WO1997037379A1

WO1997037379A1 - In-situ sensor for the measurement of deposition on etching chamber walls

Info

Publication number: WO1997037379A1
Application number: PCT/US1997/002512
Authority: WO
Inventors: Anthony John Toprae; Gerald Barnett
Original assignee: Advanced Micro Devices, Inc.
Priority date: 1996-04-03
Filing date: 1997-02-18
Publication date: 1997-10-09

Abstract

A laser interferometry technique is used to measure the thickness of deposits on the wall of an etching chamber. The measurement of deposit thickness on an etching chamber wall furnishes an accurate and useful parameter for quantifying the condition and state of an etch chamber. The measurement of deposit thickness on an etching chamber wall also supplies a parameter that is measured dynamically, over time and without interruption, while semiconductor wafers are processed, so that the need for cleaning maintenance is predicted before production loss occurs but without interruption of fabrication processing.

Description

IN-SITU SENSOR FOR THE MEASUREMENT OF DEPOSITION ON ETCHING

CHAMBER WALLS

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to the field of devices for fabricating semiconductors. More specifically, the present invention relates to a sensor for measuring the operating condition of an etching chamber.

Description of the Related Art

A common step in the process of integrated circuit fabrication is the transferring of patterns onto regions of silicon wafers. A typical technique for transferring patterns onto the wafers is an anisotropic etching process such as plasma etching or reactive ion etching. For example, plasma etching is performed using chemical etching mechanisms applied at high pressure and low excitation energy to furnish a highly selective and isotropic etch.

A plasma etching system typically has several components including an etching chamber that is evacuated to a reduced pressure, a pumping system connected to the chamber for reducing and maintaining chamber pressure, pressure gauges for monitoring chamber pressure and a variable conductance between the pumping system and chamber for independently controlling chamber pressure and flow rate. The plasma etching system also commonly includes a radio frequency (rf) power supply to create a glow discharge, and ducts and pumps connected to the chamber for measuring and controlling the flow of reactant gases in the chamber.

A useful etching process has various characteristics. For example, the useful etching process is safe, has a relatively rapid etch rate or throughput, is highly selective against etching the mask layer material or the material underlying the film to be etched, and causes minimal damage to substrates. The useful etching process is uniform across each wafer, across all the wafers in a run, and across different runs. The etch mask material is easily removable after etching is completed. The process facilitates full automation. In addition, the useful etching process is clean, having a low incidence of particulate and film contamination.

An etching system does not remain useful without maintenance. As an etching system is used over time, deposits which mainly result from the accumulation of polymers form on the walls of an etching chamber. The deposits hinder semiconductor fabrication by effecting the delivery of rf power that is used to create the glow discharge and excite the plasma in the etch chamber. As the plasma excitation is modified, the composition of chemical species in the chamber is altered, rendering the etch process unreliable and unpredictable.

One consequence of the unpredictable and unreliable nature of the process is a "stop etch" problem in which the etching time for a particular process etching step becomes unpredictable. As integrated circuit structures are reduced to a much smaller scale, the accuracy of etching of contacts and vias becomes critical. Etched structures must be fully etched so that contact structures are opened, but not over-etched so that underlying substrate and structures are harmed. In contact and via etch, proper etching is typically achieved by precisely timing the duration of etching processes. However, as deposits form on the walls of an etching chamber, the plasma excitation and etching chemistry are modified so that the etch rate falls. The modification in etch rate is highly erratic and unpredictable and proceeds at a high rate once deposits reach a critical thickness. As a result, the etch time for stop etch becomes unknowable, leading to expensive and unproductive misprocessing of semiconductor wafers.

Furthermore, the deposits are a source of particulates that result in film contamination. Proper maintenance of an etching system generally includes a thorough cleaning or scouring of the inner wall of the etching chamber.

Regular scheduled maintenance is helpful to avoid a loss in productivity resulting from deposit formation but the rate of wall deposition varies greatly with various processes and products that are fabricated using an etching chamber. Furthermore, different chambers develop deposits at different rates and the rate of deposition changes unpredictably with the age of the chamber.

What is needed is a technique for monitoring of deposits on the walls of a wafer etching chamber. Heretofore, a measurement of the state of an etching chamber has been an unmeasurable variable.

SUMMARY OF THE INVENTION

In accordance with the present invention, a laser interferometry technique is used to measure the thickness of deposits on the wall of an etching chamber. The measurement of deposit thickness on an etching chamber wall furnishes an accurate and useful parameter for quantifying the condition and state of an etch chamber. The measurement of deposit thickness on an etching chamber wall also supplies a parameter that is measured dynamically, over time and without interruption, while semiconductor wafers are processed, so that the need for cleaning maintenance is predicted before production loss occurs but without interruption of fabrication processing.

In accordance with one embodiment of the present invention, a system for measuring deposition thickness on a chamber wall of an etching chamber includes a laser source positioned external to the etching chamber for directing a laser beam toward and through an etching chamber window to a point at an interior surface of the etching chamber and a laser detector positioned external to the etching chamber and directed toward the etching chamber window substantially to the point at the interior surface of the etching chamber. The detector detects an intensity of the laser beam reflected from the interior surface of the etching chamber. The system also includes a recorder connected to the laser detector for recording the intensity of the reflected laser beam over time.

In accordance with an additional embodiment of the invention, a method of measuring deposition thickness on a chamber wall ofan etching chamber includes the steps of projecting a laser beam from a position external to the etching chamber toward and through the etching chamber window to an interior surface of the etching chamber at which the projected laser beam is reflected, detecting the reflected laser beam at a position external to the etching chamber and measuring an intensity of the reflected laser beam as detected by the detecting step. The measured intensity of the reflected laser beam is recorded over time.

The described deposit monitoring method and system achieve numerous advantages. One advantage is that misprocessing of semiconductor wafers due to substandard excitation of the plasma in the etch chamber as a result of deposits on the chamber walls is avoided. A related advantage is that wafer contamination due to particulates is prevented by early detection of chamber conditions that give rise to particulate formation before particulates are actually formed. Similarly, "stop etch" problems are avoided by accurate prediction ofa problematic condition before the condition begins. BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are specifically set forth in the appended claims. However, the invention itself, both as to its structure and method of operation, may best be understood by referring to the following description and accompanying drawings.

Figure 1 depicts a pictorial top plan view ofa plasma etch system including a system for measuring etch chamber deposition in accordance with an embodiment of the present invention.

Figure 2 depicts a pictorial top plan view of a plasma etch system including an alternative embodiment of a system for measuring etch chamber deposition.

Figure 3 is a flow chart that illustrates a method of measuring deposition thickness on the inner chamber wall of the etch chamber in accordance with an embodiment of the present invention.

Figure 4 is a timing diagram showing the operation of the system for measuring etch chamber deposition illustrated in Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Figure 1, a pictorial top plan view of a plasma etch system 100 is shown.

The plasma etch system 100 includes an etch chamber 110 and a system 120 for measuring deposits, such as polymer product deposits, within the etch chamber 110. The system 120 performs in-situ measurements of the thickness of these deposits. A typical etch chamber 110 as a generally cylindrical or hemispheric form with an inner chamber wall 112 and an outer chamber wall 114. The etch chamber 110 has an observation window 116, that is typically constructed from glass, quartz, sapphire or the like. The observation window 116 allows visual inspection of the interior of the etch chamber 110. The observation window 116 may be used, for example, to view a semiconductor wafer 102 inside the etch chamber 110.

The system 120 for measuring deposits includes a laser source 122 and a detector 124 which are positioned external to the etch chamber 110. The laser source 122 directs a laser beam 126 along a beam axis 128 toward the observation window 116, through the window 116 to a point 130 at the inner chamber wall 114. The laser beam 126 enters the interior of the etch chamber 110 at the observation window 116, traverses the chamber to the point 130 at the inner chamber wall 114, where the beam is reflected back along a detection axis 134. The inner chamber wall 112 is typically constructed from aluminum or stainless steel, which is a highly reflected surface. The reflective nature of the inner chamber wall 112 is exploited by the system 120 for the purpose of measuring deposit thickness.

The reflected beam 131 reflects back through the observation window 116 to the detector

124. In some embodiments, the reflected beam 131 is reflected back at an angle such that the reflected beam 131 passes through a second window (not shown). In this manner, the detector 124 detects the intensity of reflected beam 131 resulting from reflection of the laser beam 126 from the inner chamber wall 114. The intensity of the reflected beam 131 varies as a function of the thickness of the depositions on the inner chamber wall 114, showing either simple attenuation or variations with an interferometric effect in which interference of the deflected and reflected beams is indicative of deposition thickness. In particular, the intensity attenuates as the deposit thickness on the inner chamber wall 114 and on the chamber window 116 increases.

The laser source 122 is a conventional laser source which produces intense monochromatic coherent radiation in the visible, ultraviolet or infrared regions of the electromagnetic spectrum. The narrow laser beam 126 may be either pulsed or continuous. In various embodiments, the laser source 122 is any conventional laser type including a gas laser or an injection laser. A specific laser source 122 is selected as a function of the type of deposits formed on the inner chamber wall 114 which, in turn, depends on the type of processing, the product fabricated, the plasma gases applied to the chamber and the like. Accordingly, the specific laser wavelength that is selected depends on the optical properties of the deposition coatings and the typical deposition thicknesses that are formed. Typical deposit thicknesses range from about 0.5μ to several microns. For these thicknesses, for example, a laser source having a wavelength of 1.3 to 1.6μ may be selected.

The detector 124 has a detection surface 132 and the detector 124 is positioned and the detection surface 132 is directed toward the etching chamber window so that the detection axis 134 is disposed substantially toward the point 130 at the inner chamber wall 114. In various embodiments, the detector 124 is any conventional laser detector for detecting light intensity, such as a photocell.

The system 120 for measuring deposits also includes a chopper 140 that is positioned between the laser source 122 and the etch chamber 110 along the beam axis 128. The chopper 140 interrupts the laser beam 126 at regular time intervals to compensate for baseline background signals, such as noise signals, that are included in the laser signal detected by the detector 124. The chopper 140 is controlled to temporarily interrupt the laser beam 126 to measure a baseline or background signal at the detector 124, The baseline or background signal is indicative of the light intensity signal received when no light stimulus is applied and is therefore substantially equal to the amount of measured noise in the system 120. The baseline or background signal is subsequently subtracted from reflected beam 131 that is detected when the laser stimulus is applied to compensate for background noise.

The system 120 for measuring deposits also includes a semi-transparent mirror 142 that is positioned between the laser source 122 and the etch chamber 110 along the beam axis 128 and deflects a portion of the energy of the laser beam 126 a controlled reflection angle 144 so that a deflected beam 146 is deflected along a deflection axis 148 to a calibration detector 150. The semi-transparent mirror 142 is typically a conventional mirror formed from glass, or other transparent substrate, with a reflective coating. In one embodiment, the controlled reflection angle 144 is a 90° angle so that the deflection axis 148 is perpendicular to the beam axis 128. The calibration detector 150 is typically substantially identical to the detector 124 to allow precise compensation for changes in the laser source 122 signal strength. In some embodiments, including the embodiment shown in Figure 2, a plasma etch system 200 is configured such that the angle between the beam axis 128 and the detection axis 134 and the reflection angle 144 are selected so that the single detector 224 is positioned to receive both the reflected beam 131 and the deflected beam 146 and only a single detector is employed. A second chopper 210 that is positioned between the semi-transparent mirror 142 and the single detector 224 along the deflection axis 148. Referring again to Figure 1, the calibration detector 150 detects the intensity of the deflected laser beam 146 as reflected from the semi-transparent mirror 142. The semi-transparent mirror 142 and calibration detector 150 compensate for shifts in laser power at the laser source 122.

The detector 124 is connected to a recorder 160 to record the intensity of the reflected beam 131 over time. The recorder 160 is also connected to the calibration detector 150 so that full analysis of beam intensities, including compensation, is available. The recorder 160 is any form of conventional recorder such as an electrical signal recorder, a data acquisition system, a computer such as a personal computer, and the like. The recorder 160 is a system for receiving information from the detector 124 and calibration detector 150 and for storing the received information. In various embodiments, the recorder 160 also executes various analysis and display functions to facilitate analysis of thickness deposition.

Referring to Figure 3 in conjunction with Figure 1, a flow chart illustrates a method 300 of measuring deposition thickness on the inner chamber wall 114 of the etch chamber 110. A laser beam is projected 310 from a position external to the etch chamber 110 through the observation window 116 to an interior surface of the etch chamber 110 where the laser beam is reflected. The reflected laser beam is detected 312 at a position external to the etch chamber 110. The intensity of the reflected laser beam, as detected, is measured 314. The laser beam angle is adjusted 316 so that the intensity of the reflected laser beam is maximized. The measured intensity of the reflected laser beam is recorded 318 over time to detect variations indicative of deposition thickness. The intensity of the reflected beam attenuates as the thickness of the deposition upon the inner chamber wall 114 and upon the observation window 116 is increased.

Information that is diagnostic of deposition thickness includes both intensity amplitude information and information indicative of an interferometric effect in which interference of the deflected and reflected beams is indicative of deposition thickness. Typically, the intensity of the reflected laser beam varies in a sinusoidal manner with thickness due to constructive and destructive interference that occurs as the deposition thickness increases along the chamber wall 114.

The method 300 of measuring deposition thickness periodically compensates for background signals as directed by an activation step 320. When background compensation is enabled, the projected laser beam is temporarily obscured 322 and the measured intensity of background light reaching the detector is recorded 324 while the projected laser beam is intercepted. The baseline noise signal is determined 326 from the intensity measurement recorded while the projected laser beam is intercepted. The baseline noise signal in the recorded intensity is compensated 328 generally by subtracting the background noise signal from the measured intensity.

The method 300 of measuring deposition thickness compensates for variations in laser power by diverting 330 a portion of energy of the projected laser beam and detecting the diverted portion of energy 332. In particular, the laser energy of the projected laser beam 126 is detected by the calibration detector 150 and the laser energy of the reflected beam 131 is detected by the detector 124. Variations in laser power are compensated 334 in the recorded intensity by ratioing the laser energy of the reflected beam 131 to the laser energy of the projected laser beam 126 to determine the decline in power of the projected laser beam due to reflection off the inner chamber wall 114 of the etch chamber 110.

Referring to Figure 4, a timing diagram illustrates the operation of the deposition measurement system of the plasma etch system 200 shown in Figure 2. A constant laser illumination is supplied by the laser source 200 and, detected as shown by the detection waveform 414, by the detector 224. The detection waveform 414 varies in intensity due to the operation of the first chopper 140 and the second chopper 210. At time TO the projected laser beam 126 is blocked by both the first chopper 140 and the second chopper 210 so that substantially no laser illumination reaches the detector 224. At time Tl the chopper 140 is open so that the projected laser beam 126 passes through the observation window 116, is reflected from the inner chamber wall 114 and is detected by the detector 224. At time T2 the second chopper 210 is opened so that the deflected laser beam 146, which is deflected by the semi- transparent mirror 142, is detected by the detector 224. Thus at time T2 both the first chopper 140 and the second chopper 210 are open so that laser illumination of the projected laser beam 126 and the deflected laser beam 146 are substantially summed at the laser detector 224. At time T3 the first chopper 140 is closed so that neither the projected laser beam 126 nor the semi- transparent mirror 142 are illuminated and no laser illumination reaches the detector 224.

A wall deposition measurement is performed by detecting and comparing several laser illumination values during different time intervals. A background noise signal is measured in the interval from time TO to time Tl. A wall reflection signal is measured in the interval from time Tl to time T2. The combination of the wall reflection signal and a source illumination signal, reflected from the semi-transparent mirror 142 is measured in the interval from time T2 to time T3. Wall deposition is determined by compensating for background noise. In one embodiment, the background noise is subtracted from both the wall reflection measurement and the combined wall reflection and laser source illumination measurement. A measurement of laser source illumination strength is provided by subtracting the wall reflection signal measured from time Tl to time T2 from the combined wall reflection and laser source illumination measurement measured from time T2 to time T3. A wall deposition measurement is determined by comparing, or normalizing, the wall reflection signal to the laser source illumination measurement. While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions and improvements of the embodiments described are possible.

Claims

WHAT IS CLAIMED IS:

1. A system for measuring deposition thickness on a chamber wall of an etching chamber, the etching chamber including a window, the system comprising: a laser source positioned external to the etching chamber for directing a laser beam toward and through the etching chamber window to a point at an interior surface of the etching chamber; a laser detector positioned external to the etching chamber and directed toward the etching chamber window substantially to the point at the interior surface of the etching chamber for detecting an intensity of the laser beam reflected from the interior surface of the etching chamber; and a recorder coupled to the laser detector for recording the intensity of the reflected laser beam over time.

2. A system according to Claim 1 , further comprising: a chopper positioned between the laser source and the etching chamber along the directed laser beam for interrupting the laser beam at regular time intervals.

3. A system according to Claim 1, further comprising: a semi-transparent mirror positioned between the laser source and the etching chamber along the directed laser beam for reflecting the laser beam a controlled reflection angle; and a laser detector positioned external to the etching chamber and directed toward the semi- transparent mirror at the controlled reflection angle for detecting an intensity of the laser beam reflected from the semi-transparent mirror.

4. A system according to Claim 1 wherein: the laser source is positioned external to the etching chamber to direct the laser beam to impinge upon the etching chamber window so that the reflected laser beam has a maximum intensity.

5. An etching apparatus including a deposit monitoring system comprising: an etching chamber having a chamber interior wall and a window; a laser source positioned external to the etching chamber for directing a laser beam toward and through the etching chamber window to a point at the chamber interior wall; a laser detector positioned external to the etching chamber and directed toward the etching chamber window substantially to the point at the chamber interior wall for detecting an intensity of the laser beam reflected from the chamber interior wall; and a recorder coupled to the laser detector for recording the intensity of the reflected laser beam over time.

6. An apparatus according to Claim 5, further comprising: a chopper positioned between the laser source and the etching chamber along the directed laser beam for interrupting the laser beam at regular time intervals.

7. An apparatus according to Claim 5, further comprising: a semi-transparent mirror positioned between the laser source and the etching chamber along the directed laser beam for reflecting the laser beam a controlled reflection angle; and a laser detector positioned external to the etching chamber and directed toward the semi- transparent mirror at the controlled reflection angle for detecting an intensity of the laser beam reflected from the semi-transparent mirror.

8. An apparatus according to Claim 5, wherein the etching chamber is a plasma etching chamber.

9. An apparatus according to Claim 5, wherein the etching chamber window is a quartz window.

10. An apparatus according to Claim 5, wherein the etching chamber window is a sapphire window.

1 1. A system according to Claim 5 wherein: the laser source is positioned external to the etching chamber to direct the laser beam to impinge upon the etching chamber window so that the reflected laser beam has a maximum intensity.

12. A method of measuring deposition thickness on a chamber wall ofan etching chamber, the etching chamber including a window, the method comprising the steps of: projecting a laser beam from a position external to the etching chamber toward and through the etching chamber window to an interior surface of the etching chamber at which the projected laser beam is reflected; detecting the reflected laser beam at a position external to the etching chamber; measuring an intensity of the reflected laser beam as detected by the detecting step; and recording the measured intensity of the reflected laser beam over time.

13. A method according to Claim 12, further comprising the steps of: temporarily intercepting the projected laser beam; recording the measured intensity of the reflected laser beam while the projected laser beam is intercepted; determining a baseline noise signal from the intensity measurement recorded while the projected laser beam is intercepted; and compensating for the baseline noise signal in the recorded intensity.

14. A method according to Claim 12, further comprising the steps of: diverting a portion of energy of the projected laser beam; detecting the diverted portion of energy; and compensating for variations in laser power in the recorded intensity.

15. A method according to Claim 12 further comprising the step of: adjusting the angle of projection of the laser beam to impinge on the etching chamber window so that the reflected laser beam has a maximum intensity.