Method for growing thin silicon oxides on a silicon...

Details Method for growing thin silicon oxides on a silicon... Method for growing thin silicon oxides on a silicon...

1997-11-26

2002-05-14

Pham, Long (Department: 2823)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

C438S774000, C438S790000

Reexamination Certificate

active

06387827

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to low oxidation power, i.e. at low temperature and low oxygen concentration, thermal oxidation processes in the presence of a chlorine source. Said processes can be used during the manufacturing of semiconductor devices. Specific examples of use of such processes are the growth of ultra-thin oxide layers, the Cl-cleaning of a substrate and the temperature ramp-up cycles prior to the oxide growth.
BACKGROUND OF THE INVENTION
In thermal oxidation processes the aim is to grow SiO
2
films by exposing silicon to O
2
at elevated temperatures. Historically chlorine has been introduced in the oxidation ambient in order to improve the electronic quality of gate oxide layers. Studies have revealed that the improvements by introducing chlorine are in fact initiated by the presence of Cl
2
. Particularly the reduction of electronic instabilities, attributed to the presence of mobile ions mainly Na, has been emphasized. In addition, the use of Cl during gate oxidation was also found to result in a reduction of the density of dielectric breakdown defects and of stacking faults. It has been demonstrated that metal contamination on the wafer surface prior to gate oxidation has a distinct negative effect on the dielectric integrity of thin oxides. Particularly Ca has been identified as one of the most detrimental metals in that respect. The introduction of Cl in the oxidation ambient was found to be very efficient in removing metal contaminants, especially Ca, from the silicon wafer surface. In order to meet the stringent future gate-oxide defect density requirements, the residual concentration of metals and of Ca in particular has to be further reduced.
Most oxidation tools are now equipped for the introduction of chlorine species during silicon wafer oxidation and/or in situ tube cleaning operations. Several sources have been used to introduce chlorine. In order to compare these different methods a common parameter describing the concentration of the total amount of Cl fed to the reactor chamber, irrespective of its chemical state, is introduced. Said parameter is the “chlorine-equivalent concentration of a given Cl-source” and is defined as the ratio between “the total flow of Cl atoms [number of Cl atoms per unit time ] to the process chamber” and “the total flow of all molecules [number of molecules per unit time] to the process chamber”.
In the past it was common practice to feed HCl gas to the oxidation furnace. Although this gas was effective for this application, its use has several drawbacks. Because of its corrosive nature, this gas deteriorates the metal distribution lines as well as the metal components in the gas management system. Such corrosion phenomena result in highly undesirable metallic contamination of the gases. Moreover the handling of the pressurized gas cylinders requires special care.
Because of these drawbacks the industry has used, 1,1,1-trichloroethane (TCA) as the source for Cl in the furnace. TCA is a volatile liquid and can be introduced into process tools via Teflon™ tubing thereby avoiding the corrosion phenomena faced with HCl. Since TCA has been identified as an ozone depleting substance, attacking the stratospheric ozone layer, its production, use and/or transportation has been restricted or even banned.
In response the industry has come up with ozonelayer-friendly replacement substances for TCA such as trans-1,2-dichloroethylene (DCE) and oxalyl chloride (OC). The replacement with DCE is the subject of the United States patent U.S. Pat. No. 5,288,662. The replacement with OC is the subject of the European Patent EP 0 577262 B1. In virtually all industrial practice of Cl-oxidation, a Cl-equivalent concentration of the Cl-source of 1-3% is used as illustrated by the example1 and comparison2of the European Patent EP 0 577262 B1 and in the United States patent U.S. Pat. No. 5,288,662. In general, when Cl-carbon precursors are used, care has to be taken to get a complete enough combustion of the molecule. Regarding said combustion, the chemistry for OC is substantially different from that for either one of TCA or DCE. Because OC contains no hydrogen, all the Cl in the precursor is made available in the process tube as Cl
2
(equation 1), provided of course that water is not deliberately added. In contrast, as in HCl itself, in the TCA and DCE molecules the number of hydrogen atoms equals the number of chlorine atoms. Therefore, during combustion, TCA (equation 2) and DCE (equation 3) are sources for HCl. Only a fraction, typically about 10%, of the so formed hydrogen chloride is (further) oxidized to form Cl
2
and H
2
O, according to the equilibrium of the reaction (equation 4). It is obvious that said fraction depends on the parameters which affect the thermodynamical equilibrium like the percentage O
2
in the ambient. When this percentage is about 100%, said fraction is about 10%.
C
2
Cl
2
O
2
+O
2
→ Cl
2
+2 CO
2
  (1)
C
2
H
3
Cl
3
+2 O
2
→ 3 HCl+2 CO
2
  (2)
C
2
H
2
Cl
2
+2 O
2
→ 2 HCl+2 CO
2
  (3)
4 HCl+O
2  
2 Cl
2
+2 H
2
O   (4)
Consequently to ensure a complete combustion of TCA and DCE, the O
2
concentration should be very high (a multiple e.g 10-fold of the stoichiometrical requirement) and the temperature should be sufficiently high. Therefore in the state of the art applications of Cl-carbon precursors, the Cl-source is only on when the larger fraction of the process chamber ambient consists of O
2
. Typically almost pure O
2
is used and only a smaller fraction of N
2
is added through the introduction of the Cl-carbon using a bubbler, as illustrated by the United States patent U.S. Pat. No. 5,288,662.
The ongoing downscaling of CMOS device dimensions, in particular the gate length, demands for an ongoing reduction of the gate oxide thickness in order to meet the required device performance specifications. With this required shrinkage of the thickness of high quality gate oxides, the use of organic molecules to introduce Cl in the furnace has become more critical. In order to obtain a good thickness control the process for growing thin oxides requires a milder overall oxidation condition, especially a low temperature treatment and a reduced oxygen concentration. Consequently, the organic Cl containing molecules will undergo also a milder oxidation, yielding the risc of only partial combustion of said molecules and risc of formation of highly toxic compounds like e.g. phosgene.
In recent years a new process was introduced referred to as the “pyro-clean”, see B.-Y. Nguyen et al, in Tech. Dig. 1993 Symp. on VLSI Technol., (JSAP, Tokyo, 1993) p. 109. In this process an in-situ low temperature Cl-treatment prior to the gate oxidation process is used. The motivation for this process is based on the fact that the diffusion constant and the solubility of a number of metals in silicon increases strongly with increasing temperature. The purpose of this process is to remove metallic contamination before the onset of diffusion of metal into bulk silicon. Typically a 30 minutes treatment at 650° C. is performed using an inert (e.g. N
2
) ambient containing O
2
at a volume concentration of 2%. As a Cl source, HCl was chosen with a Cl-equivalent concentration of 3%. The addition of the small amount of oxygen is expected to be beneficial with regard to organic contamination, preventing destabilisation of the SiO
2
phase and limiting surface etching and roughening. At the same time the oxygen concentration should be kept low enough in order to limit the thickness of the oxide layer grown during this pre-oxidation step, particularly when the final oxide layers that are to be grown should be thin. The process conditions for the “pyro-clean” in B.-Y. Nguyen et al, in Tech. Dig. 1993 Symp. on VLSI Technol., (JSAP, Tokyo, 1993) p. 109 are a low temperature, a Cl-equivalent Cl-source concentration of 3% and a low oxygen concentration. In cited document HCl is used as a chlorine sour

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