Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
2000-10-02
2004-03-02
Carrillo, Sharidan (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S002000, C134S019000, C134S025400, C134S026000, C134S028000, C134S031000, C134S032000, C134S033000, C134S034000, C134S036000, C134S041000, C134S042000, C134S902000
Reexamination Certificate
active
06699330
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of cleaning articles from which contaminants must be removed, and more particularly to a method of cleaning substrates for electronic devices. More specifically, this invention relates to the removal of organic films such as photoresists used when wafers for semiconductors or substrates for liquid crystal display devices are processed, and to the cleaning of wafers to remove organic contamination caused thereon over the whole wafer processing. More broadly, it also relates to the cleaning of precision metal workpieces or glass workpieces to remove their organic contamination.
2. Description of the Prior Art
To remove photoresists used in fine processing on oxide films or polysilicon films, a method is usually used in which a mixture solution of sulfuric acid (3 or 4 parts by volume) and hydrogen peroxide (1 part by volume) (which solution is called a piranha) is heated to 110 to 140° C. and cleaning targets are immersed therein for 10 to 20 minutes. In the case when high-density ion implantation is effected via a resist mask, the resist changes in properties to become unremovable by piranha treatment, and hence ashing plasma-excited oxygen is in wide used. If, however, the whole photoresist is subjected to ashing, a trace metal due to the resist may remain on the wafer surface, and also damage which is harmful for devices may appear on the wafer surface because of high-energy plasma. Accordingly, it is common to effect ashing, leaving the resist film unremoved, followed by piranha treatment to remove the resist. In place of the hydrogen peroxide used in this piranha treatment, it has been attempted to mix ozone. However, because of a low solubility of ozone, the treatment must be made for a still longer time to remove the resist, and such a method is almost not in use.
Recently, a method of removing the resist ozone water has become available. Ozone more dissolves in water as temperature is lower. In about 5° C. ultrapure water, ozone dissolves up to a concentration as high as 70 to 100 ppm. Where the resist is removed ozone water having such a low temperature and a high concentration, it is reported that, in the case of I-ray positive novolak resin photoresist film used widely in LSI fabrication, a film of 800 nm thick can be stripped in about 10 to 15 minutes (stripping rate: 70 to 80 nm/minute).
From atmosphere in a clean room for semiconductor device fabrication, organic matters such as dioctyl phthalate (DOP), siloxanes and hexamethyldisilazane (HMDS) contaminate the surfaces of silicone wafers, oxide films and so forth. It is known that this causes deterioration of device characteristics to lower the yield of devices.
As wet-process cleaning for removing such organic matters on silicone wafers and oxide films, the above piranha treatment has been considered to be most effective. However, SO
4
2−
remains on wafers to cause fine particles under the influence of environmental atmosphere, tending to cause haze. In order to remove it completely, SC-1 treatment (standard composition: NH
4
OH:H
2
O
2
:H
2
O=1:1:5 part(s) by volume) is usually made subsequently. SC-1 treating solution has the action to decompose and remove organic matters even when made alone, and has ever been considered to be most greatly effective for the action to remove fine particles. In the SC-1 treatment, however, Fe, Al, Ca, Mg, Zn, Ni and the like in the treating solution tend to become deposited on wafers, and it is difficult to manage the cleanness of treating solutions and cleaning baths. Accordingly, it has become a conventional means for semiconductor cleaning to remove, dilute HF, chemical oxide films produced in SC-1 treatment and then make SC-2 treatment (standard composition: HCl:H
2
O
2
:H
2
O=1:1:6 part(s) by volume), which is considered to have a good metal-removing ability. This is called the RCA method. To remove the surface residual SO
4
2−
, a method is also used in which rinsing hot water in a large quantity is carried out for a long time, which, however, is inferior in the cleanness to be achievable, usually when the RCA method is made subsequently.
As cleaning methods for wafers contaminated organic matters, the treatment relying on the piranha treatment conventionally made can not be said satisfactory in view of economical advantages, productivity and safety. As a new cleaning method that can solve these problems, the method making use of ozone water has become available. This is a method in which, since ozone water having a concentration of 20 to 30 ppm is obtainable at room temperature, this oxidation power is utilized to remove organic contamination of wafers.
Higher integration of semiconductor devices, in particular, VLSI circuits, it has increasingly become important to reduce organic contamination on wafer surfaces. In roadmaps published by U.S.A. Semiconductor Industrial Society, there has been no description on surface organic carbon concentration till recently. In those published at the end of 1997, they approve a surface organic carbon concentration of 1×10
14
atoms/cm
2
, while this concentration must be made to 1.8×10
13
atoms/cm
2
by 2009. Of course, this cleanness must be achieved also after the stripping of resist. Cleaning solutions for piranha treatment are repeatedly used in view of economical advantages. However, in an attempt that methyl silicon layers ascribable to HMDS in positive-resist primers are removed to such a high level of cleanness, it is difficult to do so for piranha cleaning solutions having deteriorated as a result of repeated use. Accordingly, the number of times for their use must be severely restricted. Hence, this results in an increase in the quantity of sulfuric acid used, bringing about not only an economical disadvantage but also a difficulty in waste-water disposal.
Since also the removal of the resist on a metal film may damage the film when treated with a strong acid, the treatment is made by dissolving the resist at about 70° C. for about 15 minutes using N-methylpyrrolidone (NMP) as a remover. In such a case, rinsing with ultrapure water is carried out after rinsing with an organic solvent such as isopropyl alcohol. This treatment requires to use the organic solvent in a large quantity, and is undesirable in view of economical advantage and besides costly for waste-water disposal.
Accordingly, the treatment with ozone water attracts expectation. High-purity ozone water on the level of semiconductor purpose is produced by making ozone-containing high-purity gas absorbed in ultrapure water. Now, when ozone-containing gas is injected into a container holding a liquid, and where ozone concentration in the gas is represented by C
G
[mg/L] and ozone concentration in the liquid standing saturated with it by C
L
[mg/L], a partition coefficient is given as D=C
L
/C
Q
. Here, when the liquid is water, some research information gives values of D=0.2 at 25° C., D=0.28 at 20° C. and D=0.47 at 5° C. Since the ozone concentration attained by means of a usual high-purity ozone gas generator is about 200 mg/L, calculation made thereon gives saturation concentrations of 40 ppm at 25° C. and 94 ppm at 5° C. In practical use, only concentrations a little lower than these concentrations are attained. Moreover, since ozone tends to decompose in water, the ozone concentration in an ozone water cleaning bath can not be maintained at the highest level unless ozone gas is always injected while the ozone water is circulated. Also, when there is any obstacle to flow, such as a wafer carrier, in the cleaning bath, some part on the wafer surface may come to lack in ozone to cause a decrease in resist stripping rate. Even if the resist itself can provide the stripping rate at a value of about 100 nm/minute, it takes a time twice or more the treatment time calculated from this stripping rate, in order to completely remove the resist completely up to the methyl silicon layer in respect of
Carrillo Sharidan
Nomura Micro Science Co., Ltd.
Pillsbury & Winthrop LLP
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