Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
2000-09-29
2003-07-08
Gulakowski, Randy (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S001000, C134S001300, C134S002000, C134S021000, C134S025400, C134S032000, C436S169000, C436S169000
Reexamination Certificate
active
06589356
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method for cleaning a silicon-based substrate in an ammonia-containing solution and more particularly, relates to a method for cleaning a silicon wafer in a cleaning solution containing ammonia without incurring ammonia vapor damages to the silicon surface of the wafer.
BACKGROUND OF THE INVENTION
In the manufacture of semiconductor devices, a large quantity of deionized (DI) water is required to process silicon wafers. The consumption of DI water increases with the size of the wafers. For instance, the consumption at least doubles in the processing of 200 mm size wafers when compared to the consumption in the processing of 150 mm size wafers. DI water is most frequently used in tanks and scrubbers for the frequent cleaning and rinsing of wafers in process. It is desirable that the surface of a wafer be cleaned by DI water after any process has been conducted on the wafer, i.e., oxide deposition, nitride deposition, SOG deposition or any other deposition or etching process. Such wafer cleaning step is accomplished by equipment that are installed either in-line or in a batch-type process.
For instance, a cassette-to-cassette wafer scrubbing system is one of the most used production systems for wafer cleaning prior to either a photoresist coating, oxidation, diffusion, metalization or CVD process. A typical automated wafer scrubber combines brush and solution scrubbing by DI water. The scrubber utilizes a hyperbolic high pressure spray of DI water with a retractable cleaning brush. A typical mechanical scrubbing process consists of rotating a brush near a wafer surface that is sprayed with a jet of high pressure DI water at a pressure between about 2,000 and about 3,000 psi. The brush does not actually contact the wafer surface, instead, an aquaplane is formed across the wafer surface which transfers momentum to the DI water. The movement of the DI water thus displaces and dislodges contaminating particles that have been deposited on the wafer surface. Contaminating particles are thus removed by a momentum transfer process. As a result, larger particles become more difficult to dislodge and remove from a wafer surface.
A typical wafer scrubbing process consists of a DI water spray step followed by a spin dry and nitrogen gas blow dry step. In a typical wafer scrubbing equipment, production rates are generally between 60 to 120 wafers per hour depending on the program length. The spinning speed of the wafer is between 500 to 10,000 rpm while under a water pressure of up to 6,000 psi.
In more recently developed wafer scrubbing systems, inline systems are used which provide high pressure DI water scrubbing only while eliminating the possibility of wafer contamination by overloaded brushes. The water pressure in these systems range between 3,000 to 6,000 psi which are ejected from a nozzle mounted on an oscillating head. The wafer is spun when the oscillating spray is directed onto the wafer surface. After the cleaning step, wafer is dried by a pure nitrogen gas purge to promote rapid drying. After the scrubbing operation, wafers can be loaded into an in-line dehydration baking system for thorough drying. Batch-type systems are also used with DI water for cleaning, rinsing and drying prior to many IC processes. The systems can be programmed wherein wafers are loaded in cassettes before each cycle. One disadvantage of the batch system is their inability to be integrated into part of an automated wafer processing line.
In the conventional DI water cleaning systems, the basic requirements for the DI water cleaning system are that it provides a continuous supply of ultra-clean water with very low ionic content. It is believed that ionic contaminants in water, such as sodium, iron or copper when deposited onto a wafer surface can cause device degradation or failure. It is therefore desirable to eliminate all such ionic content from a DI water supply prior to using the water for cleaning wafers. A conventional method of measuring the ionic content in DI water is by monitoring the water resistivity. A water resistivity of 18×10
6
Ohm-cm or higher indicates a low ionic content in the DI water. In a conventional water purifying system, several sections which include charcoal filters, electrodialysis units and a number of resin units to demineralize the water are used for purifying the water.
Deionized water is frequently used in a wet bench process after a metal etching process has been conducted on a semiconductor wafer. When residual etchant chemical must be removed, deionized water rinse is used in a wet bench process for semiconductor wafer processing to perform two major functions of a quick dump rinse (QDR) and a cascade overflow rinse. Conventionally, the two functions are carried out in separate tanks in order to produce the desirable result. One of the major processing issues presented by the conventional dual-tank process is the particle re-deposition problem during a withdrawal step when cassettes are transported from a quick dump rinse tank to a cascade overflow tank. A second major issue is the large floor space required for accommodating the two tanks.
A conventional wet bench wafer cleaning process is shown in FIG.
1
. The wet bench wafer cleaning process
10
for cleaning wafer
12
is carried out in six separate cleaning and rinsing tanks sequentially of a HF cleaning tank
14
, a first quick dump rinse (QDR) tank
16
, a SC-1 cleaning tank
18
, a second quick dump rinse tank
20
, a SC-2 cleaning tank
22
and a third quick dump rinse tank
24
. The first HF cleaning tank is used to hold a diluted HF solution, for instance, at a concentration of 0.5% HF in H
2
O for removing a thin native oxide layer from the wafer surface. After the diluted HF cleaning process, the wafer
12
is rinsed in a first quick dump rinse tank
16
with deionized water. Wafer
12
is then cleaned in a second cleaning tank filled with SC-1 cleaning solution, i.e. a mixture of NH
4
OH, H
2
O
2
and DI water at a ratio of 1:1:5. The SC-1 cleaning solution is used at a temperature between 70~80° C. for a suitable time period. The wafer
12
is then rinsed again in a second quick dump rinse tank
20
that is filled with DI water. In the final stage of cleaning, the wafer
12
is cleaned in tank
22
filled with a cleaning solution of SC-2 which is a mixture of HCl, H
2
O
2
and DI water at a ratio of 1:1:5. The wafer
12
is then rinsed in a third quick dump rinse tank
24
with DI water.
The wet bench wafer cleaning process
10
shown in
FIG. 1
is conventionally used for pre-diffusion clean, pre-gate oxidation clean, pre-CVD clean, etc. For instance, in the ULSI fabrication of integrated devices, the conventional wet bench wafer cleaning process
10
can be advantageously used for wafer surface cleaning before a coating process in a CVD chamber or an oxidation process in a furnace.
Despite the fact that the conventional wet bench wafer cleaning process
10
is widely used, serious processing disadvantages of the process has been observed. For instance, during the SC-1 cleaning process carried out in tank
18
, since SC-1 contains about 28% NH
4
OH which tends to form NH
4
OH vapor in the tank cavity over the surface of the solution. When wafer
12
is removed from the SC-1 cleaning solution and taken out of tank
18
, NH
4
OH vapor attacks the clean wafer surface, i.e. the fresh silicon surface of the wafer. As a consequence, a defect known as “silicon hole” frequently occurs wherein craters in the silicon surface are formed due to NH
4
OH vapor attack.
It is therefore an object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process without the drawbacks or shortcomings of the conventional wet bench process.
It is another object of the present invention to provide a method for cleaning a silicon-based substrate in a wet bench cleaning process in which NH
4
OH vapor attack on the silicon surface is avoided.
It is a further object of the present invention to provide
Chang Juin-Jie
Kao Rong-Hui
Twu Jih-Churng
Gulakowski Randy
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
Winter Gentle
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