wafer that is in contact with the storage elevator, while other portions of the wafer lose heat only by lateral conduction through the silicon. This results in a large thermal gradient within the wafer. This is believed to be the cause of numerous wafer breaks in the storage elevator. Each portion of the wafer in contact with the storage elevator contracts as it cools, while the other non-contact portions do not. This differential cooling causes a tensile load in the cold part and a compressive load in the hot part. These loads can crush or shear the wafer, depending on the orientation of this gradient.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus for controlling the thermal gradient of a hot silicon wafer or the like supported inside a load lock chamber.

[0005]

According to a method embodying the present invention that achieves the above and other objects, a heated object is placed in a vacuum-sealed load lock chamber, Next, put the chamber in two, not once
The gas is injected in stages. First, a particularly limited amount of gas is introduced and the chamber is maintained in this state for a preselected time so that the pressure in the chamber reaches a preselected intermediate level, for example 10 torr. As mentioned above, the gas thus introduced into the load lock chamber has a cooling effect on the heated object, which effect is particularly strong in the part where the object is supported by the holder, This is because the thermal conductivity of the holder is high, and the distance that gas molecules travel to take heat from the heated object and conduct it to the holder is short. When only a limited amount of gas is introduced, the rate of cooling at the part in contact with the object is
Lower than when the load lock chamber is completely ventilated. Since this introduced gas contributes little to the cooling of the non-contact part of the object, this implies that the thermal gradient in the object is reduced by reducing the rate of heat transfer to the holder at the contact part. means. At the end of this processing time, the chamber is completely vented.

[0006] Such a gradual ventilation process can be realized in both software and hardware. In a software implementation of the method described above, a valve is provided between the chamber and the high pressure supply of vent gas, and a control mechanism, which may include a computer, is first activated for a limited period of time, for example by outputting a pulse signal. Only open the valve and program after the experimentally selected residence time to finally open the valve completely. In a hardware implementation, it may include a vent reservoir and an air sequencer circuit. First, vent gas in the vent reservoir is introduced into the chamber such that its internal pressure increases to a predetermined intermediate level, and the sequencer delays and opens another valve connected to the vent gas supply, Vent the chamber completely. By using a switch valve and a volume chamber, such a sequencer can be made much like an electrical delay circuit made up of transistors and capacitors.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the thermal conductivity of silicon is 100.degree.
Is about 0.25 cal / sec · cm · ° C.
At 0 ° C. it is based in part on the observation that it is less than half this value. Another observation underlying the present invention is that the radiant transmission of a 200 mm bare wafer is greater than 85%, for example, at wavelengths of 3-10 microns. This, combined with the highly polished condition of the wafer surface or each surface thereof, means that the wafer is generally a weak source of infrared radiation. Thus, when a pressurized, hot silicon wafer is placed in a load lock chamber and placed on a holder, the removal of heat from the wafer in this vacuum environment is based on its close contact with the holder. Become. However, as explained in the background section,
Tight contact is unlikely to occur between the wafer and the holder.

When a vent gas is introduced into the load lock chamber, gas molecules that contact and absorb heat from the wafer strike the holder and thus tend to conduct heat. In the peripheral area supported by the wafer holder, the molecules travel from the wafer to the holder and thus transfer heat, and the distance from the holder to the wafer surface is reduced. In contrast, the portion of the wafer that is not in contact with the holder has a lower lateral conductivity through the silicon and much longer the vent gas molecules travel before they can contact other surfaces to transfer heat. Because of both becoming, heat cannot be lost efficiently. This causes a steep thermal gradient inside the wafer, causing stress inside the wafer and eventually breakage. The present invention is based on the observation that this thermal gradient can be controlled by adjusting the pressure (or gas density) inside the load lock chamber.

According to the present invention, the gas is injected into the load lock chamber in stages, not all at once. A limited amount of vent gas (such as nitrogen) is first introduced into the load lock chamber and before the load lock chamber is completely vented, for example to bring its internal pressure to 1 atm.
During a predetermined amount of time (location time), the interior of the load lock chamber (much lower than the final pressure level reached by the interior of the load lock chamber after complete ventilation).
Maintained at an intermediate pressure level. Therefore, the internal pressure P
FIG. 1 schematically shows the transition over time. This is in contrast to the graph of FIG. 2, which shows the change in pressure when operating the load lock chamber in the prior art mode under the same conditions.

The purpose of temporarily maintaining the interior of the load lock chamber at such a low pressure level is to reduce the thermal gradient that occurs within the wafer between the portion where it contacts the thermally conductive holder and the portion that does not. It is in.
Since the average velocity of the gas molecules decreases as the gas pressure decreases, the number of reciprocating movements of the vent gas molecules between the wafer and the holder becomes extremely low during this time, and the contact portion of the wafer Does not drop as rapidly as when the load lock chamber is completely ventilated at one time. Therefore, the thermal gradient in the wafer is
It is relatively gradual and much less prone to breakage due to the thermal gradient of the wafer.

The limited amount of gas to be injected (and thus the intermediate pressure level mentioned above) and the duration are determined experimentally. For example, a stepped vent with a 10 second duration at an intermediate pressure of 0.8 torr has a maximum slope in a wafer cooled from 300 ° C. Excessively long durations have a negative effect on production. However, the drop in production caused by the 15 second duration is only about 0.78%, and if the number of broken wafers is reduced by one per week, this duration is considered to be a good match. It is. In FIG. 1, the intermediate pressure level and duration are denoted as P _d and t _d , respectively. (FIG. 1 is not intended to be quantitative, but merely schematic, and the levels shown in FIG. 1 are not intended to indicate a preferred intermediate pressure level.) Shows an apparatus capable of implementing the method of the present invention described above. In the block diagram of FIG. 3, reference numeral 10 denotes a load lock chamber provided with support means 12 for supporting an object such as a wafer to be taken in and out of a processing chamber (not shown). Load lock chamber 10
Storage means, such as storage elevators, which are connected to the system are also omitted in FIG. 3, both for simplicity and because they are not part of the present invention. The support means 12 is depicted as a holder with a protruding portion on which the peripheral portion of the wafer W is mounted. However, this diagram is schematic and does not represent accurately. A vacuum duct 14 connects the load lock chamber 10 to a vacuum pump, and a nitrogen gas supply 16 is connected to the duct 14 through a conventional valve 18 and a filter 20 commonly used in this type of pneumatic system. Connected. Reference numeral 22 denotes a means for controlling the opening and closing of the valve 18 by the electric signal 19 as a whole, and includes a computer 24. Computer 24 is increased initially, the internal pressure to a predetermined intermediate pressure level P _d by appropriate predetermined amount of nitrogen gas flows from source 16 through a vacuum inside the duct 14 of the load lock chamber 10 A pulse signal for opening the valve 18 is output for the predetermined duration t.
After _d , it is programmed to output another signal to completely vent the load lock chamber. The program for controlling the valve 18 in the preferred manner explicitly described above can be created by any person skilled in the art and will therefore not be described in detail. Experiments with two variables (pulse length and duration at intermediate pressure) show that controlling the pulse time yields favorable consistent results in the relationship between pressure and pulse duration. This control law is linear since it regulates the weight flow to the load lock chamber.

FIG. 4 is a diagram showing another apparatus that can implement the method of the present invention by hardware. In the block diagram of FIG. 4, parts that are substantially the same as those already described above in connection with FIG. 3 are indicated by the same numerals. In this hardware implementation, each upstream valve 32
And two pneumatically controlled valves connected in series, shown as a downstream valve 34 and having a vent reservoir 34 therebetween, connect the duct 14 to the nitrogen gas source 16.
Is connected to The opening and closing of the valves 30 and 32 is not electrically controlled by software, but is controlled pneumatically by a sequencer 40, the function of which is that the pressure inside the load lock chamber 10 is not controlled at one time as shown in FIG. Instead, the valves 30 and 32 are opened and closed so as to increase in two stages as schematically shown in FIG. The sequencer 40 is essentially a pneumatic delay circuit similar to an electrical delay circuit with transistors and capacitors. In the embodiment shown in FIG. 4, the sequencer 40 includes a delay valve 42 (Clippard Instrument Labora).
tory, Inc. Model R343 etc.), switch valve 44
(Such as Clipper's Model R401) and a volume chamber 46 (such as Clipper's Model R821). Of the four terminals of the sequencer 40, two serve to connect a high pressure (eg, 60 psi) air supply 38 to a lag valve 42 and a switch valve 44, respectively, and the other two connect lag valves. The downstream valve 32 and the switch valve 44 are connected to the upstream valve 30, respectively. The switch valve 44 allows high pressure air from the source 38 to pressurize the upstream valve 30 therethrough when no pressure is applied to its piston (not shown), thus opening it and causing the vent reservoir to open. 34 is designed to be filled with vent gas (in this example, nitrogen gas) from the source 16. The pipe between the lag valve 42 and the downstream valve 32 is connected through a normally closed air pilot valve 36 to a source of high pressure air (also shown in FIG.
(Which may be another source). The retard valve 42 allows high pressure air from a source 38 to fill the volume chamber 46 therethrough while the air pilot valve 36 is closed and thus no pressure is applied to its piston (not shown), and It is designed to apply pressure to the piston of the switch valve 44. As a result, the air flow path passing through the switch valve 44 is closed, and the upstream valve 30 is also closed when the pressure transmitted through the switch valve 44 disappears.

While the air pilot valve 36 remains closed, no pressure is applied to the downstream valve 32, which causes the downstream valve 32 to remain closed, causing the load lock chamber 10 to reach its vacuum state. Means to keep. When the air pilot valve 36 opens (at time t ₀ ), for example, due to an electrical signal 41, the high pressure air passing therethrough exerts pressure on both the downstream valve 32 and the piston of the retard valve 42. The pressure on the downstream valve 32 causes it to open, causing a limited amount of vent gas stored in the vent reservoir 34 to enter the load lock chamber 10 and thus increase its internal pressure to a predetermined intermediate level. Let it. During this time, the pressure applied to the piston of the delay valve 42
This has the effect of closing the air passage between the air supply source 38 and the volume chamber 46 while allowing the air stored therein to flow through the delay valve 42. When the pressure on the switch valve 44 piston drops to a certain limit, the piston is pushed back by the biasing force on it, re-establishing the pressure transmission relationship between the high pressure air supply 38 and the upstream valve 30. Thus, after opening the upstream valve 30 and delaying as indicated by the pressure profile of FIG. 1, vent gas flows from its source 16 into the load lock chamber 10 for complete ventilation. The time of this delay is determined by the rate at which air flows out of the volume chamber 46. This outflow rate is
Speed control valve with variable orifice (Clippard Ins
trument Laboratory, Inc. Model JFC2 48).

After the load lock chamber 10 is completely vented, the air pilot valve 36 is closed again (eg, at t ₁ ). Thereby, the downstream valve 32
Is closed and the vent reservoir 34 is refilled with vent gas. At the same time, air from the high pressure source 38 fills the volume chamber 46 through the delay valve 42,
Pressure is applied to the piston of the switch valve 44, thereby closing the valve 30 on the upstream side. During this time, the load lock chamber 10 is evacuated through the duct 14 for the next cycle of operation. FIG. 5 is a pressure diagram showing changes in pressure with time in the air pilot valve 36, the delay valve 42, and the switch valve 44.

The present invention has been described in terms of only some of its embodiments. However, these examples are illustrative and not limiting. For example, intermediate pressure levels can be as low as 1 torr or as high as 50 torr, depending on circumstances and other factors. The holder for the heated object inside the load lock chamber need not be of the construction schematically depicted in the drawings. In fact,
It need not be of the type that holds the wafer horizontally. It should be understood that modifications and variations obvious to those skilled in the art to the above-described embodiments are within the scope of the present invention.

[Brief description of the drawings]

FIG. 1 is a graph showing the change over time of the pressure inside a load lock chamber ventilated by a method embodying the present invention.

FIG. 2 is a graph showing the change over time of the pressure inside a load lock chamber vented by a prior art method.

FIG. 3 is a block diagram schematically illustrating an apparatus for venting a load lock chamber by a method according to the present invention.

FIG. 4 is a block diagram schematically illustrating another apparatus for venting a load lock chamber by a method according to the present invention.

FIG. 5 is a pressure diagram showing a pressure change in the operation of the apparatus of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Load lock chamber 12 Support means 14 Vacuum duct 16 Nitrogen gas supply source 18 Valve 19 Electric signal 20 Filter 22 Control means 24 Computer 36 Air pilot valve 38 High pressure air supply source 40 Sequencer 41 Electric signal 42 Delay valve 44 Switch valve 46 Volume chamber 48 Speed control valve

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Glen D. Alexander San Jose Edelweiss, California, USA 590 (56) References JP-A-60-197878 (JP, A)

Claims (4)

(57) [Claims] 1. A method for controlling a temperature difference generated when a heated silicon substrate is cooled, wherein a vacuum pressure is formed in a load lock chamber having a substrate supporting means capable of conducting heat. Placing the substrate on the support means in the load lock chamber, and providing a sufficient amount of vent gas to provide an intermediate pressure between the vacuum pressure and atmospheric pressure into the chamber for a preselected time. And introducing an additional amount of vent gas into the chamber sufficient to bring the intermediate pressure to atmospheric pressure. 2. The method of claim 1, wherein said vent gas is stored in a vent gas reservoir coupled to said chamber via a control valve. 3. An apparatus for controlling a temperature difference generated when a heated silicon substrate is cooled, comprising: a vacuum load lock chamber having a substrate supporting means capable of conducting heat; and a vent gas supply. A source, valve means for allowing and blocking the flow of vent gas from the vent gas supply to the chamber, and an amount of vent gas sufficient to obtain a preselected pressure in the chamber. And a valve control means capable of introducing said gas into said chamber, maintaining said pressure for a preselected time, and introducing additional vent gas into said chamber until atmospheric pressure is reached. 4. The valve means includes a downstream valve, a vent gas reservoir and an upstream valve, wherein the downstream valve is coupled between the chamber and the vent gas reservoir, and wherein the upstream valve is connected to the vent gas reservoir. A vent reservoir is coupled between a gas reservoir and the vent gas supply, and the vent gas reservoir is configured to pre-select the pressure in the chamber when the vent gas is introduced into the chamber via a downstream valve. 4. The apparatus of claim 3, comprising a preselected amount of vent gas increasing to a predetermined pressure.