FIELD OF THE INVENTION
The present invention relates to a device for drying/curing of screen printed
patterns on silicon wafers in a conventional belt drying furnace preventing the
silicon wafers from direct contact with the belt to eliminate contamination of the
wafers.
BACKGROUND OF THE INVENTION
In solar cell processing, the conveyor belt furnaces are widely used for various
process steps such as drying/curing of screen printed metal contacts on diffused
wafers, screen printed etch paste for selective emitter design etc. In these
process steps, the screen printed wafers are placed on a conveyor belt and
passed through high temperature zones of the furnace.
The conventional conveyor furnaces are used for drying/ curing of the screen
printed metal contacts, and screen printed etch paste for emitter etch back at
elevated temperature (>200 °C). In these furnaces, different types of wire mesh
conveyor belts made up of high carbon steel or galvanized iron or 304 stainless
steel are used. Over a period of time, these wire mesh conveyor belts become
prone to corrosion resulting in formation of rust over them. When a Silicon wafer
is placed on such a conveyor belt and passed through the heating zone of the

furnace, the belt leaves its rusty impression on the wafer leading to
contamination. In semiconductor/PV manufacturing, the drying/curing process
steps are very critical and surface contamination due to rusty conveyor belt can
be detrimental.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a device for drying/curing of
screen printed patterns on silicon wafers in a conventional belt drying furnace
preventing the silicon wafers from direct contact with the belt to eliminate
contamination of the wafers.
SUMMARY OF THE INVENTION
Accordingly, there is provided a device for drying/curing of screen printed
patterns on silicon wafers in a conventional belt drying furnace preventing the
silicon wafers from direct contact with the belt to eliminate contamination of the
wafers.
In the invention disclosed herein, an innovative stainless steel (SS) wafer carrier
is provided for example, 156 mm x 156 mm silicon (Si) wafer which prevents
contamination from the conveyor during drying/curing at high temperature in
conventional furnaces. The SS carrier is designed to make only point contact at
four corners of the wafers. The device does not act as any kind of barrier for
uniform heating of the wafer.

The Si wafer can be placed securely on wafer carrier and loaded on to the
conveyor of the furnace. Thus the wafer doesn't come into direct contact with
the conveyor or belt and the belt does not contaminate the wafer. As the wafer
carrier is made with two mm diameter SS 304 wire, it does not add any
significant additional thermal load on the furnace and also the uniformity of
heating remains unaffected.
A slip-preventing means is provided to the device that prevents slipping of the
wafer during transport on the conveyor and while unloading the carrier from
furnace.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a conveyor belt furnace having silicon wafers on the belt for
drying and curing.
Figure 2 - shows a schematic illustration of a device for drying and curing of Si-
wafers constructed according to the invention.
Figure 3 - shows a Si-wafer disposed on the device ready for drying/ curing in
the conveyor belt furnace.
Figure 4 - shows the inventive device disposed on the conveyor belt and
accommodating a Si-wafer for drying/curing.

DETAILED DESCRIPTION OF THE INVENTION
In conventional furnaces, wire mesh conveyor belts of carbon steel or galvanized
steel or 304 stainless steel are used. Over a period of time, these wire mesh
conveyor belts get corroded and rusted. When a wafer is placed on such a
conveyor (shown in Fig. 1) and passed through heating zone of the furnace, the
rusted belt leaves its impression on the wafer which leads to contamination. In
semiconductor/PV manufacturing, the drying/curing process steps are very
critical and surface contamination due to conveyor is detrimental. In the present
invention, a device has been provided for silicon wafers for drying/curing of
screen printed patterns in a conventional conveyor furnace without allowing the
Si to touch the belt, and avoid any contamination from the conveyor.
SS wire with 2 mm diameter is used for making the device. The SS wire is cut
into three types of segments. The length of the segments (A, C and B) are 220
mm, 152 and 80 mm respectively. The two A- segments are bent in "U" shape
with extended arm lengths of 45 mm on both sides of "U" and the three C
segments are bent in "L" shape. The heights of U and L shaped segment are 10
mm and 12 mm respectively. The two B-segments are each having axial length
of 40 mm. The dimensions and shapes of various wire segments used in the
fabrication of the device are shown in Fig. 2. The wafer carrier is fabricated by
TIG welding of all the three segments at nine points as shown in Fig.2 as welded

joints (WJ). The height of B segments is kept 2mm more compared to that of A-
segments so that while loading the wafer on the carrier, the wafer is pushed
against the L shaped B-segments and allowed to rest at four corners of A-
segments (shown in Fig. 3). This helps in loading the wafer securely and also
prevents slippage of wafer during transport in the furnace.
The SS carrier loaded with wafer is placed on to the conveyor (shown in Fig. 4).
The carrier along with wafer moves along with the conveyor and transported in
to heating zone of the furnace. As the bottom of the carrier is fully open, it does
not hinder heat from bottom heaters of the furnace to reach the wafers.
Therefore, the wafer carrier allows uniform heating of the wafer from both the
top and bottom heaters of the furnace.
USES OF THE INVENTION:
Drying and curing of screen printed patterns on Si wafers at elevated
temperatures (>200°C) using SS carriers in conventional furnaces without
contacting the conveyor belt so as to avoid contamination from conveyor.

WE CLAIM:
1. A device for drying/curing of screen printed patterns on silicon wafers in a
conventional drying belt furnace preventing the silicon wafers from direct
contact with the belt to eliminate contamination of the wafers comprising:
- producing at least three different types of segments by cutting a stainless
steel wire about 2 mm diameter;
- the first type of segment (A) formed in "U" shape with extended arms on
both sides, the second type of segment (C) formed in "L" shape, and the
third type of segment (B) formed as a horizontal member;
- welding of the three types of segments (A, B, C) at nine points (WJ),
wherein the number of the segments formed are two in case of the first
segment (A), in case of third type segment (B), and three for the second type
(C), wherein the length of the - each segments A, B and C is selected in a
ratio of 2.75 :1 : 1.90, wherein the height of the first and the second
segment (A, C) is in the ratio of 1: 1.20, and wherein the extended arm
length of the first segment (A) on both sides of the "U" is 21% of the length
of first segment (A).

2. The device as claimed in claim 1, wherein the length of first, second and
third segments (A, B, C) is 220 mm, 80 mm, and 152 mm respectively,
wherein the extended arm length on both sides of the "U" for the first
segment (A) is 45 mm, and wherein the height of the first and second
segments (A, C) is 10 mm and 12 mm respectively.
3. The device as claimed in any of claims 1 or 2, wherein the axial distance
between the second segments (C) is 40 mm.

ABSTRACT

A device for drying/curing of screen printed patterns on silicon wafers in a
conventional drying belt furnace preventing the silicon wafers from direct contact
with the belt to eliminate contamination of the wafers comprising producing at
least three different types of segments by cutting a stainless steel wire about
2 mm diameter; the first type of segment (A) formed in "U" shape with extended
arms on both sides, the second type of segment (C) formed in "L" shape, and
the third type of segment (B) formed as a horizontal member; welding of the
three types of segments (A, B, C) at nine points (WJ), wherein the number of the
segments formed are two in case of the first segment (A), in case of third type
segment (B), and three for the second type (C), wherein the length of the - each
segments A, B and C is selected in a ratio of 2.75 :1 : 1.90, wherein the height of
the first and the second segment (A, C) is in the ratio of 1: 1.20, and wherein
the extended arm length of the first segment (A) on both sides of the "U" is 21%
of the length of first segment (A).