Endpoint detection in the fabrication of electronic devices

Details Endpoint detection in the fabrication of electronic devices Endpoint detection in the fabrication of electronic devices

1999-04-05

2002-06-18

Powell, William A. (Department: 1765)

Semiconductor device manufacturing: process

Including control responsive to sensed condition

Optical characteristic sensed

C156S345420, C216S060000, C356S329000, C438S723000, C438S743000

Reexamination Certificate

active

06406924

ABSTRACT:

BACKGROUND
The present invention relates to the detection of an endpoint during processing of a substrate.
In the fabrication of devices for electronic applications, semiconductor, dielectric and conductor materials, such as for example, polysilicon, silicon dioxide, and metal containing layers, are deposited on a substrate and etched to form features such as patterns of gates, vias, contact holes and interconnect lines. These features are typically formed by chemical vapor deposition (CVD), physical vapor deposition (PVD), oxidation and etching processes. For example, in a typical etching process, a patterned mask of photoresist or oxide hard mask is formed on a deposited layer by photolithographic methods and exposed portions of the deposited layer are etched by an energized halogen gas, such as Cl
2
, HBr and BCl
3
. In a typical CVD process, a gas provided in the chamber is decomposed to deposit a layer on the substrate. In PVD processes, a target facing the substrate is sputtered to deposit the target material onto the substrate.
One problem with conventional deposition and etching processes is that it is difficult for the operator to determine when to end the process. For example, in deposition processes, it is sometimes desirable to change process conditions to change the nature of the film being deposited on the substrate or to stop deposition after a desired film thickness has been obtained. In etching processes, it is often desirable to stop etching of a particular layer without etching through or otherwise damaging one or more underlayers, especially when the underlayer is thin, such as for example, a gate oxide underlayer below a polysilicon or silicon layer. It is also desirable to stop the etching process on the layer because the underlayer is easily damaged by impingement of energetic plasma species.
Endpoint detection methods are used to determine completion of a stage of the etching or deposition process. In one method, a change in the emission spectra of a plasma is analyzed to determine a compositional change in a layer being deposited or etched, for example, one that occurs after a layer is etched through and an underlayer having a different chemical composition is exposed, as for example, taught in U.S. Pat. No. 4,328,068, which is incorporated herein by reference. However, conventional plasma emission methods detect a process endpoint only after the layer has been etched through and the underlayer exposed, and exposure to the plasma can damage the underlayer. Also, changes in the intensity of the plasma and absorption of selected wavelengths of the plasma emission by the chamber window reduces detection accuracy.
In another endpoint measurement method, known as interferometry, a light beam directed onto a layer on a substrate is partially reflected from the surface of the layer, and partially transmitted through the layer and reflected by one or more underlayers, substantially throughout the process being conducted on the substrate. Constructive and destructive interference of the multiple reflections give rise to an interference pattern which undergoes periodic maxima and minima depending upon the path length of the radiation through the thickness of the layer being processed on the substrate. Before processing of the substrate, an initial thickness of the layer is assumed or measured. During processing, the observed periodic maxima and minima of a measured interference pattern is directly correlated to a calculated reduction in thickness of the layer (which would result in a changing pathlength of the radiation transmitted through the layer being processed) to estimate an endpoint of the process. However, this process requires an accurate knowledge of the initial thickness and refractive index of the layer being etched, which are often difficult to obtain and which can change across the surface of a substrate or from one substrate to another. Inaccurate measurements of either the initial thickness or refractive index of the layer throws off the entire detection method because the calculation of the residual thickness of the layer (from the periodic maxima and minima of the detected interference fringes) will lead to an erroneous estimate of the final thickness of the layer. In addition, variations in the thickness and refractive index of the layer that normally occur in manufacturing will require accurate measurement of these parameters for each and every substrate that is etched or deposited upon, which is an impractical solution. Thus, this method has limited utility, especially for measuring residual thickness of polysilicon layers that are superimposed on thin gate oxide layers, because the margin of error is very small due to the thinness of the underlying gate oxide layer.
In yet another endpoint detection method, known as ellipsometry, a polarized light beam that is at least partially reflected from a layer being processed on a substrate is analyzed to determine changes in the phase and magnitude of the reflected light beam that occur as the layer is being processed, as for example disclosed in U.S. Pat. Nos. 3,874,797 and 3,824,017, both of which are incorporated herein by reference. However, it is difficult to obtain accurate measurements of the thickness of a layer on the substrate without using a polarized light beam having multiple wavelengths, as for example, described in Multiwavelength Ellipsometry for Real-Time Process Control of the Plasma Etching of Patterned Samples, Maynard Layadi and Tseng-Chung Li,
J. Vac, Sci. Technol. B.
15(1), January/February 1997. The multiple wavelengths and complex phase and magnitude measurements are difficult to make. In addition, deposits formed on the window of the chamber change the polarization of the light beam passing through the window which can result in erroneous ellipsometric measurements.
Thus, it is desirable to have an endpoint detection method that detects a change in a stage of a process being conducted on a substrate immediately before or after processing of the layer is completed and without damaging underlayers. It is further desirable to have an endpoint detection apparatus that provides a detection signal prior to etching through or deposition of a layer to allow the etching or deposition process to be changed at a suitable time. It is also desirable to have an endpoint measurement system which measures remaining thickness or a change in thickness of a layer being processed, with high resolution, low signal to noise ratio and independent of the strength of the radiation transmitted through the chamber.
SUMMARY
Embodiments of the present invention satisfy the above identified needs, in principle, by detecting an endpoint during processing of a substrate with accuracy and repeatability. In one embodiment, the present invention comprises a chamber for processing a substrate, the chamber comprising a radiation source capable of emitting radiation having a wavelength that is substantially absorbed in a predetermined pathlength in a thickness of the layer and a radiation detector adapted to detect the radiation. In another embodiment, the chamber further comprises a filter in a path of the radiation, the filter adapted to selectively pass through the radiation.
In another aspect, the present invention relates to a method of processing a substrate, in which, the substrate is placed in a process zone, and process conditions are maintained in the process zone for processing a layer on the substrate, the process conditions including, without limitation, one or more of gas composition, gas flow rate, an operating power level of a gas energizer, gas pressure and temperature. Radiation having a wavelength that is absorbed in a predetermined pathlength of a thickness of the layer being processed on the substrate is provided. A change in the radiation is detected after processing of a predetermined thickness of the layer.
The method is particularly useful for etching a layer on a substrate, in which, the substrate is placed in a process zone which is maintained at process conditions suitable for et

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