System for cleaning a semiconductor wafer

Details System for cleaning a semiconductor wafer System for cleaning a semiconductor wafer

1999-06-10

2004-03-30

Spisich, Mark (Department: 1744)

Brushing, scrubbing, and general cleaning

Machines

Brushing

C015S088300

Reexamination Certificate

active

06711775

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor wafer cleaning and, more particularly, to techniques for more efficiently applying cleaning fluids over a wafer and improving wafer cleaning throughput.
2. Description of the Related Art
In the semiconductor chip fabrication process, it is well-known that there is a need to clean a wafer where a fabrication operation has been performed that leaves unwanted residuals on the surface of the wafer. Examples of such a fabrication operation include plasma etching (e.g., tungsten etch back (WEB)) and chemical mechanical polishing (CMP). If left on the surface of the wafer for subsequent fabrication operations, the unwanted residual material and particles may cause, among other things, defects such as scratches on the wafer surface and inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is therefore necessary to clean the wafer adequately yet efficiently after fabrication operations that leave unwanted residue on the surface of the wafer.
FIG. 1A
shows a high level schematic diagram of a wafer cleaning system
50
. The cleaning system
50
typically includes a load station
10
where a plurality of wafers in a cassette
14
may be inserted for cleaning through the system. Once the wafers are inserted into the load station
10
, a wafer
12
may be taken from the cassette
14
and moved into a brush box one
16
a
, where the wafer
12
is scrubbed with selected chemicals and water (e.g., de-ionized (DI) water). The wafer
12
is then moved to a brush box two
16
b
. After the wafer has been scrubbed in the brush boxes
16
, the wafer is moved into a spin, rinse, and dry (SRD) station
20
where DI water is sprayed onto the surface of the wafer and spun to dry. During the rinsing operation in the SRD station, the wafer rotates at about 100 rotations per minute or more. After the wafer has been placed through the SRD station
20
, the wafer is moved to an unload station
22
.
FIG. 1B
shows a simplified view of a cleaning process performed in brush box one
16
a
. In brush box one
16
a
, the wafer
12
is inserted between a top brush
30
a
and a bottom brush
30
b
. The wafer
12
is capable of being rotated to enable the rotating brushes
30
a
and
30
b
to adequately clean the entire top and bottom surfaces of the wafer. In certain circumstances, the bottom surface of the wafer is required to be cleaned as well because contaminants from the bottom may migrate to the top surface
12
a
. Although both the top surface
12
a
and the bottom surface of the wafer are scrubbed with the brushes
30
, the top surface
12
a
that is scrubbed with the top brush
30
a
is the primary surface targeted for cleaning, since the top surface
12
a
is where the integrated circuit devices are being fabricated.
After typical CMP operations, a wafer is placed into the cleaning station
50
. In brush box one
16
a
, the top brush
30
a
and the bottom brush
30
b
are preferably concentrated with a cleaning chemical, which is received from a source
32
. Once scrubbing is performed with the chemicals, it is generally desired to have the wafer surface
12
a
cleaned with water. The water cleaning is carried out such that substantially all of the chemicals used during the scrubbing are removed from the surface of the wafer
12
a
. In the prior art, the standard process is to pass the water through the brush (TTB).
However, because the chemical scrubbing was just completed, the brushes will be highly saturated with the cleaning chemicals. Consequently, in order to properly clean the surfaces of the wafer with water, the brushes are typically flushed with large amounts of water in an effort to remove the chemicals from the brushes and from over the wafer surfaces. Unfortunately, although the brushes are flushed with a large amount of water, a lower concentration of the cleaning chemicals remains in the brushes themselves and on the wafer surfaces. Accordingly, such a cleaning process is noticeably flawed because some chemicals used in the cleaning operation itself may remain on the wafer when the wafer is moved to the next brush box.
In some cases, the remaining chemicals can have the disadvantageous effect of causing unwanted reactions with the cleaning chemicals applied in the next brush box, and in other cases, some cleaning chemicals may remain on the wafer surface when the wafer is moved to the SRD station
20
. Unwanted reactions can also have the downside of generating or introducing particulates. Furthermore, if hydrofluoric (HF) acid is used in the cleaning system
50
, it is very important that substantially all of the HF be removed before the wafer is introduced to the SRD station
20
. In situations where some HF remains on the wafer surfaces, the HF can have the destructive effect of eating away at the interior mechanical parts of the SRD station
20
.
Assuming that the scrubbing is complete for a given wafer in brush box one
16
a
, and that the wafer is moved to a next station, another wafer will be introduced into brush box one
16
a
from the load station
10
. Before the new wafer can be cleaned with the cleaning chemicals, a time must pass while the brushes
30
are brought up to the appropriate chemical concentration. This replenishing of the chemicals is necessary because during the cleaning of the prior wafer the brushes were flushed with water to remove the chemicals and perform the DI water cleaning. After some time passes, the brushes will once again be ready to be applied to the wafer so that the chemical cleaning can be performed with the brushes.
It should be apparent that the aforementioned cleaning technique is unduly inefficient. Such a cleaning process has the downside of taking more time to load the brushes with chemicals to the desired chemical concentration, flush the chemicals from the brushes to perform the water cleaning, and then re-loading the brushes with chemicals again. Not only is the process inefficient, this process can be unsafe, in that unwanted chemical reactions can occur, particulate generation can be promoted, and the mechanical components of the cleaning station
50
can thereby be placed at risk of degradation.
In view of the foregoing, there is a need for a cleaning process that avoids the problems of the prior art by improving cleaning fluid application techniques and increasing wafer cleaning throughput.
SUMMARY OF THE INVENTION
Broadly speaking, the present invention fills these needs by providing an improved method for cleaning a semiconductor wafer. The method implements a technique for maintaining the chemical concentration in the brushes at a substantially constant level throughout the wafer cleaning process. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a method is disclosed for cleaning a surface of a wafer. The surface of the wafer is generally scrubbed with a cleaning brush that applies a chemical solution to the surface of the wafer. In this embodiment, the cleaning brush implements a through the brush (TTB) technique to apply the chemicals. The scrubbing is generally performed in a brush box, with a top cleaning brush and a bottom cleaning brush. The top cleaning brush may then be removed from contact with the top surface of the wafer. The flow of chemicals through the top brush is preferably stopped, and the chemical concentration in the top brush is preferably maintained at substantially the same concentration that was in the brush during the scrubbing operation. Next, a flow of water (preferably de-ionized water) is delivered to the surface of the wafer. The delivery of water is preferably configured to substantially remove the chemical solution from the surface o

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