Semiconductor device manufacturing: process – With measuring or testing
Reexamination Certificate
2002-02-27
2003-01-21
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
With measuring or testing
C438S016000, C438S018000
Reexamination Certificate
active
06509200
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of appraising a dielectric film, a method of calibrating the temperature of a heat treatment device, and a method of fabricating a semiconductor memory device; and in particular, to an ideal method of appraising a dielectric film, this appraisal in turn being used to appraise the crystallinity of a dielectric film that is grown on a DRAM capacitor; a method of calibrating the temperature of a heat treatment device; and a method of fabricating a semiconductor memory device.
2. Description of the Related Art
In recent years, the development of large-capacity memory elements has been advancing with the development of semiconductor fabrication technology. The progress toward higher integration of DRAM (Dynamic Random Access Memory) in which one memory cell is constituted by a single capacitor and a single transistor has been particularly rapid. Although capacitors of higher capacitance are required in order to maintain reliability such as resistance to soft errors in semiconductor memory devices such as DRAM, the area that can be occupied by capacitors is decreasing as semiconductor memory device become more highly integrated.
Since the charge storage capacity of a capacitor increases in proportion to the electrode area of the capacitor and the dielectric constant of the dielectric film and in reverse proportion to the thickness of the dielectric film, various methods have been proposed for increasing the electrode area of capacitors. Known methods include methods in which the (storage node) electrodes themselves are processed to a fin shape or crown shape, or methods in which HSG (Hemi-Spherical Grains) are formed.
As a simplified explanation of this HSG formation mechanism, when an amorphous silicon film having a clean surface is heated to a temperature sufficient to cause crystallization, the silicon atoms are dispersed within the film with high mobility, and collisions between these silicon atoms result in the formation of crystal nuclei. Crystallization centering on these crystal nuclei progresses from the surface of the film in the direction of depth of the film, and hemispherical crystals having a diameter of several tens of nanometers are formed, thereby producing a minute roughness is produced on the surface.
The use of this HSG technique enables the formation of capacitors having more than twice the surface area of a flat surface.
Alternatively, as methods of increasing the capacitance of a capacitor without changing the electrode area, research is in progress both regarding the reduction of the thickness of the dielectric film and regarding dielectric films that have a high relative dielectric constant. In recent years, tantalum pentoxide (Ta
2
O
5
), yttrium oxide (Y
2
O
3
), and hafnium dioxide (HfO
2
) are receiving attention as films having a high dielectric constant. These materials have a relative dielectric constant that is markedly higher than a silicon oxide film, which has a relative dielectric constant of 3-4, or a silicon nitride film, which has a relative dielectric constant of 6-8. Tantalum pentoxide, which is a material having inherent-thermodynamic stability, is particularly promising as a material for DRAM capacitors.
Tantalum pentoxide has a relative dielectric constant on the order of 22-25 even as a thin film, and a film can therefore be grown by sputtering, CVD (Chemical Vapor Deposition), or sol-gel. After formation of the tantalum pentoxide film, the tantalum pentoxide is next subjected to a heat treatment in an oxygen atmosphere to raise the relative dielectric constant to approximately 40. The chief objective of this heat treatment is to improve the crystallinity of the tantalum pentoxide film, and the treatment is carried out in an oxygen atmosphere to maintain the crystallinity of the tantalum pentoxide film.
Thus, an increase in the capacitance of the capacitors is achieved by: forming HSG on the capacitors of a semiconductor memory device such as DRAM to increase the surface area, growing a film of a material having a high dielectric constant such as tantalum pentoxide on this roughened surface, and then subjecting the surface to a heat treatment. In a DRAM fabrication method that employs this tantalum pentoxide film, however, there is the problem that the effect of the heat treatment on the tantalum pentoxide film cannot be adequately judged.
If the area of the electrode is fixed as described above, the electrostatic capacity of the capacitors varies depending on the relative dielectric constant and film thickness of the-dielectric film. Although reproducibility of the film thickness of the dielectric film can be secured by adjusting the conditions of the CVD film-forming device, the dielectric constant of the dielectric film varies depending on the crystallinity of the dielectric film itself, and therefore varies greatly according to the temperature condition of the heat treatment.
This problem is further explained with reference to
FIG. 3
, which shows the correlation between the temperature (RTO temperature) of the heat treatment and the refractive index of the dielectric film. As shown in
FIG. 3A
, the refractive index, which indicates the crystallinity of the dielectric film, does not gradually increase with increase in the temperature of the heat treatment, but rather, changes abruptly with a particular temperature as a line of demarcation and then increases to a fixed value. This phenomenon occurs due to a rapid improvement in the crystallinity of the tantalum pentoxide from an amorphous state to a crystalline state when the heat energy exceeds a particular threshold value, and after this improvement in crystallinity, density does not change despite further increase in temperature.
Accordingly, to improve the crystallinity of a dielectric film and set the dielectric constant to a desired value, the temperature of the heat treatment is preferably set as high as possible. However, components such as transistors are formed in layers below the capacitors of a semiconductor memory device, and setting the temperature of the heat treatment to a high level results in problems such as change in the distribution of impurity concentration in diffusion layers and the alteration of the characteristics of transistors due to the diffusion of impurities into unintended areas. Thus, when actually fabricating a semiconductor memory device such as DRAM, the temperature of the heat treatment is preferably set to the vicinity of the border between area II and area II in FIG.
3
A. Since this is an area in which the refractive index changes rapidly with respect to change in temperature, the temperature of the heat treatment must be: set accurately.
In semiconductor fabrication devices such as heat treatment devices, the treatment temperature is typically controlled based on the indication of a temperature sensor that is installed inside the device, but factors such as the position of installation of the temperature sensor inside the device or the shape and amount of the sample that is being used may result in divergence between the temperature that is indicated on the device and the actual treatment temperature. This divergence may further vary over time according to factors such as the operation time of the device.
In order to correct this divergence in temperature, a method may be adopted in which a sample for temperature calibration on which a dielectric film has been grown is used to actually carry out the heat treatment, following which the crystallinity of the dielectric film after the heat treatment is then appraised by, for example, an x-ray diffraction method to estimate the actual treatment temperature and adjust the set temperature of the device. However, this approach is problematic both because the x-ray diffraction method takes time to perform and because a correspondence cannot be established between the x-ray diffraction data and the treatment temperature after the crystallinity of the dielectric film has been improved, whereby calibration da
Luk Olivia
NEC Corporation
Niebling John F.
Young & Thompson
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