FIELD OF THE INVENTION:
The present invention relates to profiling of contact firing furnace used in Solar cells production. In particular, a data recorder which may record the temperature profile in-situ.
BACKGROUND OF THE INVENTION
In a solar cell production line, metal contacts are made on solar cells to collect photo-generated carriers efficiently. A metallic grid is printed using a silver (Ag) paste on that wafer and printed silicon wafers are passed through a pre-heated drying cum firing furnace. This furnace has to have a particular heating profile which is generally given by the paste manufacturer. This information is important because the Ag paste has to make a firm and low resistance contact with the diffused emitter which is generally less than 0.3 um thick. Typically the furnace consists of a drying section and a firing section (total length 28 feet) with total nine heating zones which can reach up to 1000° C. The printed silicon wafer is loaded on an SS belt which can move with a linear speed ranging from 25 inch per minute (ipm) to 250 ipm. All the zones and belt speed are set at a particular value to achieve the desired profile.
It requires a very precise measurement of firing temperature which can be measured using a thermocouple. If done by a conventional means, it requires a very long thermocouple connected to the Si wafer, which is passed through the furnace and the

temperature is recorded outside. This method does not give accurate result and is a very difficult in operation, especially considering the length of the furnace. Therefore, a new type of data recorder has been developed, which is battery powered, can be sent through the furnace and can record the temperature profile in-situ.
One document related to the subject matter is

In this invention the sample is carried within the reflow furnace. The temperature is recorded and is transmitted to a radio wave sensor placed outside the reflow furnace. The received radio waves are processed by the receiver and data is generated. It is different from present invention as in this case the thermocouple is placed on the sample and it directly records the temperature. The data is instantly processed by the micro-controller.
OBJECTS OF THE INVENTION
The object of the invention is to perform an in-situ measurement of temperature of High temperature (HT) conveyorised belt furnace locally.
Another object of the invention is to develop a data recorder for accurate profiling of contact firing furnace used in Solar cells production/R&D.

Yet another object of the invention is to develop a system with accurate and precise measurement of temperature profile with an accuracy of < ± 1° C.
SUMMARY OF THE INVENTION
In solar cell production line a metallic grid is printed using a silver paste on Si wafer and fired in a belt furnace using a particular profile. Profiling this long firing furnace using a conventionally available thermocouple and a recorder is cumbersome. Therefore, a new type of data recorder has been invented which can record the temperature profile in-situ. The data recorder consists of a micro-controller, a thermocouple, a memory card, a cold junction compensator chip and few other components assembled on a PCB. The PCB with battery is put inside a thermally insulated box with one hot junction end of the thermocouple hanging out of the box. This end of the thermocouple is stuck to a Si wafer and is placed on the moving belt of the HT furnace. When the data recorder passes through different zones of the furnace, the temperature values are saved on a memory card. The data may be retrieved using a computer and the profile is plotted.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: Shows schematic diagram of a typical drying cum firing furnace.
Fig. 2: Shows block diagram of the card..

Fig. 3: Shows schematic showing placement of the controller-thermocouple-
sample assembly on the furnace belt.
Fig. 4: Shows typical firing profile.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
In solar cell production line, metal contacts are made on solar cells to collect photo-generated carriers efficiently. This is one of the last process steps in a solar cell production line, which is generally done after Anti-Reflection Coating (ARC) deposition. This step is divided into two parts
1. Drying (A) of back Aluminium printed paste
2. Drying (A) and co-firing (B) of front printed grid of silver paste
First an aluminium paste is printed at the rear side of the full size (6" X 6") silicon wafer through a screen printer and is then dried at about 200° C in a conveyorised furnace. Then, a metallic grid is printed using a silver paste on the front side of that wafer and the printed silicon wafer is passed through a pre-heated drying (A) cum firing (B) furnace. This furnace has to have a particular heating profile which is generally given by the paste manufacturer. The profile has to show the following information precisely: 1. The peak temperature which the passing silicon wafers see.

2. The time for which it remains in different temperature zones especially temperatures >800° C. Generally this time interval is about 2-3 s. This temperature profile information is important because the Silver paste has to make a firm and low resistance contact with the diffused emitter which is generally less than 0.3 un thick.
Typically, the furnace consists of a drying (A) and a firing (B) section as shown in Fig. 1. The drying (A) section consists of 3 heating zones (Zl, Z2, Z3) which goes up to a maximum temperature of 500° C and firing (B) section consists of 6 heating zones (Z3, Z4, Z5, Z6, Z7, Z8, Z9) which can reach up to 1000° C. The printed silicon wafer is loaded on an SS conveyor (11) belt of the furnace which can move with a linear speed ranging from 25 ipm to 250 ipm. All the zone temperatures and belt speed are set to a particular value to achieve the desired temperature profile.
It requires a very precise measurement of firing (B) temperature with respect to time duration, which can be measured using a thermocouple (20) and a stop watch. Conventional measurement involves using a very long thermocouple (greater than the length of the furnace, > 28 feet in this case) and its hot junction is pasted on a Si wafer (22) and then it is passed through the furnace and the temperature is recorded outside manually. This method is very cumbersome. The result of conventional method has errors as it is recorded manually and also handling of lengthy thermocouple on a moving conveyor (11) inside a furnace is messy. Therefore, a new type of data recorder

(10) has been developed, which is battery powered and has a specially programmed chip that can be sent through the furnace for recording temperature values in-situ.
The data recorder (10) consists of a micro-controller, a 'K" type thermocouple (20), a memory card (40), cold junction compensator chip (30) and few other components assembled on a PCB (100). The block diagram of PCB is given in Fig-2. The PCB (100) has a connector to connect the thermocouple (20), a power supply (a battery of 4.5 V using 3 dry AA cells of 1.5 V each) and a serial port. The PCB (100) has two selector switches (41, 42): one for ON/OFF and other for READ/WRITE data. It has one push button to reset the controller.
When the card is powered, the micro-controller starts logging the temperature data from thermocouple (20) using a computer program. The powered PCB (100) and the battery are put inside a thermally insulated box (50). One hot junction (200 end of the thermocouple (20) is allowed to come out of that box through a small opening and is placed on Si wafer (22) as shown in Fig. 3. The wafer & thermally insulated box (50) are then placed together on the moving belt (11) of furnace and are allowed to go inside. The zone temperatures and belt speed of the furnace are fixed at desired values. When the data recorder (10) passes through different zones of the furnace, the thermocouple (20) (TC) directly reads the exact temperature which is seen by the wafer. It sends the values of voltage as generated by TC in micro volts to microcontroller which is amplified 1000 times by the cold junction compensator chip

(30). It gets recorded to a memory card (40) using the programmed micro controller (25). When the data recorder (10) comes out of the furnace and connected to a computer, the recorded data is taken into an MS excel format from where the profile of the furnace is printed. A typical profile of the furnace is shown in Fig. 4. The total time taken for the data recorder (10) to travel through furnace and profile to be seen on PC is not more than 5 minutes. After examining the current profile, necessary changes are made in the settings of furnace such as zone temperatures and belt speed to match the profile of the furnace to that of the paste manufacturer.
Once the desired profile is obtained, the printed wafers are allowed to go inside the furnace for drying (A) and co-firing (B). A correct and optimized profile helps in optimizing the drying (A) and firing (B) process parameters leading to gain in efficiency of solar cells.

WE CLAIM
1. A data recorder (10) which measures, records and stores instantaneous temperature
directly at the furnace which comprises of:
a controller (25) to log the temperature data from a thermocouple;
a memory unit (40);
a junction compensator means (30)
and a battery powered PCB (100) wherein the above components are mounted which is sent through the furnace to record the temperature locally with a especially designed program unit.
2. The data recorder (10) as claimed in claim 1, wherein the temperature measuring sensor is a 'K' type thermocouple (20) which is readily available.
3. The data recorder (10) as claimed in claim 1, wherein the PCB (100) comprises at least two selector switches (41,42).
4. The data recorder (10) as claimed in claim 1, wherein the PCB (100) and the battery is encapsulated inside a thermally insulated enclosure (50).
5. The data recorder (10) as claimed in claims 2 to 4, wherein one hot Junction (200 of the thermocouple (20) is exposed out of the enclosure to position the same on a Si wafer (22).

6. The data recorder (10) as claimed in claim 4 or 5, wherein the enclosure engaged with Si wafer (22) is positioned on a conveyer belt and passed through the furnace.
7. A method to measure, record and store temperature profile information directly from the furnace wherein a data recorder (10) operates by powering a memory unit (40), when the controller starts logging the exact temperature at various zones (A & B) of the furnace which wherein is retrieved from the thermocouple (20) using a computer program;
wherein the thermocouple (20) reads the temperature as seen by the wafer and sends it to cold junction compensator chip (30);
wherein it is amplified by chip (30) by about 1000 times and is stored to memory unit through controller (25)
the recorded data is retrieved when the data recorder (10) comes out of the furnace.