Method for detecting end point in plasma etching by...

Details Method for detecting end point in plasma etching by... Method for detecting end point in plasma etching by...

2001-05-02

2003-07-29

Goudreau, George (Department: 1763)

Semiconductor device manufacturing: process

Including control responsive to sensed condition

C438S010000, C438S011000, C438S014000, C438S714000, C438S720000, C438S017000, C438S018000, C216S084000, C216S086000

Reexamination Certificate

active

06599759

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a method for detecting end-point in a plasma etching process and more particularly, relates to a method for detecting end-point in a plasma etching process by monitoring an impedance change in the RF power of the plasma as an indication that an interface between two different material layers has been reached.
BACKGROUND OF THE INVENTION
In the fabrication and processing of semiconductor devices, such as silicon wafers, a variety of different semiconductor equipment and/or tools are utilized. These tools and equipment are well-known in the art, and include for example, photolithographic machines, etchers, deposition equipment, furnaces, as well as a variety of sensors and control equipment. Although the capabilities of these types of semiconductor processing equipment have improved over the years, the technique of monitoring the ongoing process has not necessarily kept pace with the improvements. In the area of monitoring the ongoing semiconductor manufacturing process, current practices generally utilize ex-situ process monitoring. A problem with ex-situ monitoring is that the results are not available until the end of the process, or if in-situ readings are required, the ongoing process must necessarily be interrupted in order to obtain the required reading. Moreover, where a number of parameters are monitored for a given process, it is difficult to determine the dependency of one parameter to the others. Such processing parameter correlations are difficult to obtain, and are made even more difficult when measurements are being taken for the purpose of providing in-situ control of the ongoing process.
As mentioned above, one of the processes involved in manufacturing semiconductor devices is etching. A number of etching technologies may be employed, such as reactive ion etching (RIE) for etching fine line or small via patterns in a silicon wafer. RIE involves positioning a masked wafer in a chamber containing plasma. The plasma contains etchant gases which are vertically disassociated in an RF so that the reactive ions contained in the etchant gases are accelerated to the wafer surface. The accelerated reactive ions combine chemically with unmasked material on wafer's surface.
In connection with the plasma etching process, it is known to monitor the progress of the etching process by measuring the intensity of the plasma emissions at a specific wavelength. Changes in the level of intensity of the plasma at the wavelength of interest can be correlated to the progress of the etching process, consequently this technique may be employed to determine the time at which the etching process should be ended, such time point being commonly referred to in the art as the “end-point” time. It is further known that during normal, stable operating conditions, the end-point, as determined by a change in the monitored wavelength, should be within a certain range. However, certain processing conditions, indicative of an unstable processing condition or other problems affect the end-point time. For example, incorrect process parameters, wrong recipes, improper part installation during maintenance, chamber or line leakage and other similar problems result in an unstable process which is normally not detected until a batch, or even a complete lot of wafers has been processed. This after-the-fact detection of unstable processing conditions results in substantial scrap and decreased yield.
Although it is known that a change in the monitored wavelength of the plasma is correlatable to the end-point time, such information has not been effectively employed for early detection of unstable processing conditions, and particularly with respect to batch-to-batch and lot-to-lot processing variations that reduce yield.
In U.S. Pat. No. 6,117,348 issued Sep. 12, 2000 and assigned to the common assignee of the present invention, a method for real time monitoring of a plasma etching process as well as an apparatus for performing the same are disclosed. The method monitors a plasma etching process employed to produce multiple batches or lots of semiconductor devices, such as silicon wafers. The method broadly comprises the steps of detecting a change in a characteristic of the plasma during etching of a wafer or a wafer batch; recording the time when the change in the characteristic is detected, such recorded time representing the duration of the etching and defining an end-point value; comparing the end-point value with one or more reference values corresponding to stable process conditions; and issuing a notice of unstable process conditions based on the results of the comparison. The characteristic to be detected preferably comprises a change in the intensity of a specific wavelength generated by the plasma employed to perform the etching. The method also includes the step of storing a plurality of end-point values respectively recorded during the etching of a plurality of corresponding wafers, and employing these stored values as a reference with which a monitored end-point value is compared.
The patent also discloses an apparatus for carrying out the method, including means for sensing a particular wavelength of interest emitted by the plasma, means for analyzing the monitored wavelength, and for detecting a change in the intensity of such wavelength, and a program controller for calculating an end-point valve using the measured changes in wavelength intensity and for comparing the measured end-point with one or more reference values corresponding to stable processing conditions.
FIG. 1
depicts the primary components of a typical reaction ion etching system, as well the components forming the real time monitoring system. A wafer
14
to be etched, typically a masked wafer, is placed on a cathode
16
inside a plasma chamber
12
filled with plasma, all of which components form a part of a plasma etching apparatus
10
. Although the reactive ion etching system shown in
FIG. 1
employs a cathode
16
as the heating source, a system in which both the anode and cathode are powered can also be utilized. A gas supply
20
provides the necessary etchant gas to the plasma in the chamber
12
, and a pump
22
is employed for evacuating plasma discharge during etching. A RF generator
18
supplies RF power to the cathode
16
so as to form an RF field in the plasma. The RF field causes reactive ions contained in the etchant gas to accelerate to the surface of the wafer
14
. The thus accelerated reactive ions combine chemically with unmasked material on the surface of the wafer so as to form volatile etch product. The volatile etch product is released into the plasma and a plasma chamber
12
and a plasma discharges is formed. This discharge includes the emissions of light at specific wavelengths which are determined by the particular gases and materials employed in the process. For example, a CO plasma employed to etch an oxide surface emits light at a wavelength of 480-485 nm, an Al plasma used to etch metal substrates emits light at a wavelength of 396 nm, and a SF plasma used to etch a polysilicon substrate emits light at a wavelength of approximately 400 nm.
As shown in
FIG. 1
, changes in the level of intensity of the wavelength emitted by the plasma in the chamber
12
are continuously monitored by a suitable sensor
24
which transmits the monitored wavelength via a line, which may comprise an optical fiber
26
, to a device such as a spectrometer
28
, which in turn outputs a signal to the device such as a multi-channel analyzer
30
which isolates the wavelength of interest and measures its intensity. The measured intensity, as well as changes therein, is transmitted by the analyzer
30
to a programmed controller
32
which includes a microprocessor controller
34
provided with suitable memory
36
for storing end-point time reference values. The microprocessor
34
operates in accordance with a set of programmed instructions which receives data from the analyzer
30
relating to changes in the monitored wavelength, determines the

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