Methods and apparatus for processing the backsides of wafers

Abstract

In one aspect, a method of processing the backsides of a batch of semiconductor wafers includes receiving a cassette for wafers in its inverted orientation in which the wafers are horizontally orientated with their front faces facing downwardly, removing the wafers sequentially, or as a batch, and processing the upwardly facing backsides of the wafers. In another aspect, an apparatus for processing wafers from a cassette holding wafers horizontally includes a cassette support where the cassette support is formed to receive an industry standard cassette in its inverted position but not in its upright position.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for processing the backsides of wafers.

2. Description of Related Art

The semiconductor wafer industry uses standard carriers (“cassettes”) for storage, transportation and automated loading of wafers into equipment. These standards are set by SEMI (Semiconductor Equipment and Materials Institute) and defined cassette dimensions for various wafer sizes. In all cases these cassettes have a defined bottom, typically with an ‘H’ bar shape for accurate location upon a matching platform and a top without an ‘H’ bar and sometimes with a handle. The relevant SEMI standards include but are not limited to SEMI E1, E1.7 and E1.9. The ‘H’ bar is also referred to as ‘bar’ and ‘crossbar’.

It is becoming more common to process, inspect or otherwise access the backside (bottom) of wafers and this frequently requires wafers to be inverted after unloading from a cassette, or to be inverted and loaded into a cassette upside down. Such processes might include optical inspection, sputter deposition chemical vapour deposition or plasma etching. These processes are most efficiently done with the wafer lying horizontal upon a support with the process taking place above the wafer. For backside processing the wafer must therefore be facing front-down. An example written procedure taken from Stamford University Nanofabrication facility as at Aug. 29, 2003 is as follows: “Flip Transfer” of wafers:

1. Obtain an empty receiver cassette.

• • Make sure it is the appropriate kind and level for cleanliness for your process.
  • Make sure you are wearing clean vinyl gloves

2. Place the receiver cassette upside down over the donor cassette.

• • The protruding pins should mate with the alignment holes on both sides of the cassettes.

3. Grasp both cassettes in both hands and “flip” the donor cassette upside down onto the receiver cassette. Make sure you have a firm grip on both cassettes and that they are properly aligned and mated; otherwise, you risk dropping (and thus damaging) all your wafers.

4. Your wafers have now been transferred.

For automated wafer inversion, batch transfer equipment is available to lift all the wafers from a cassette typically lying on its back, and transfer to another cassette that is the opposite way around.

SUMMARY OF THE INVENTION

From one aspect the invention consists in a method of processing the backsides of a batch of semiconductor wafers including:

(a) receiving a cassette for wafers in its inverted orientation in which the wafers are horizontally orientated with their front faces facing downwardly.

(b) removing the wafers sequentially, or as a batch; and

(c) processing the upwardly facing backsides of the wafers.

Preferably the wafers are returned to an inverted cassette in their “removed” orientation and the unloaded cassette may then be re-inverted or not depending on the next processing step.

From another aspect the invention consists in apparatus for processing wafers from a cassette holding wafers horizontally including a cassette support wherein the cassette support is formed to received an industry standard cassette in its inverted position but not in its upright position.

The reception of a cassette in its correct inverted orientation may be determined by optical means.

The support may include means for varying its upper surface profile between a profile which will accept an upright cassette and a surface that will receive an inverted cassette. This can be simply an adaptor plate to be located on the cassette support or it may be a formation which can be moved proud of and beneath the support surface.

The apparatus may include means for rotating a cassette through 180°, in which case the profile varying means may operate in synchronism with the cassette rotating means.

From a still further aspect the invention may consist in apparatus for processing a wafer from a cassette including cassette support, means for determining the orientation of a cassette and cassette handling means for placing the cassette on the cassette support in a desired orientation. The orientation determining means may include an optical scanner for reading a barcode or other visual identity on the cassette.

Although the invention has been described above it is to be understood that it includes any inventive combination of the features set out above or in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways and specific embodiments will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of a metal cassette (prior art);

FIG. 2 is a schematic of a top plate of a cassette elevator for accepting the bottom of a cassette from 100 mm to 200 mm in size (prior art);

FIG. 3 illustrates an embodiment of the invention;

FIG. 4 is a reproduction of SEMI standard E1 FIG. 1A (prior art); and

Exhibit 1 is a reproduction of pages 3 to 8 of SEMI standard E1.9 (prior art).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As has been mentioned above the semiconductor industry uses standard ‘carriers’ better known as cassettes with defined bottoms for locating cassettes in a vertical orientation on a matching plate for removal of wafers parallel with the plate. It is a characteristic of these plates that they will only accept the cassette in one bottom-down orientation. In FIG. 1 a cassette 10 has slots to accept a plurality of wafers and a bar 14 which together with the bar ends 13 on the bottom 16 of the cassette forms an ‘H’ shape referred to in the SEMI standards. As will be seen in FIG. 2 a cassette receiving plate 11 of an elevator schematically shown, in part, at 12 has a formation shown in particular at 17 in its upper surface 15. In this particular example a bar 19 fits within a slot 20 and when a cassette is placed correctly upon the plate 11 the bar 19 is displaced and actuates a switch or optical device (not shown) to signal the correct placement of a cassette.

It can be seen from the above description that the cassette can only be placed bottom side down upon the plate for the elevator system 12 to operate.

The inventors have appreciated the difficulties, additional wafer handling and/or additional equipment requirements of ‘flip transfer’ discussed above and have devised a method and apparatus for overcoming these as illustrated in FIG. 3.

Here an elevator system shown partially at 121 having top plate 111 with top surface 151 has a formation 171 to receive to top face 18 of a cassette 10. Correct inverted location is signalled e.g. by a bar 191 being displaced through a slot in top plate 111. movement of the bar is sensed and signalled to the wafer processing system (not shown) of which the elevator system 121 is a part.

Additionally or alternatively to the physical parts of the top plate 111, an optical system 22 may detect visual indicators such as bar codes, cut out or any other suitable mark upon the cassette to signal the correct inverted orientation and location of the cassette.

Additionally a gripper 23 may be part of or attached to the wafer processing system and may be connected to a system incorporating the optical system 22 such that a cassette may be delivered e.g. robotically to the wafer processing system and be inverted before being located upon the top plate 111.

At exhibit 1 is shown part of the SEMI standard for 300 mm cassettes showing that, whilst these cassettes are visually quite different than for the smaller sizes, they still have a defined ‘bottom’ and ‘top’, and aspect of their design to enable optical sensing of orientation (including inversion) and location.

It is an advantage of this invention that wafers are always in cassettes the correct way up i.e. backside to bottom of cassette and the processing apparatus detects and/or receives an inverted cassette. It is therefore much less likely to mistakenly process a wafer front side when a backside process was intended than when wafers have to be loaded into cassettes the wrong way up, as is current practice when using deposition or etching equipment e.g. for depositing backside metals or etching through a wafer vias.

Claims

1. A method of processing the backsides of a batch of semiconductor wafers including: (a) receiving a cassette for wafers in its inverted orientation in which the wafers are horizontally orientated with their front faces facing downwardly. (b) removing the wafers sequentially, or as a batch; and (c) processing the upwardly facing backsides of the wafers.

2. A method as claimed in claim 1 wherein the wafers are returned to an inverted cassette.

3. A method as claimed in claim 1 wherein an unloaded cassette is re-inverted.

4. A method as claimed in claim 1 including inverting the cassette from its normal orientation in which the wafers have their front faces facing upwardly.

5. Apparatus for processing wafers from a cassette holding wafers horizontally including a cassette support wherein the cassette support is formed to received an industry standard cassette in its inverted position but not in its upright position.

6. Apparatus as claimed in claim 5 wherein the support includes means for varying its upper surface profile between a profile which will accept an upright cassette and a surface that will receive an inverted cassette.

7. Apparatus as claimed in claim 5 further including means for rotating a cassette through 180°.

8. Apparatus as claimed in claim 7 wherein the profile varying means operates in synchronism with the cassette rotating means.

9. Apparatus for processing a wafer from a cassette including a cassette support, means for determining the orientation of a cassette and cassette handling means for placing the cassette on the cassette support in a desired orientation.

10. Apparatus as claimed in claim 9 including a cassette support formed to receive an industry standard cassette in its inverted position, but not in its upright position.