WO2011078686A1

WO2011078686A1 - Cleaning stack of wafers

Info

Publication number: WO2011078686A1
Application number: PCT/NO2010/000473
Authority: WO
Inventors: Pouria Homayonifar; Anita BØRVE
Original assignee: REC Wafer Norway AS
Current assignee: REC Wafer Norway AS
Priority date: 2009-12-21
Filing date: 2010-12-20
Publication date: 2011-06-30
Anticipated expiration: 2012-06-21
Also published as: GB2476315A; GB0922363D0

Abstract

The invention provides apparatus for cleaning wafers in a stack (10) comprising a multiplicity of wafers, and comprising a generally horizontal trough (17) into which the stack of wafers is to be placed, the axis of the trough being perpendicular to the planes of the individual wafers, supports (12) within the trough to hold the wafers off lower parts of the trough, a fixed support (14) for one end of the stack and sufficient space in a direction away from this end of the stack to allow the wafers to separate by a distance such that capillary action between them is broken, in which there is provision (16) to introduce jets of liquid upwardly from lower parts of the trough (17), whereby those jets of liquid maintain the wafers clear of direct contact with the supports (12) to hold the wafers off the lower parts of the tough, and separate the wafers laterally in a direction parallel to the axis of the trough so moving the wafers in a direction away from the fixed support (14), and those jets of liquid also clean the wafers.

Description

CLEANING STACK OF WAFERS

Field of the Invention

The invention relates to an apparatus and a method for use in the process of solar cell manufacturing, and is particularly concerned with cleaning a multiplicity of thin wafers arranged in a stack.

Background of the Invention

Thin wafers of silicon for photovoltaic panels are produced by slicing a silicon block into a multiplicity of wafers. These may be between 50 μηι and 300 μπι thick. Thin wafers are desirable to reduce material requirements and manufacturing costs, but such wafers are fragile. Before cutting, an adhesive layer is spread on one surface of the block to hold the wafers in place during cutting. The action of cutting is effected with an array of fast moving wires. During cutting the block is cooled and the debris material from cutting is removed in a liquid or slurry. The slurry and debris remain on the surface of the wafers after cutting, and so cleaning of the wafers is essential. The presence of slurry and water on the wafers results in some capillary action, which tends to hold adjacent wafers together. Cleaning is necessary to remove the slurry, any remains of the adhesive layer, and related chemicals.

It is known to clean wafers individually. The wafers are arranged in a horizontal line, and a significant length of equipment is necessary (e.g. 5 m - 15 m) to carry out the cleaning process. One of the main disadvantages of single wafer cleaning is that the wafers may be damaged on the rollers or moving pins which carry the wafers through the cleaning process. WO2009129989A1 and EP2060659A2 describe such cleaning systems. Any misalignment of the rollers or pins which support and move the wafers through the equipment may lead to wastage through the creation of defects in the wafers.

To avoid the problems associated with single wafer cleaning, it has been proposed to carry the wafers through the cleaning process in a basket or cassette with a 4 mm - 10 mm gap between the wafers. WO2009098042A1 shows an embodiment of such a solution. US6001 191 shows a similar basket cleaning method. However, this method needs more than one tank for washing and rinsing, and again results in equipment of significant length, as with the single wafer concept described above.

Wafers have been fixed in fixed comb arrangements for spraying. US20040262245 shows an example of such a cleaning method. Summary of the Invention

The invention provides apparatus for cleaning wafers in a stack comprising a multiplicity of wafers, and comprising a generally horizontal trough into which the stack of wafers is to be placed, the axis of the trough being perpendicular to the planes of the individual wafers, supports within the trough to hold the wafers off lower parts of the trough, a fixed support for one end of the stack and sufficient space in a direction away from this end of the stack to allow the wafers to separate by a distance such that capillary action between them is broken, in which

there is provision to introduce jets of liquid upwardly from lower parts of the trough, whereby those jets of liquid maintain the wafers clear of direct contact with the supports to hold the wafers off the lower parts of the tough, and separate the wafers laterally in a direction parallel to the axis of the trough so moving the wafers in a direction away from the fixed support, and those jets of liquid also clean the wafers.

It is preferred that there is a movable support for the other end of the stack, the movable support being free to move a limited distance in a direction perpendicular to the planes of the individual wafers to allow the wafers to separate such that capillary action between them is broken.

In this form it is further preferred that there is provision to move the moveable support back towards the fixed support, so that when the pressure from the jets of liquid has been reduced, but before the wafers settle back onto the supports to hold the wafers off the lower parts of the trough, the wafers can be pushed back to their original positions on the axis of the trough.

Advantageously there is a regulating valve by which the volume of liquid fed to the liquid feed supply can be controlled, so to regulate the pressure in the jets of liquid to hold the wafers off the supports which hold the wafers off the lower parts of the trough.

Conveniently the provision to introduce jets of liquid is formed of closed chambers with a liquid feed supply and small holes to form the jets directed towards the interior of the trough.

It is preferred that the supports within the trough to hold the wafers off the lower parts of the trough are rods of sufficient rigidity to hold the wafers in position.

It is also preferred that the end supports and supports to hold the wafers off the lower parts of the trough are formed as a magazine, which is separate from the provision for jets of liquid.

The invention includes a method of cleaning wafers in a stack comprising a multiplicity of wafers, including the step of supporting the wafers in vertical planes, supplying jets of liquid from beneath the wafers, the jets being of sufficient strength to lift the wafers off the supports, allowing the wafers to spread horizontally, so that the jets of liquid can clean the faces of the wafers, and then, when the wafers have been cleaned, urging the wafers together for removal from the supports.

The invention also includes wafers when cleaned in the apparatus of the invention as described above, or by the method of the invention as described above.

Brief description of the Drawings

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which :- Fig 1 is a side view of a stack of wafers supported in a magazine;

Fig 2 is an end view of that stack, showing nozzle chambers beneath the wafers;

Fig 3 is a side view of the arrangement showing the extent of the nozzle chambers;

Fig 4 is an end view showing the stack holder and wafers after the spray has started so that the wafers have been lifted off the supports in the magazine;

Fig 5 is a corresponding side view showing the wafers spreading out to the right;

Fig 6 is an end view showing cleaning liquid escaping between the stack and the nozzle chambers; and

Fig 7 shows the wafers uniformly scattered along the full length of the magazine. Detailed description of the Drawings

Fig 1 shows a stack of wafers 10 supported in a magazine generally indicated as 1 1. As shown for this specific embodiment, the wafers are square, and are arranged so that their diagonals run horizontally and vertically. However, the wafers may have other shapes, and the magazines for those wafers will have appropriate

configurations. The magazine comprises support rods 12, fixedly connected to an end support 14. At the end of the support rods 12 away from the fixed end support 14 there is a movable end support 15. The movable end support is arranged to move away from the fixed end support 14 in a direction perpendicular to the planes of the wafers in the stack 10.

Initially, in this example, individual wafers in the stack 10 are spaced apart by the thickness of cutting wires used earlier in the process. However, the gaps between the wafers may be smaller. During the wire sawing process (preceding cleaning) the wafers were held in place by an adhesive coating on one surface of the block that was sawn to form the wafers. The gaps between the wafers are filled with slurry and water which result in capillary forces which tend to hold adjacent wafers together.

In the end view shown in Fig 2, there are nozzle chambers 16 over which the lower two sides of the stack 10 can be positioned (when the magazine is in position for the stack to be cleaned). These nozzle chambers are arranged to form the sides of a generally horizontal 'V shaped trough 17, within which the pairs of rods 12 on either side of the trough 17 support the stack of wafers 10 just clear of the spray surfaces of that trough. It is not necessary for the nozzle chambers to underlie all of the lower sides of the wafers.

In Fig 2 there is a diagrammatic representation of liquid feed pipes 18 to feed cleaning liquid to the nozzle chambers 16. A collection channel (not shown) is arranged below the nozzle chambers 16 to collect liquid after spraying to clean the wafers. The magazine, trough and collection channel can be contained in a tank with a total volume of 1 cubic metre.

This Fig 2 shows the initial configuration of the magazine 1 1 within the 'V shaped trough formed by the nozzle chambers 16.

Fig 3 shows the magazine 1 1 and the stack of wafers 10 with the rods 12 holding the stack of wafers off the nozzle chambers 16. This is the situation when the magazine has been positioned within the trough 17, but before the supply of cleaning liquid is switched on.

In Figures 4 and 5, the supply of cleaning liquid under pressure has been switched on, and cleaning liquid is being sprayed upwardly and inwardly through the nozzle chambers 16. High pressure liquid from the nozzles causes the wafers to float within the tank. Fig 4 shows the passage of the cleaning liquid (grey arrows 21) and the consequent lifting of the stack of wafers off the rods 12 (black arrow 22). As may be seen from Fig 5, pressurized spray from the nozzle chambers 16 also moves the wafers to the right (small black arrows 23A), so breaking the capillary attraction between wafers and spreading the wafers apart. This moves the wafers at the outer end of the stack towards the moveable support 15. As the wafers spread, the movable support 15 moves slowly to the right (small black arrow 23B).

Spreading of the wafers allows space between them, so that the cleaning liquid has easy access to effect a thorough cleaning action.

Figures 6 and 7 show the final position of the wafers, which are disposed uniformly along the entire length of the magazine. Cleaning liquid can now escape between the stack 10 and the nozzle chambers 16. The movable support 15 has now reached its limit of movement. The wafers can no longer spread out in a horizontal direction. In this condition forces in a vertical direction reach a balance, with pressure, buoyancy, gravity and drag forces in equilibrium

Pressure of the cleaning liquid is then reduced, but is still sufficient to hold the wafers clear of the rods 12. In this state the stack is compressed by leftward movement of the movable end support 15.

Primary cleaning and then a separate rinsing process can be carried out with the stack held in the same magazine, and the liquids being applied in the same tank. All wafers are washed simultaneously. During the cleaning process the wafers have no contact with the rods 12, as the wafers 'float' within the tank. Depending on the amount of contamination, wafer thickness, size and liquid consumption, the float height is adjustable by regulating the liquid pressure.

In the example illustrated in the drawings, the magazine 1 1 is separate from the nozzle chambers 16. This is one preferred form, but alternatively the nozzle chambers may be linked to the rods 12, so that the magazine and nozzle chambers can be moved together from tank to tank. The example shown in the drawings is believed to be the preferred form. This makes it possible to have just one washing station with fixed nozzle chambers, so that several magazines loaded with stacks of wafers can be moved through that station in sequence.

Specific dimensions applicable to one particular process (and without limitation) are described below.

The size of the nozzles should be small (e.g. <1 mm) to prevent damage to the wafers. The distance between nozzles could be 2-20 mm. The length of the drilled plate (in which the nozzles are formed) could be 10-2000 mm and with a width of 20-2000 mm. A typical arrangement for one stack of 700 wafers, each wafer being 156x156x0.18 mm, could have nozzle chambers of 180 mm, with hole diameters of 0.8 mm, spaced from adjacent holes by 5 mm. In this case the movable support could travel through a distance of around 40 mm.

Advantages of the Invention

Because there is no direct contact between the wafers and the rods during cleaning, the nozzles will inject narrow streams of liquid between the wafers. These streams cannot significantly distort the edges of the wafers, and consequently there will be thorough cleaning with reduced wafer edge defects.

While it is difficult to assess the influence of various effects from upstream processes, the reduction in defects arising from the washing process of the invention can be assessed broadly. For instance, with a wafer thickness of 180 μηι, it has been found that there is an 0.8 % crack and breakage level with basket and horizontal line cleaning, but only 0.2 % with the present invention.

Claims

1. Apparatus for cleaning wafers in a stack comprising a multiplicity of wafers, and comprising a generally horizontal trough into which the stack of wafers is to be placed, the axis of the trough being perpendicular to the planes of the individual wafers, supports within the trough to hold the wafers off lower parts of the trough, a fixed support for one end of the stack and sufficient space in a direction away from this end of the stack to allow the wafers to separate by a distance such that capillary action between them is broken,

in which

2. Apparatus as claimed in claim 1 , in which there is a movable support for the other end of the stack, the movable support being free to move a limited distance in a direction perpendicular to the planes of the individual wafers to allow the wafers to separate such that capillary action between them is broken.

3.. Apparatus as claimed in claim 2, in which there is provision to move the moveable support back towards the fixed support, so that when the pressure from the jets of liquid has been reduced, but before the wafers settle back onto the supports to hold the wafers off the lower parts of the trough, the wafers can be pushed back to their original positions on the axis of the trough.

4. Apparatus as claimed in any one of the preceding claims, in which there is a regulating valve by which the volume of liquid fed to the liquid feed supply can be controlled, so to regulate the pressure in the jets of liquid to hold the wafers off the supports which hold the wafers off the lower parts of the trough.

5. Apparatus as claimed in any one of the preceding claims, in which the provision to introduce jets of liquid is formed of closed chambers with a liquid feed supply and small holes to form the jets directed towards the interior of the trough.

6. Apparatus as claimed in any one of the preceding claims, in which the supports within the trough to hold the wafers off the lower parts of the trough are rods of sufficient rigidity to hold the wafers in position.

7. Apparatus as claimed in any one of the preceding claims, in which the end supports and supports to hold the wafers off the lower parts of the trough are formed as a magazine, which is separate from the provision for jets of liquid.

8. Method of cleaning wafers in a stack comprising a multiplicity of wafers, including the step of supporting the wafers in vertical planes, supplying jets of liquid from beneath the wafers, the jets being of sufficient strength to lift the wafers off the supports, allowing the wafers to spread horizontally, so that the jets of liquid can clean the faces of the wafers, and then, when the wafers have been cleaned, urging the wafers together for removal from the supports.

9. Wafers when cleaned in the apparatus of claims 1 to 7, or by the method of claim 8.