Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
1999-12-20
2003-07-15
Markoff, Alexander (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S026000, C134S029000
Reexamination Certificate
active
06592676
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a method for reducing the metal contamination on a surface of a semiconductor substrate which is submitted to a wet cleaning process.
BACKGROUND OF THE INVENTION
One of the challenges in semiconductor device processing is the problem of metal contamination on the semiconductor surface. Semiconductor devices are extremely sensitive to minute quantities of metals adsorbed to the semiconductor surface. These metallic species are well known to deteriorate electrical performance. Particularly, Group IIA metals have been found to be very detrimental to the gate oxide layers. There is an important need to reduce metallic contamination to low levels.
The RCA cleaning sequence which is still standard in semiconductor (silicon wafer) device processing was devised in the years 1970s. The SC1 or Standard Clean 1 process, using a mixture of concentrated NH
3
and H
2
O
2
mixed with H
2
O in a ratio of 1:1:5, is used largely to remove particulates. The SC2 or Standard Clean 2 process, using a mixture of HCl, H
2
O
2
and H
2
O, is used to remove metallic contaminants from the silicon surface. These SC1 and SC2 cleaning steps, in either order are each followed by a rinsing step in DI water, whose purpose is to remove acidic or caustic species from the wafer and to remove metallic contaminants remaining in solution near the wafer surface from proximity with the wafer.
Current cleaning and rinsing techniques have proven to be unsuccessful at reducing metallic contaminants to the low levels required without significantly increasing at great the cost of the DI rinse water supply purity. The final rinse of the cleaning process contributes to metal contamination, even in ultra pure water (UPW), and must be improved. The microelectronics industry also needs to lower its costs. In cleaning, this can be done through the introduction of simplified cleaning recipes, including a single chemistry clean that has both good particle and metal performance. Ammonia-peroxide-water mixtures (APM) today have good particle performance, but their performance with respect to metallic contamination needs much improvement.
It was shown by Hall and co-workers (Hall et al., IEST 1998 Proceedings, the ICCCS 14
th
Int. Symp. On Contamination Control, Apr. 26-May 1, 1998) that calcium contamination and contamination by other metals is worse for dilute SC1 chemistries, where the concentrated chemicals NH
3
, H
2
O
2
and H
2
O are mixed in a ratio of 1:1:30, than for SC1 solution, where the concentrated chemicals NH
3
, H
2
O
2
and H
2
O are mixed in a ratio of 1:1:5. Hall's explanation for the reduced adsorption of Ca and other metal ions is that they form amine complexes, that is M(NH
3
)
n
+
, where n typically is between 1 and 6.
The Applicants have previously suggested that H
+
always is present in the environment of the silicon wafer so that metals do not attach to the silicon substrate (see EP-96309145.9 incorporated herein by reference). This is indeed a possible solution to the recontamination problem, but it is not applicable in all circumstances. Notably, it is not possible to keep the wafer environment always acidic during a rinse or during APM or SC1 treatment. Coming out of an SC1 process or a similar process, wafers would go through various levels of alkaline solution, and then pass through neutrality, before reaching an acid rinse condition, even if an acidic rinse is carried out.
U.S. Pat. No. 5,290,367 discloses a surface cleaning solution for use in semiconductor processing, said solution containing as a complexing agent a compound having one or more phosphoric acid groups or a salt thereof and a polyphosphoric acid or a salt thereof. The complexing agent is binding to metals and therefore reduces the adsorption of metals to the semiconductor surfaces.
U.S. Pat. No. 5,466,389 discloses an alkaline cleaning solution for use in semiconductor IC processing. The invention is aimed to avoid the presence of unstable compounds such as H
2
O
2
in cleaning solutions used for semiconductor processing. The invention furthermore aims to achieve semiconductor wafer cleaning with such cleaning solutions without causing undue wafer roughness.
The compound N(CH
3
)
4
OH is known to be a possible alternative for NH
4
OH in SC1 solutions. N(CH
3
)
4
OH (TMAH) is strongly dissociating in aqueous environments.
Other prior art patents disclosing methods for reducing the adherence of metallic impurities to semiconductor surfaces include EP649168 and EP528053. The solutions as disclosed in these patents are successful in reducing the contamination level of only a limited number of metal ions. The addition of phosphonic acid to the cleaning solution disclosed in EP528053 can introduce undesired organic residue on the substrates and the phospor element may lead to an undesired doping of the semiconductor substrate. EP649168 furthermore is limited to acid solutions having a pH of lower than about 5.
AIMS OF THE INVENTION
The present invention includes a method for enhancing the metal removal capability of a semiconductor substrate in a substrate cleaning process, without requiring greatly increased chemical purities.
By “enhancing the metal removal capability”, it is to be understood for the purpose of the present patent application, reducing surface concentrations on the surface i.e. removing the metal contaminants from the surface of the semiconductor substrate and/or preventing the adsorption of metal contaminants present in the chemical solution onto said surface.
The present invention therefore aims to enable a cost-effective single chemistry cleaning.
SUMMARY OF THE INVENTION
The present invention is related to a method for reducing the metal contamination on a surface of a semiconductor substrate wherein said substrate is submitted to a wet cleaning process in a solution capable of oxidising said surface and containing a substance for substantially reducing the amount of metal ions bound to the oxidised surface.
In a first aspect of the present invention, a method for reducing the metal contamination on a surface of a semiconductor substrate is disclosed. According to the method of the invention, said substrate is submitted to a wet cleaning and/or rinsing process in a solution capable of oxidising said surface and containing a substance strongly ionising in said solution whereby creating an amount of ions of at least one species in said solution. At least one of the ion species is having such size and shape that the ions of this species are binding strongly to the oxidised surface and said amount of ions is sufficient to substantially reduce the amount of metal ions bound to the oxidised surface.
The substance is strongly ionising in said solution whereby creating an amount of ions of at least one species in said solution. Thus the substance is dissociating, or possibly hydrolysing, into ions. The ions preferably are plurality NH
4
+
ions and/or derivates and/or compounds thereof that have the beneficial impact on reducing the amount of metal ions bound to the oxidised surface. With the term strongly ionising, it is meant that at least about 6% of the molecules of the substance is dissociating into ions. Under the term strongly ionising, it is also understood that about 7 to 8 or 9-10-15-20-30-40-50-60-70-80-90-100% of the molecules of the substance are dissociating into ions. These numbers of strong ionisation are given for room temperature conditions, equivalent numbers being relevant for higher or lower temperature conditions of practising the invention. The invention can be practised at temperatures lower (above 0° C.) or higher (for instance 50 or 60 or 80 or 100° C. or higher) than room temperature. The invention can further be practised at ambient temperature.
Preferably the solution is at a pH value of about 5 or at a pH value of about 5.5 or 6 or 6.5 or 7 or higher. Preferably also said solution does not contain in combination a nonionic surfactant and an effective amount of a pH reducing chemical component to reduce or control
Loewenstein Lee
Mertens Paul
Vereecke Guy
Interuniversitair Micro-Elektronica Centrum
Markoff Alexander
McDonnell & Boehnen Hulbert & Berghoff
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