WO2003092049A1

WO2003092049A1 - Improvement in process control for etch processes

Info

Publication number: WO2003092049A1
Application number: PCT/US2003/012462
Authority: WO
Inventors: Michael D. Bodger; Mark Burton Halbrook; David Heason; David Robert Reeve
Original assignee: The Boc Group Inc.
Priority date: 2002-04-23
Filing date: 2003-04-23
Publication date: 2003-11-06
Also published as: GB2406639B; GB2406639A8; GB2406639A; AU2003228646A1; GB0423267D0

Abstract

A film structure being monitored, such as a silicon wafer undergoing etch, is monitored by examination of the reflection of an incident light beam. The invention is concerned with the situation where a feature of interest, such as an etch groove having a reflective surface (2), is small in relation to the size of the achievable light spot. The detection and processing of the reflected light is based on the detection and examination of evanescent waveforms which are generated at surfaces (10) and (11).

Description

Improvement in Process Control for Etch Processes

This invention relates to the control of etch and deposition processes in the manufacture of semiconductor devices, micro electronic machines (MEMs) and waveguides.

BACKGROUND

It is well known that interferometric techniques can be applied to determining the endpoint in thin film deposition or etch. These techniques have been limited in their application to feature sizes of a few microns or greater, however, since the probe light is incapable of resolving smaller structures due to the diffraction limit of the probe light. Contemporary feature structures are becoming so small that they are less than the diffraction limit in dimension and the prior art techniques are becoming less useful and applicable because of this limit.

An object of the present invention is accordingly to provide a method of monitoring semiconductor manufacturing processes such as etch and deposition involving small feature sizes. Desirable and achievable outcomes of proper use of these techniques are elimination of the etch stop layer in dielectric etch, an improvement in control of shallow trench isolation etch, an improvement in gate oxide etch, and an improvement in gate spacer etch. The invention is also applicable to the control of a range of micro- machining applications.

BRIEF DESCRIPTION OF INVENTION:

The invention, in one aspect, provides a method for improved control of etch or deposition in a semiconductor manufacturing process to produce a structure having a small feature size, the method comprising: providing a spectrally narrow illumination source selected to operate at a wavelength region in which the film to be processed, together with any overlying films or masks which are of the same small structure dimension, are substantially transparent, generating from said illumination source an optical probe measurement beam, illuminating an article undergoing processing with said beam, the article having within the area of illumination an ordered feature arrangement having a feature size of the same order as the structure to be produced, detecting an interference pattern which is derived substantially from the interaction of the evanescent wave with said ordered feature arrangement resulting from the illumination, and using the interference pattern information to detect or predict the desired endpoint or monitor the progress in real time the of the etch or deposition.

From another aspect the present invention provides apparatus for use in a semiconductor manufacturing process, the apparatus comprising: a vacuum enclosure; a workpiece location within the enclosure for locating a semiconductor workpiece to be processed to produce a structure having a small feature size, said semiconductor workpiece having an ordered feature arrangement having a feature size of the same order as the structure to be produced; a spectrally narrow illumination source producing light at a wavelength region in which the film to be processed, together with any overlying films or masks, are substantially transparent; optical projection means cooperating with the light source to produce an optical probe measurement beam directed to said workpiece location; optical detection means arranged to detect an interference pattern which is derived substantially from the interaction of the evanescent wave resulting from the beam falling upon a said semiconductor structure with said ordered feature arrangement; and data processing means arranged to compare the output of the optical detection means with a- predetermined signal behaviour to produce a process control signal. Preferred features and advantages of the invention will be apparent from the following description and the claims.

DETAILED DESCRIPTION OF INVENTION:

An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which:

Fig. 1 is a cross-section of a typical prior art semiconductor construction; Fig. 2 is a cross-section illustrating a semiconductor fabrication process forming one embodiment of the present invention; Fig. 3 is a front view of a silicon wafer showing structures used in the process of Fig. 2; and Fig. 4 is a schematic cross-section of one form of apparatus for carrying out the invention.

A typical section of the etched dielectric for the semiconductor conductor deposition scheme known as 'Damascene' is shown in profile in Figure 1. Typically the structure is etched down to an etch stop layer 1 which layer provides for a slowing down of the etch so that the etch may be terminated by reference to time or alternatively the distinguishing chemical composition of the etch stop layer 1 may be determined by reference to specific wavelengths of light emitted within the plasma used to carry out the etch.

It is desirable to optimise the performance of the semiconductor device by eliminating the etch stop layer and decreasing the geometry of the device and improving the permittivity of the dielectric material.

It is known (Ref: FR-2718231) that interferometric techniques which derive measurements from interfering optical signals (Figure 2) reflected from the top of the etched surface 2, the top of the mask 3, the bottom of the etched film 4, and the bottom of the mask 5 can yield data throughout the etch. Furthermore, it is known (Ref: US 6,226,086 Bl) that processing the data relative to a mathematical model of the physical situation provides additional useful information so that remaining thickness and etch rate can be determined with high accuracy providing an improvement in process control and possible elimination of the need for an etch-stop layer.

An analogous situation exists where a film is being deposited rather than etched.

It is common practice to deliver the optical signal as a focussed spot in such a way that the illumination substantially falls on the surface being etched. Although common this practice has the disadvantage that the spot size is practically limited by diffraction to about 5 microns. This size is no longer compatible with the development of semiconductor, MEMs and waveguide devices, which are now below one micron in feature size.

An alternative is to illuminate a larger area: this has the advantage of illuminating a number of structures and some diffraction effects will provide a modulation of the signal, which can enable endpoint detection. However, with known techniques very little of the signal couples into the substrate and the etched films and the endpoint signatures are consequentially weak and ill defined.

It is a prime objective of this invention to provide a means for efficient coupling of an interferometric probe beam into the combined structure of mask, etched film and/or substrate by using a spectrally narrow illumination means with a wavelength which is deliberately chosen so that the mask and film into which the small structures are to be etched maximise their interaction with the evanescent wave and thus continue to provide strong modulation by means of interference between the incoming and reflected waves even though the structures themselves are below the diffraction limit of the illuminating probe beam.

Proper choice of wavelength involves consideration of the structure dimension, and its spacing and repeat to the materials surrounding it, and ensuring that the film to be processed and any mask are transparent to the illumination. If mathematical analysis does not yield a suitable wavelength choice using the design features to be etched or deposited, then a repetitive test structure can be incorporated, typically in the scribe lines of a semiconductor wafer. If a test structure is used then it is selected to have a geometry that simultaneously meets the requirements of optimising the evanescent wave coupling to the substrate at a feature size that is fully representative of the feature size to be monitored during the thin film etch or deposition process.

It is preferred that the structure unit repeat is less than 1/3 of the illumination wavelength, and that the etched region of the structures accounts for between 10% and 90% of the illuminated area.

The efficiently coupled illumination means is then reflected usefully by the interface of the effective refractive index layer that exists between the top of the structure that is being etched and the bottom with the illumination that is propagating to the bottom of the structure and back suffering a phase delay that is produced by the depth of the etch together with the index of refraction that results, not from the bulk index of the etched material, but rather from the volumetric average of the bulk index together with the voids resulting from the etch itself.

These interfaces only exist for structures probed by the evanescent wave and are shown by the dotted lines 10 and 11 in Figure 2. It will be appreciated that the surfaces illustrated by the dotted lines 10 and 11 form continuous unbroken surfaces for the purposes of this interference using evanescent waves, and that this is distinct from the reflection processes which occur at 3, 5, 2 and 4. By tracking the interference fringes resulting from this effect resulting from the reflection at the continuous unbroken plane 10 and 11 which is uniquely sensed by the evanescent waves then the etch depth can be determined. Consider the example wafer structures shown in Figure 3. The structures 6 that it is desired to etch have a line width of 0.2 microns.

The test structure 7 that would have previously been required has a dimension of 10 microns. This would accommodate a focussed spot diffraction limited at 5 microns from a monitoring interferometer, but the large size of the feature would mean that the etch process would proceed at a different rate in the test feature from that within the structure that requires to be manufactured. As such the monitoring technique will not return a useful measure.

Now consider the array of features shown in the test structure 8 on the example wafer. These have a feature size (0.2um) that is representative of the size of the process feature 6 that requires to be monitored, but in addition they have a geometrical arrangement that has been carefully chosen to optimise coupling of the incident interferometric monitor beam into the region below the mask. It will be appreciated that a suitable arrangement may naturally follow from the circuit design as an alternative to optimising the effect by use of a test structure. A number of benefits now flow.

The probe light wavelength is chosen to be in a region where the etched film and the mask are substantially transparent. In addition, however, the probe wavelength can also be selected to be in a region where the material underlying the etched film itself is also transparent to the incident probe light. If this is the case then the probe light can, after suffering a partial reflection at the top of the plane formed by the effective medium (10) and the bottom of this plane (11) , proceed into the underlying films and/or base wafer material . Provided that an interface deeper in the structure causes reflection of this beam so that it can interfere, then information is provided from deep in the structure below the etch itself. In this way, and in particular if a tuneable narrow band source is used, then information can be gained about remaining thickness.

It will be apparent to the skilled reader that if the remaining thickness is known, then an alternative measurand is width of structure and in this way the width of a gate structure can be optimised.

Alternatively of course if the probe light is chosen to be at a wavelength where the material underlying the etched film is opaque then the previous effect can be suppressed.

It will be readily apparent to the skilled reader that a combination of these effects may also be present and that proper application of the technique can maximise different aspects depending on the key process parameter that requires to be measured. In addition it will be appreciated that the additional information, particularly if the underlying material is transparent to the probe wavelength, will permit stopping the etch on a remaining thickness calculation without the need to resort to an etch stop layer. Furthermore since the physical size of the test structures 8 is deliberately chosen to be similar to that of the process features 6 (or alternatively the actual arrangement of process feature is used itself) the measure is fully representative of the film etch or deposition as it occurs within the process features themselves - the test structure acting as a true surrogate for the process features whose measurement is the key process measurand.

If in use these test structures can be conveniently located in the scribing lines of the semiconductor wafer as shown at 9, then they can function without a negative impact on the device yield from the wafer.

Thus, the invention selects a process control means that is simultaneously compatible in size with the main features on the substrate - and as such behave representatively in etch and deposition - and at the same time optimise evanescent coupling and/or sub- wavelength optical effects so that the incident radiation can couple with the effective medium apparent to the evanescent wave which consists of the mask and etched film, and if desired subsequently on into the underlying films and/or substrate, and yield an interferometric measure of the etch or deposition which can then be used to endpoint or control rate and uniformity.

Figure 4 shows by way of example an apparatus suitable for carrying out the invention. A vacuum chamber 40 is evacuated via an exhaust line 42 by a vacuum pump (not shown) . A support 44 holds a silicon wafer 46 for processing at a predetermined location. A narrow- bandwidth light source, such as a laser 48, and an optical system 50 both located outside the chamber 40 provide a light beam 52 incident via a window 62 on a selected portion of the wafer 46. A photodetector 54 is positioned to receive the reflected light and to provide a signal 56 containing information arising from interference by the evanescent wave. In Fig. 4, the photodetector 54 is shown as positioned close to the wafer 46; however it could be at a more remote location, such as outside the vacuum chamber 40, as the interference caused by the evanescent wave at the wafer surface modulates the reflected beam and can be detected at any location on the beam. The signal 56 is processed by a signal processing circuit 58 to provide a process control signal 60. The signal processing circuit may conveniently comprise analog- to-digital conversion followed by numerical processing. Suitable forms of apparatus for forming the beam, detecting the reflected signal, and processing the detected signal are well known in the art and not described in detail herein.

The basic purpose of the signal processing is to compare the real-time performance with a model of the desired process, which model may be derived by mathematical analysis or from a trial run which is known to have produced an acceptable result.

The signal processing may, in one example, comprise applying a shape or pattern recognition algorithm to the data stream. In a preferred form, the data stream is first subjected to digital filtering using a digital filter applied to one or more time windows as the signal develops, the digital filter having first been derived from a mathematical prediction of the signal behaviour.

The apparatus may be used to measure depth of etch, remaining film thickness, rate of etch, and a figure of merit giving an average width of etch. Such measurements can be used to control the progress of the etch process; indicate the endpoint of the etch; give early warning of the endpoint approach so that the etch can be slowed down or the chemistry of the etch changed to fine-tune the process (commonly called a 'soft landing'); or to permit the etch to be stopped part-way through a film, eliminating the use of an etch stop layer.

It will be understood that the invention is based upon optimising the arrangement of illumination, detector, signal processing means together with the illuminated feature distribution so that interference resulting from the film interface whose refractive index sensed by the illumination is not that of the bulk material but rather, due to the evanescent wave, is that resulting from a volumetric mean of the film and the etched structure.

The invention thus provides a means for monitoring and endpointing etch and deposition processes in situations where the feature size is small in relation to light beams which can be practically provided.

Claims

1. A method for improved control of etch or deposition in a semiconductor manufacturing process to produce a structure having a small feature size, the method comprising: providing a spectrally narrow illumination source selected to operate at a wavelength region in which the film to be processed, together with any overlying films or masks which are of the same small structure dimension, produces substantial evanescent coupling of the source into the film together with interference modulation of the reflected signal, generating from said illumination source an optical probe measurement beam, illuminating an article undergoing processing with said beam, the article having within the area of illumination an ordered feature arrangement having a feature size of the same order as the structure to be produced, detecting an interference pattern which is derived substantially from the interaction of the evanescent wave with said ordered feature arrangement resulting from the illumination, and using the interference pattern information to detect or predict the desired endpoint or monitor the progress in real time of the etch or deposition.

2. The method of claim 1, in which the ordered feature arrangement is a test structure applied to the article for the purpose of the process.

3. The method of claim 1, in which the ordered feature arrangement comprises structural features of the desired article itself.

4. The method of any preceding claim, in which the film to be processed and any overlying mask is substantially transparent to the wavelength of the illumination source.

5. The method of claim 4, in which the ordered feature arrangement has a feature size equal to or less than one-third of the wavelength of the illumination source, and a ratio of feature open to etch to features masked from the etch of at least 1:10.

6. The method of claim 5 in which the ordered feature arrangement has a simple repeat of the etch structure.

7. The method of claim 5 in which the ordered feature arrangement has no simple repeat of the etch structure.

8. The method of any preceding claim, in which the probe beam has a linear transverse dimension of 0.05mm or more.

9. The method of any preceding claim, further including comparing the interference pattern information with a model of predicted behaviour.

10. The method of claim 9, in which said model is created by analysing the process critical features, which analysis takes as its input the design of the features and their arrangement with other features in the three dimensions of the overall component together with the optical properties of the materials and the illumination wavelength of the illumination source.

11. The method of claim 9, in which said analysis includes analysis of the behaviour of the evanescent wave together with their polarisation modes and the interference resulting from the etched (or deposited) film as its thickness varies .

12. The method of any of claims 9 to 11, in which said analysis is used to provide an optimised endpoint approach using the spectrally narrow illumination source illuminating an area of an article being processed.

13. The method of any preceding claim, in which the article being processed has a mask defining areas to be etched, and detection of the reflected beam discriminates against radiation that has not optimally coupled into the region below the mask.

14. The method of any preceding claim, in which the article being processed comprises a substrate carrying a mask/film structure, and the beam is at a wavelength which propagates within the substrate as well as the mask/film structure, thus permitting the use of reflections from within the substrate itself.

15. The method of any of claims 1 to 13, in which the article being processed comprises a substrate carrying a mask/film structure, and in which the beam is at a wavelength which precludes propagation within the substrate, thus permitting discrimination against reflections within the substrate.

16. The method of any preceding claim including the further step of tuning the illumination means to a selected wavelength.

17. The method of claim 16, in which said selected wavelength is chosen in dependence on the material being examined and remains constant throughout the process.

18. The method of claim 16, in which said selected wavelength is tuned to a number of different wavelengths during the process, and the detected signals are compared with a family of predictions.

19. The method of claim 18 in which the family of predictions includes predictions for feature width as well as depth, and in which the results derived from tuning to different wavelengths are compared with the best fit of a family of predictions to give an estimate of the width of the etch feature.

20. The method of any preceding claim, in which the spectrally narrow illumination source is provided by combining a spectrally broad source with a wavelength discriminating means.

21. The method of any of claims 1 to 19, in which the illumination source comprises light generated by the deposition or etch process itself.

22. The method of claim 21, in which the deposition or etch process is a plasma process.

23. Apparatus for use in a semiconductor manufacturing process, the apparatus comprising: a vacuum enclosure; a workpiece location within the enclosure for locating a semiconductor workpiece to be processed to produce a structure having a small feature size, said semiconductor workpiece having an ordered feature arrangement having a feature size of the same order as the structure to be produced; a spectrally narrow illumination source producing light at a wavelength region in which the film to be processed, together with any overlying films or masks which are of the same small structure dimension, produces substantial evanescent coupling of the source into the film together with interference modulation of the reflected signal, in which the film to be processed, together with any overlying films or masks which are of the same small structure dimension, produces substantial evanescent coupling of the source into the film together with interference modulation of the reflected signal; optical projection means cooperating with the light source to produce an optical probe measurement beam directed to said workpiece location; optical detection means arranged to detect an interference pattern which is derived substantially from the interaction of the evanescent wave resulting from the beam falling upon a said semiconductor structure with said ordered feature arrangement; and data processing means arranged to compare the output of the optical detection means with a predetermined signal behaviour to produce a process control signal .

24. Apparatus according to claim 23, in which the illumination source or the detection means or both is provided with polarisation means.

25. Apparatus according to claim 24, in which said polarisation means is fixed.

26. Apparatus according to claim 24, in which said polarisation means is rotating.

27. Apparatus according to any of claims 23 to 26, in which the illumination means is tunable.

28. Apparatus according to claim 27, in which the illumination source is tuned to a plurality of wavelengths during production of a given product, and the data processing means is arranged to compare the detected signals with a family of predictions at said plurality of wavelengths.