Invited Paper

Applications of optical emission spectroscopy in plasma inanufacturing systems

George G. Gifford

IBM East Fishkill Facility, R/300-40E IJopewell Junction, New York 12533

ABSTRACT

Optical emission spectroscopy (00s) is an established laboratory diagnostic technique for plasma processes. By detecting light from the electronic transitions of atoms and molecules it is possible to identify and monitor the chemical species in a plasma. ‘I’his technique has been extended to semiconductor manufac- turing to determine the endpoint of plasma processes. ‘l’he production of semiconductor devices relies heavily on plasma etching and dcposition processes. I3ecausc OES is a fairly simple technique, its use as a continuous tool arid proccss honitor has been investigated. I Jltimately, this technique could provide imme- diate feedback for automatic adjustment of individual process parameters. This embodiment has been referred to as adaptive process control.

2. INIRODUCTION

Reccntly, the value of OBS as a plasma process diagnostic instrument has been demonstrated in manufac- turing. A low cost, high resolution ( < 0.2nm 1;WIJM) optical emission spectroscopy system was used to dctect process and tool contamination with excellent results. Examplcs from this system show the advan- tages of high resolution OUS for diagnostics. To determine the simultaneous dynamic behavior of several process specics, a lower resolution photodiode array based system has been used. An example of a moni- toring and control system ckveloped from the characterization of a manufacturing process will be shown.

3. EXPLANATION

Manufacturing engineers need to ensure that each product wafer going through a process receives, as much as possible, the same treatment. I,argcly, this reyuircs that the plasma tool functions repeatably and that each wafer is exposed to the same processing environment. I’rocess uniformity is a measure of the ability to reproduce this rrivironment over the critire surface of a wafer and process reproducibility is the ability to achieve the same results from wafer-to-wafer. The processing environment ran be characterized not only by the process and tool parameters (chamber pressure, RI; power, flow rates, DC. bias, etc.) but also by the resriltiiig plasma glow.

OIB is a tneans of characterizing the cl~mical cornposition of the plasma. It relies on the de-excitation of atoms and simplc molcculrs in the plasma to reveal the chemical composition and plasma excitation charac- trristics. Iiach spccics gives off light at a uniyuc set of wavelengths depending on the energy levels to which i t was nri@nally cxcitcd and thr energy Icvd to which it relaxes. Thus, with sufficient spectral resolution and range, it is possiblr to obst:rve the light coming from each of these transistions and determine both the iden- tity of the species, and its excited states in the plasma. IJnfortunately, the complexity of the excitation mc~hanisms makcs it very difficult to quantify the ground state population. Relative changes it1 concen- tration are more easily determined. Cobum (1) and atomic actinometry can be used with caution in comparing the concentration of an atomic species, like fluorine, to that of a known quantity of added argon.

Several exceptions have been noted by J .

Soinctimes, residual gas analysis (RGA) is preferred for the study of gas composition and the detection of impurities because partial pressures can be determined. rl’Vyi> major advantages of OES are that it is a non-

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invasive technique, and that sampling effectively takes place from the process chamber itself rather that at a sampling port downstreatn from the wafer. Once a quartz window has been installed in the wall of the vacuum chamber a wealth of information about the plasma process is available.

4. APPLICATIONS 4.1. Process development

'There are several distinct ways that O L X can bc used to enhance plasma processes for semiconductor fabri- cation. In process development efforts, OES can provide a major improvement in process understanding because it is an in-situ method for studying the process. It can reveal different types of information depending on the spectral bandwidth of interest and the length of the plasma process step. One use of OES in both process development and process monitoring is to study the dynamic behavior of a process. A photodiode array based system can be used to look at the changing relationship among several species during a process arid to monitor large bandwidths with an effectivc time resolution of less than 50 milliseconds. A bandwidth of greater than 600nm can be monitored, but with a substantial reduction in resolution. This type of OT!S can also be used to study thc reproducibility of chemical and plasma changes that occur during the process. O n r disadvantage of the photodiode array is a loss of rcsolution due to leakage currents between adjacent diodes and the resulting broadening of spcctral basc widths.

'1'0 properly identify a specific chemical species in a complex gas mixture, higher resolution optical emission equipment can be uqed. Figure 1 illustrates the high signal- to-noise ratio and spectral line clarity that high resolution OF3 can provide. A 0.32m scanning monochromator equipped with adjustable width slits and a high dispersion grating can provide a useful resolution of about 0. Inm (FWIIM). A balance, though, must be maintained between the need for high resolution an3 the desire to obtain data in real time. With a resol- ution of O.lnm, it can take scveral minutes to covcr a 50nm batidwidth. This emphasiLes thc importance of first characterizing the spectrutn with a broad bantl\vidth photodiock array and then choosing specific regions to study at a higher resolution.

In-situ monitoring techniques, in general, can assist the process engineer in optimizing a process for manu- facturing. Because OES can be uscd to determine tlie endpoint of a process, the effect of various parameter changes on the process etch rate and reproc1ucit)ility can be easily studied. In this way the throughput of a process can be optimized. Also, thc CES signal is affected by the process uniformity. For example, when an etch by-product that decreases in concentration at endpoint is monitored, the slope of its decrease indi- catcs thc relative unifbrtnity of the process. A mor(: uniform process will show a more abrupt level change in the intcnsity of the species.

'Traditional optical endpoint scherncs call for thc monitoring of a specific wavelength with a bandpass filter or short focal length monochromator. This is difficult when the emission band chosen is too close to irrel- evant or competing emission signals. Tkcreasing the slit width and increasing the monochromator focal length arc two simple ways to improve resolution. Alternatively, multiple wavelength or multiple species schemcs cart iocrcase the arriiracy 0 1 the endpoint determination. This can be accornplishcd with either a scrirs of filtrrs, monochrornators, or ;I photodiodc array. If the group of species to be monitored is in a narrow hand, 75nm or less, an intensified photodiode array can be used 011 a 0.28tn spectrograph with an effective resolution of less than 0.3iiin (FWIIM). At this resolution it is possible, in simple spectra, to differ- rntinte between the broader molecular emission hands and the narrow atomic emission lines.

Also, 01% can provide information o i i thc interaction of the process with the product wafer and process chamber. I b r cxamplc, the detection o f alurninum in a oxygen ashing plasma is a clear indication of alu- minum spui tcring from thc rharnbcr or product wafer. Excessive sputtering and possible redeposition can clamage product wafcrs. Ion boinbardincnt is an intcgral component of reactive ion etching, and 017s can bc used to monitor this unintentional sputtering. Ily monitoring the emission of sputtered atoms while

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adjusting process parameters and tooling configurations, OES can be used to optimize the process and tooling design.

4.2. Failure recovery and tool qualification

Once a reproducible means of obtaining high quality spectra is established for a manufacturing tool it is advantageous to the manufacturing and equipment engineers to obtain an OES ”fingerprint” of the manufac- turing process. This fingerprint is then combined with information about the typical process and tool parameters during a production run. If OES is used to characterize a plasma process when it is running smoothly, then this fingerprint can be used as a starting point after the tool is repaired or if a failure occurs. Problems such as a vacuum leak or tool contamination can be more readily identified using this technique.

‘I’his fingerprint is a reference to which spectra taken at later dates are compared. It is important to acquire both high resolution spectral data, which can be used to reveal the exact identity of a species, and dynamic information about the behavior of many different species using a photodiode array. Ideally this fingerprint is first acquired when the plasma tool is first qualified for manufacturing. For etching processes it may be helpful to take an additional fingerprint using a wafer which is mirumally affected by the process, for example, using an oxide coated wafer in an oxygen ashing process. By eliminating the intentional dynamic change of the plasma constituents, in this case oxidized carbon, the stability of the etching tool becomes more apparent. ‘l’hc number of product wafers that are risked is also reduced. These tests then establish a baseline condition for the tool arid process t o which future comparisons and tool corrections may be made.

This type of database is a very powerful aid in the diagnosis of problems with a plasma tool and process. With the complexity of plasma tools and processes, the titne needed to diagnose a problem is often as time consurning as the actual repairs or adjustmentq. ‘I’hough the engineer must manually set up the diagnostics cyuipment and collect the data, ideally the means for comparing the most recent spectrum with this fitigcrprint is automatic. Software routines need to be developed which make this automatic comparison in a statistically meaningful manner. The ratio of the current spectrum to a reference is much more useful than the difference. Subtraction does not highlight the sudden appearance of trace species that may have a large iinpact on the process.

‘I’he quantitative determination of impurity levels or pas mixtures is not readily obtained with OES. Ilowcver, comparisons of thc intensity ratios of two specics can he used to show changes in the gas mixture. One application could be to verify the accuracy, using a standard, and the repeatability of the flow control- lers. Iiigure 2 shc~ws two low resolution spectra from tfifluorochloromethane (Freon 13) and hydrogen plasmas. The highlighted peaks on the left side of each spectrum are the prominent emission bands of the CF2 molecule. ‘I’he darkened peaks to the right are the most prorninent emission bands of €IC]+. By comparing the ratio of their intensities, I(CF2)/1(1ICl +), one can see a relationship between this ratio and the ratio of 1;rcon 13 t o hydrogen in the feed gas mixture. As the ratio of Freon 13 to hydrogen is increased from 0.3 t o 0.48 the nortn:ilized intensity ratio, I(CF2)/1(IICl+ ), increases from 0.89 to 1.04. Changes as small as a five percent increase in the Freon 13 to I12 ratio have been detected using this technique.

Figure 3 qhows a portion of the emission spcctrutn for two of the most common process contaminants, watcr and nitrogrn, taken of a water plasma to which nitrogen had been added. This figure shows the real breadth of the major 1JV emission band for the 011 molecule. ‘I’his dissociation fragment of water is often simply listed in rcferenccs by the location of its band head at 306.4nm. A 700 channel UV intensified photodiode array at room tcmperature was used to obtain this spectrum. The subtraction of the spectrum from a pure water plasma from this combined spectrum yields the nitrogen spectrum (shown as the dark region) in Figure 3.

OK3 can also be used to check the stability of the plasma. Figure 4 shows the sensitivity of the N emission line at 396. I Snrn to instabilities in the matching network of an argon sputter etching tool. This instability is

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most readily detected in the argon emission line itself as argon is the only feed gas in this process. The N signal must then be corrected for changes in the Ax signal, by monitoring the ratio Al/Ar, before it can be reliably used to determine the endpoint of a process. The Ar signal has been monitored to detect RP tuning mismatches, arcing, and changes in RF losses. ‘J’hough Figure 4 shows an example using high resolution OES, this technique has also been demonstrated with a photodiode array based system.

‘lo be of greatest use to the engineer, an 00s system must be able to provide both dynamic information about the plasma process and specific information about the identity of trace impurities or etch by-products. If one OES system is to be used to periodically monitor several production tools, portability and ease of set-up are prerequisites. These two requirements exclude the use of long focal length spectrometers and spectrometers that require that the optical cavity be opened when a grating is switched or when the unit is switched from monochromator to spectrograph mode. Spectrographs that meet all of these requirements have recently becotne commercially available.

4.3. OES intplcmciitatioii

4.3.1 Hardware issucs

IJntil recently it has been vcry difficult for manufacturing arid equipment engineers to acquire spectral infor- mation. Substantial improvenients it1 both hardware and software have been necessary. First, a standard is needed for the optical access to each type of plasma tool. The reproducible placement of the optical fiber is essential to fhe developinent of an accurate database. Hecause an optical fiber accepts a cone of light, an optimal sampling location must be determined and thcn an easy me‘ans of repeatably monitoring the tool must be established. Permanently mounting a11 01% systetn to a production tool simplifies this task.

One standard for fiber optics is the use of SMA (subminiature type A) bushings as optical connectors. This bushing provides for a I.0mm bundle or single fiber to be carried with silica or other cladding. ‘I’he high thread count and the tight tolerance specifteations for this connector (required for its use in communication systems), tnake it ideal for plasma tools. Its relatively small size makes it easy to develop interfaces to small, single wafer chambers. A paper by M. Webb (2) explains the optimization of fiber optic interfaces for spectroscopic uses.

Another issue whcn setting up an optical monitoring systcm in a manufacturing facility is the fragility of thc optical components and detector. A n intensified photodiode array can easily cost $15000. If the unit is thermoelectrically cooled then provisions musl bc made for a dry nitrogen purge. ‘I’his prevents moisture condensation on the intensifict which can cause corrosion. The use of argon as a purge gas must be avoided. Argo11 readily ionizes in the high voltage section of thc iritcnsifier and causes arcing which can destroy the intensifier. In gcnerd, the use of special color coding for all fragile components is recommended. ?‘his will help plasma tool mairitcnancc pcrsonncl who have not been trained in the care and replacement of optical components. ‘I’he use of a rcd outside jacket for commonly used fiber optics can substantially reduce the possibility of breakage bcrausc i t can be easily sccn in yellow clean room lights.

4.3.2 Software rlrvclopnien t

Software for 0 1 3 3 syt;.tema has, until recently, been written for the spectroscopist. The recent availability of user friendly 0 1 % software has advanced the use of OES as a general process monitor and diagnostic instru- ment. Manufacturing eiigitiecrs, in gcwcral, need instrumentation that does not require frequent trips to the user’s matiual. ‘Iaqk oriented software for the semicoriductor fabrication line is certainly a requirement for any futurv instrurncntation.

Automated C)V,S data collection has recently been demonstratcd using a commercially available OF3 system. Many suggestions and alterations were made to the software of this photodiode array controller to improve

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the user interface for the production environment. One example of this was the simplification of the proce- dure to subtract the detector's dark current noise. This background subtraction technique is now a five second task done once per manufacturing shift. Default background levels are used if none is supplied.

By automating the data collection procedures, though, it is now possible to fill the computer hard disk very quickly. l'his has led to two areas of development. Data reduction can be accomplished quite effectively once a process has been fully characterbed and its dynamic behavior can be generalized. By only storing occasional full bandwidth scans and the intensity versus time data for the species of interest, the number of production runs that can be stored on the hard disk has been increased by four times. Second, the means for effective data compression are now readily available through the use of packing routines.

With this increase in thr availability of data and the need for improved process monitoring in manufacturing, better data analysis routines are nceded that can make use of the data and allow for improved real time response. Statistical process control (SPC) software could be quickly implemented to assist in the determi- nation of a tool baseline and measure subsequent deviations.

4.4 Continuous process moiiitoriiig and roiitrol

IIigh resolution OES was used to characteria the plasma of an in-situ Ar RF sputter cleaning process in manufactut-ing. ( 3 ) A spectral region betwceri 280 and 340nm was found that revealed important informa- tion about the condition of the process and tool. Because improvements in the reproducibility of this process and the performance of the sputter etch tools were sought, a continuous process monitoring and control system was developed.

In this process, a photodiode array OES system is used to simultaneously monitor the emission from seven chemical species, Ar, AI+ , CO, 0 1 1 , AI, Cu, and N2. IJsing a 0.28m spectrograph with a 1200 grooves/" grating, it is possible to monitor the entire spectral region from 290 to 3401m using a 700 channel photodiode array. This equipment combination provides an effective resolution of about 0.26nm (FWI IM). I'igure 5 shows three tune slices from a typical sputter etch run. In this particular case, a sample time of 33 seconds was chosen and 20 samples were taken over this 660 second etch. Samples 1, 3 and 20 are shown.

nased on the behavior of these species, the photodiode array controller has been programmed to determine the endpoint of the process, detect process and tool contamination, and monitor RF stability all in real time. 'The typical dynamic behavior of the integrated spectral arcas for four of these species (AI, Ar, CO, and 011) is shown in Figure 6. The signal for 011 has been corrected for interference with the A1 signal.

In this process, an oxidized aluminum and copper surface is cleaned by the sputter etch. As the oxide is removed, the intensity of these two species increases dramatically. CO is a by-product of this sputter etch process in the prcsencc of photoresist. The endpoint of this process can be readily determined by the plateau in the increase of the A1 line and the decrease of the CO band as seen in Figure 6. The Ar and Ar+ signals (only Ar is sliown in Figure 6) reveal the stability of-the RI; components. 'There is a natural decrease in their intensity over time due to changes in the chamber impedance as the process progresses.

Water and nitrogen are contamitiants in this process. OH, formed by the dissociation of water, is readily discerned in Figure Sa. 'The tool for this process is vented to atmosphere after each run and this first sample shows the water which is desorbed from both the wafers and the chamber when the plasma is fvst ignited. Nitrogen is another impurity present in this example. The N2 signal drops throughout the run and suggests a problem with outgassing. 'I'he wafers in this run were baked at a lower temperature than those of previous runs, and the OES scans from those runs do not suggcst the presence of any nitrogen. A constant N2 signal level woulcl indicate a vacuum leak or a purge valve problem.

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Thresholds can be set in the controller software which allow the system to react to these changing species signal levels in the form of visual alarms and messages. Three different color coded alarms have been devel- oped. If excessive contamination or any major instabilities in the RF are detected, then an alarm message to imtnediately abort the process run is shown in red on the monitor of thc OES controller. The determination of the process endpoint invokes an alarm message, shown in green, instructing the operator to shutoff the RI? and continue with the process. ‘I’he third type of alarm warns, in blue, that preventive maintenance should be done on the tool. This alarm is triggered by rising nitrogen levels or a shift in the typical time needed for process endpoint. l h e means for immediate programmed feedback to the process tool is planned for the OES system. This will allow the controller to send commands directly to the process tool. Cur- rently, at the end of each process run, a data arid alarm summary is autotnatically stored on the hard disk of the OFJS controller.

5. SUMMARY

OES has been implemented as a plasma process monitor and diagnostic instrument in semiconductor manu- facturing. Several key improvements in the hardware and software available to the manufacturing and equip- ment engineer have made this possible. Additional improvements are needed in the area of data analysis including statistical process control, spectra comparisons and species identification. The integrated use of OES with other real-time process monitoring teclmiques is a natural progression from its use as a stand- alone instrument.

6. AC KNOWLEDG EMENI‘S

The author would like to thank Ilr. Gary Selwyn, Mr. Ivan I)olgov, Mr. George Wohh, Mr. Lawrence nauer and Mr. Frank hlarinacchio of 113M arid Mr. Doug Malchow of F,G&G Princeton Applied Research for their involvement with this work.

1. J. W. Coburn, and M. Chen, .I. Appl. I’hys. 2 (6), 3134 (1980).

2. M. J . Wehh, Spectroscopy, Vol. 4, No. 6, 1989.

3. G . Srlwyri, G. Gifford, 1. Ilolgov, 11. \Vildtnnn, J . Rapp, submitted to Plasma Chemistry and Plasma Processing.

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26 I

22 24 1

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N2 Molecular Emission Bands

Second Positive System C 3 lT,-B3 TIg N2'

350 Wavelength (nm)

Fig. 1 High resolution OES spectrum of nitrogen plasma.

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a) Freon 13/H2, 15/50 SCCM ratio, 15mT I

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Fig. 2 Photodiode array OES spectra of Freon 13/H2 plasmas.

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900 I

700 "*i VI e - 5 500 6ool Z E

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Fig. 3 Spectrum of water plasma with added nitrogen. Darkened areas are nitrogen bands determined by the subtraction of the spectrum of a pure water plasma from this combined spectrum.

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Fig. 4 RF stability monitored with high resolution OES of AI line.

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I- U

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c) 627 to 660 secs. Intensity divided by 8.

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Fig. 5 Time integrated array spectra of Ar RF sputter etch.

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Fig. 6 Dynamic behavior of integrated spectral regions.

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