FIELD OF INVENTION
The present invention relates to an arrangement for and a method of measuring temperature of a moving wafer carrier in a vacuum chamber used for the deposition of amorphous silicon layers for the fabrication of passivated interface heterojunction solar cells.
BACKGROUND OF THE INVENTION
The amorphous silicon layers are deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) process in vacuum chambers. The substrate temperature is one of the most critical process parameters. The substrate temperature affects the process results greatly, so it is important to monitor the temperature of wafer / wafer carrier inside vacuum chamber to obtain desired properties of amorphous silicon layers.
In patent no. JP3977244B2 2007-09-19 titled “Method of measuring infrared emissivity of semiconductor wafer”, temperature measurement of semiconductor wafer by method of infrared emissivity is discussed. It explains about method of measuring temperature of semiconductor wafer during the heat treatment process inside a vacuum chamber. This method is capable of measuring the infrared emissivity ε in a short time.

The back side of wafer is coated with graphite and its temperature is measured by the infrared radiometer which acts as a sensor.
In US patent no. US 6,462,640 B2 titled “Sensor with a temperature dependent measuring element” published in 2002, a temperature sensor is discussed which is used in high voltage ovens for measurement of temperature of more than 550oC. The temperature sensor is arranged inside a quartz tube which provides electrical insulation of temperature sensor from power supply of the oven. The quartz glass tube is itself surrounded by a protective tube made of stainless steel. A spacer made of mica is provided between the body and the measuring element.
Another prior art is described in U.S. Pat. No. 3,461,216 titled “Apparatus for temperature measurement in vacuum melting and casting installations” filed in 1969. In that an apparatus for sensing the temperature of molten metal in a vacuum melting furnace is presented. The apparatus comprises a thermocouple mounted at one end of the furnace. During melting, the metal will not make contact with the thermocouple, but when desired to take the temperature of the molten metal, the furnace is tilted to bring the molten metal in contact with the thermocouple.

In patent no. DE3528161C1 1986-10-23 titled “Thermocouple for the measurement of temperatures in vacuum furnaces”, a thermocouple for the measurement of temperatures in vacuum furnaces is discussed. A thermo element which rapidly determines temperature and which is protected from disturbing signals during the measurement of temperature in a vacuum furnace is presented. This is composed of a thermocouple protectively sheathed by a transparent quartz glass protective tubing.
In none of prior arts mentioned above, the method for measuring temperature of a moving carrier inside a vacuum chamber is presented. An arrangement is conceived in which temperature of wafer carrier can be measured in moving mode. The present technique is simple, inexpensive and more accurate than other methods.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose an arrangement for and a method of measuring temperature of a moving wafer carrier in a vacuum chamber, which is simple, inexpensive and accurate.

Another object of the invention is to propose an arrangement for and a method of measuring temperature of a moving wafer carrier in a vacuum chamber, which is capable of measuring temperature of wafer carrier in moving mode.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 : Shows a Thermocouple (TC) arrangement inside a vacuum chamber showing the TC stand in a vertical position when there is no contact with the carrier.
Fig. 2: Shows a Thermocouple arrangement inside a vacuum chamber showing the tilted position of TC stand when carrier passes over it.
Fig. 3: Shows a Thermocouple arrangement inside a vacuum chamber showing TC stand again coming back to the vertical position when the carrier clears the Thermocouple junction.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The present invention disclosed a novel approach for the temperature measurement of moving wafer carrier in vacuum chamber for precise control of properties of amorphous silicon layers deposited by PECVD process.

Amorphous silicon (a-Si) layers are deposited by PECVD process in vacuum chamber for fabrication of solar cells. The substrate i.e. silicon wafer is placed on the stainless steel wafer carrier and that wafer carrier is placed in the vacuum chamber for heating. The substrate is heated before a-Si layer deposition either in the preceding chamber or in the process chamber itself. The substrate temperature is very critical for obtaining the optimum process results. Therefore it is important to monitor the temperature of wafer / wafer carrier inside vacuum to obtain desired properties of a-Si layers. The wafer carrier needs to be moved in chamber for uniform deposition of a-Si layers. Therefore an arrangement is conceived in which temperature of wafer carrier can be measured in moving mode.
Schematic of the arrangement for thermocouple (TC) & its stand inside the vacuum chamber is shown in following figures. The thermocouple is arranged in such a way that thermocouple junction is protruding out of thermocouple stand so that it can make a physical contact with the carrier as shown in figures 1, 2 and 3. The thermocouple arrangement is placed in vacuum chamber and the wires are drawn out of chamber through a feed through. The wires from thermocouple are attached to a temperature readout.

In vacuum chamber there are heaters (H) to heat the carrier and substrates. The carrier moves on rollers inside the chamber. As shown in fig 1, 2 & 3 when the carrier moves in the chamber it makes a contact with the thermocouple junction and temperature is sensed on the readout.
The thermocouple is fixed in a stainless steel stand with just junction protruding out, sufficient to make contact with moving carrier.
The bottom of the TC stand is supported by a heavy SS weight (W) of about 2 kg. The value of weight (w) has been selected such that when carrier passes and touches the thermocouple junction, the TC stand tilts and maintain the contact with the carrier during it movement and keeps displaying temperature of carrier. The contact time varies from 15 seconds to 50 seconds depending on carrier speed. The TC stands comes back to vertical position when the carrier clears. The height of TC stand and bottom weight have been designed in such a way that the TC stand does not topple when tilted due to very low center of gravity.
The arrangement has been successfully used in determining the substrate temperature in situ in a vacuum chamber, which has been pumped down to 10-7 torr.

The technique is simple, inexpensive and more accurate than other methods such as Infrared (IR) emissivity, temperature measuring tapes, inks etc.
The arrangement can be installed in any chamber having a feed through point.
This does not adversely affect the base pressure (10-7 torr) of vacuum chamber.
The temperature is measured very accurately within ± 20C at 1300C.

WE CLAIM
1. An arrangement for measuring temperature of a moving wafer carrier in a
vacuum chamber, comprising;
a moving stainless steel wafer carrier (C) for carrying silicon wafer placed in a vacuum chamber (V) for heating of said substrate;
a thermocouple (TC) with a thermocouple stand (T) disposed in the said vacuum chamber (V);
the said thermocouple (TC) having thermocouple junction (J) protruding out of the thermocouple stand (T) for allowing a physical contact with the moving carrier (C);
a plurality of rollers (S) for moving the carrier;
a plurality of heaters (H) for heating the substrate in the vacuum chamber (V);
a feed through (F) for drawing out wires from thermocouples;
characterized in that,
a temperature read out (R) is disposed for sensing the temperature of the carrier when the movement of carrier (c) is configured for making a contact with the thermocouple junction (J), making the TC stand (T) tilt and configured for maintaining the contact with the carrier during its movement displaying the temperature of said carrier on a temperature readout (R) , when a heavy weight (W) is disposed at the bottom of the TC stand (T) for preventing topple of TC stand (T).

2. The arrangement as claimed in claim 1, wherein the weight supporting the TC stand (T) is 2 kg.
3. A method of measuring temperature of a moving wafer carrier in a vacuum chamber by an arrangement claimed in claim 1, comprising;
arranging a stainless steel vacuum chamber (V);
arranging a carrier (C) with rollers (S) to carry silicon wafer and to move inside the said vacuum chamber (V) for heating;
disposing a plurality of Heaters (H) inside the vacuum chamber to heat the carrier and substrates;
disposing a thermocouple (TC) with thermocouple stand (T) having a thermocouple junction (J) protruding out of the said stand (T) to make a physical contact with the said carrier (C);
drawing out wires from thermocouples out of the vacuum chamber (V) through a feed through (F) and attaching the said wires to a temperature readout (R);
disposing a heavy weight (W) to support the bottom of TC stand (T);
a plurality of rollers (S) attached to the carrier, wherein the carrier moves in the vacuum chamber making a contact with the thermocouple junction when the temperature is sensed on the readout (R) wherein the TC stand (T) remains tilted so

long the carrier (C) maintains contact when the stand returns to vertical position without being toppled as the carrier (C) clears the junction (J).
4. The method as claimed in claim 3, wherein the contact time varies from 15 seconds to 50 seconds depending on speed of carrier.
5. The method as claimed in claim 3, wherein the pressure in the vacuum chamber is maintained at 10-7 torr.
6. The method as claimed in claim 3, wherein the temperature is measured within ± 2°C at 130°C.