Optics: measuring and testing – Dimension – Thickness
Reexamination Certificate
2002-02-22
2003-11-11
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Dimension
Thickness
C438S005000, C438S007000
Reexamination Certificate
active
06646752
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method and an apparatus for measuring the thickness of a thin film and more particularly, relates to a method and an apparatus for measuring the thickness of a thin gate oxide layer accurately before moisture and organic residue are deposited on the film to cause erroneous readings.
BACKGROUND OF THE INVENTION
In the fabrication process for semiconductor devices, the process of growing a gate oxide layer for insulating a gate is an important step. In ULSI fabrication, the thickness of the gate oxide layer grown has been reduced to less than 100 Å by using the 0.35 &mgr;m technology. The process control for growing such thin gate oxide layers is therefore more critical than those used in the 0.7 &mgr;m technology for growing gate oxide thicker than 200 Å. To grow the ultra-thin gate oxide layers, particle, organic and metal contaminations must be reduced in a super-clean room technology and with improved cleaning processes. It has been found that in order to improve the integrity of ultra-thin gate oxide layers, the surface of a silicon wafer must be free of native oxide or other contaminants.
When the fabrication technology progresses into 0.1 &mgr;m, the thickness of the gate oxide layer may well be under 50 Å. At such small thickness, the surface micro roughness at the SiO
2
/Si interface also becomes an important factor on channel electron mobility as well as other gate oxide qualities, for instance, the breakdown voltage of the gate oxide layer. Different contaminants may cause different detrimental effects on the device reliability when the gate oxide integrity is in question. The problems are important since gate oxide quality is one of the critical steps that determine the yield, reliability and performance of a ULSI circuit. Problems that may occur due to roughness, impurity and contamination can be the result of insufficient cleaning technology, poorly controlled oxidation technology and how the silicon wafer was prepared. Although various cleaning processes have been developed to remove contaminants, an important consideration is to avoid contamination rather than to clean it up during processing.
The ultra-thin gate oxide layers used in ULSI devices can be formed by many different techniques. One of such techniques is a rapid thermal oxidation process for forming an ultra-thin gate oxide layer of 60~70 Å for a 0.25 &mgr;m technology, or a thickness of 40 Å or less for a 0.18 &mgr;m technology. In a rapid thermal oxidation process, the equipment for performing the oxidation is similar to that used in a rapid thermal processing technique so that a process chamber can rapidly increase the temperature of a wafer, rapidly changing various gas requirements in the chamber and achieving a high vacuum without causing contamination to the wafer surface. In a rapid thermal oxidation process, the gate oxide formation can be carried out at a temperature between 950° C. and 1200° C. with reasonable growth rates.
In order to accurately control the quality of the ultra-thin gate oxide layer formed, the thickness of the layer grown must be accurately monitored. The monitoring or measuring of the ultra-thin gate oxide thickness becomes more important as the film thickness becomes smaller with the 0.25 &mgr;m or the 0.18 &mgr;m technology. Since the gate oxide layer is transparent at such small thickness and is formed over a highly absorbing substrate of silicon, a technique of ellipsometry is frequently used to determine the film thickness. While ellipsometry is used to determine the thickness of thin transparent dielectric layers by utilizing a visible light source, semiconductor layers that are transparent only to infrared light source can also be measured by using infrared. It has been found that for very thin semiconductor layers, i.e., such as in the ultra-thin gate oxide layers, even visible light penetrates deep enough for useful ellipsometric measurements to be made.
The ellipsometry operates by the principle that when an incident beam is plane polarized, the two perpendicular components will have different amounts of phase shift during reflection and therefore different reflection coefficients. The ellipsometry is usually used for the measurement of films of a thickness that is less than one wavelength of the viewing light. When ellipsometry is used on greater thicknesses as an interferometry, multiple number of thickness may have the same ellipsometric data.
The basic arrangement of an ellipsometer optics
10
is shown in FIG.
1
. The optics
10
includes a monochromatic light source
12
, a filter
14
, a polarizer
16
, a quarter wave plate
18
, a specimen holder
20
, an analyzer
22
and a detector
24
. The polarizer
16
, the analyzer
22
and the quarter wave plate
18
can all be rotated independently and their angular position with respect to the instrument frame closely monitored. By using appropriate initial settings of the three optical elements, namely the analyzer, the polarizer and the quarter wave plate, and then rotating the quarter wave plate and the analyzer until a light transmission minimum is observed, the various parameters required for calculating the film thickness can be determined. When different light wavelengths is used for the measurement, different quarter wave plates
18
must be used since the thickness of the plate must be tailored to the specific wavelength.
While the ellipsometer shown in
FIG. 1
generally provides a reliable technique for measuring the thickness of an ultra-thin gate oxide layer, problems in obtaining accurate measurements are frequently encountered which are not related to the ellipsometric technique. For instance, it has been observed that after an ultra-thin gate oxide layer, i.e., about 20 Å, is formed on a silicon wafer, the thickness measurement continuously increases with time. It has also been found that when ultra-thin gate oxide film of 20 Å is formed on a silicon wafer, the maximum deviation measured from its supposed thickness is about 0.4 Å, or about a 2% deviation. It is therefore impossible to measure the real thickness of the ultra-thin gate oxide layer by the traditional ellipsometric technique when the thickness measured is time dependent.
In modern IC devices where the thickness of a gate oxide layer is extremely small, i.e., between about 20 Å and about 50 Å, the measurement problem presents a serious drawback in the quality control of the devices. The cause for the continuing thickness increase on the ultra-thin gate oxide layer has been attributed to moisture and organic residue absorption on the gate oxide film surface, instead of any further oxide growth. The traditional ellipsometer therefore cannot be reliably used to monitor the thickness of an ultra-thin gate oxide layer.
A typical time-dependent measurement curve obtained on an ultra-thin gate oxide layer by a conventional ellipsometer is shown in FIG.
2
. Data plotted in
FIG. 2
are obtained in three separate tests on similar samples. It is seen that within the first 6 hours of deposition, thicknesses measured by the ellipsometer increase continuously from about 19.8 Å to about 20.2 Å, resulting in a 0.4 Å increase, or approximately a 5% deviation from the original thickness measurement of 19.8. The film thickness further increases after 6 hours to approximately 20 hours, even though at a slower rate, to a final thickness of about 20.5 Å. Such variations in the thickness measurements cannot be tolerated for reliability reasons.
It is therefore an object of the present invention to provide a method for measuring a thickness of a thin film that does not have the drawbacks and shortcomings of a conventional measurement technique.
It is another object of the present invention to provide a method for measuring a thickness of a thin film that has a surface sensitive to moisture or organic residue.
It is a further object of the present invention to provide a method for measuring a thickness of
Chen Pu Fang
Lee Tung Li
Leu Chao Po
Perng Bing Huei
Font Frank G.
Nguyen Sang H.
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
LandOfFree
Method and apparatus for measuring thickness of a thin oxide... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for measuring thickness of a thin oxide..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for measuring thickness of a thin oxide... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3150105