, MEASUREMENT CAPABILITIES OF X-RAY FLUORESCENCE FOR BPSG FILMS K.O. Goyal, J.W. Westphal Semiconductor Equipment Group Watkins-Johnson Company Scotts Valley, California 95066 Abstract Deposition of borophosphosilicate glass (BPSG) films as dielectrics is one of the many steps involved in semiconductor device manufacturing. Accurate control of boron and phosphorus concentrations in these films, is essential in maintaining desired film properties. The narrowing process specification limits with the industry trend towards shrinking device sizes, is putting an increasing demand on the precision and accuracy of metrology tools used for film characterization. This paper evaluates the ability of x-ray fluorescence (XRF) technique in measuring boron and phosphorus concentrations in BPSG films deposited by chemical vapor deposition. Repeatability and reproducibility studies were conducted to evaluate precision of the x-ray fluorescence measurement technique. The spread and shift of the normal probability distribution curves from this data is quantified in terms of the statistical indices for capability potential (C,), and for process control and centering (Crk). These indices are used as an indication of the precision and accuracy of dopant concentration measurements provided by the XRF technique. The 6-sigma capability of this technique has also been evaluated. Introduction Deposition of dielectric films by chemical vapor deposition (CVD) is one of the various steps involved in IC device fabrication. The dielectric films deposited by CVD techniques, often need to be doped with boron and phosphorus to modify film properties. Boron is typically added to lower the glass transition temperature while phosphorus acts as a gettering agent for mobile metal ions. Accurate control of these dopant levels in the deposited film is essential for maintaining high yield of the IC devices that constitute the end product. A highly capable metrology tool in terms of its accuracy, precision, and stability becomes necessary to keep these dopant concentrations within the process specification limits. The degree of uncertainty in the measurement must be low in comparison to the uncertainties characteristic of the CVD process. Wet chemical analyses (WCA), Fourier Transform Infrared Spectrometry (FTIR), X-ray fluorescence (XRF), are some of the common dopant measurement techniques used by the semiconductor industry.
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Experiments Borophosphosilicate glass (BPSG) films of different boron (B) and phosphorus (P) compositions were deposited on a Watkins-Johnson TEOS999 APCVD system using TEOS-Ozone chemistry. The as-deposited films were measured for B & P using the Rigaku 3630 Wafer Disk Analyzer. This XRF tool was equipped with total reflection mirror optics, automatic pressure controller, and a dry pump. A spot size of 35 mm and single point measurements at 0.099 Torr vacuum, were used in these experiments. Each signal was collected over a time interval of 120 seconds. Phosphorus concentration was determined by measuring the characteristic P-&X signal corresponding to electron transitions between the L and K shells in the phosphorus atom. The Rigaku Fundamental Parameter0 method was used to convert the intensity of the measured signal to P,O,-wtO/o. Similarly, the boron concentration was determined using the B-Ka signal by the Empirical method0 set up in the Rigaku SYS3630 software. The results reported in PZ05- wt% and B,O,-wt% are converted to the desired units of P-wt% and B-wt%, respectively. The BPSG film thickness is a relevant parameter in determining the concentration of phosphorus in the film. It was measured on a Prometrix SM300 thin film measurement system. Accuracy of B and P dopant measurements in the BPSG films was evaluated by measuring 12 wafers of different B dz P compositions and film thickness by the XRF technique, and also analyzing a wafer from the same lot for each of these 12 different processing conditions, by wet chemical analyses (WCA). WCA are primary detection techniques involving dissolution of the BPSG film off of the silicon wafer substrate. B-w-t% was estimated from the solution by Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) which involves evaporation of the solution in an Argon plasma. Photons emitted by the electronically excited B atoms produced in the plasma are used to quantify B-wtO/o in the BPSG film sample. The P-wt% was estimated by calorimetry, which involves conversion of all phosphorus containing species to a blue molybdate complex for subsequent quantification. Precision of the XRF technique was evaluated by repeatability and reproducibility tests. Repeatability testing was performed by measuring the same wafer 10 times without removing the sample from the optical path. Reproducibility testing involved unloading and reloading of the same sample into the measurement chamber. Upper limits of 4.5 wt% were set on the B and issues concerning stability of the as-deposited thickness ranged from 3000 to 9000 A. P concentrations used for experimentation due to thin film in ambient conditions. The BPSG film Statistical Evaluation of Data Dopant measurements made by WCA were treated as the baseline for evaluating capability of the XRF technique. The absolute error between XRF and WCA measurements for the 12 wafer samples arranged in increasing order of the dopant level, is plotted in Fig. 1. This data shows
that the absolute errors are within f0.20 wt% for boron and ko.15 wt% for phosphorus measurements. Sample # Fin. 1 Absolute Errors in Dopant The WCA results used as a baseline are reported to have an accuracy and precision of f3% of the absolute value of the measured B-wt% and P-wt%. Relative errors in dopant measurements by XRF can be obtained by assigning error bars of f3% to the WCA results and replotting the data as in Fig. 2 & Fig. 3. Fig. 2 shows that P-wt% measured by XRF is within the 23% error limits of the WCA results. This implies that the XRF technique in turn has a f3% relative error on phosphorus measurements. Fig. 3 shows that the relative error limits on boron measurements are nearly twice as large. The B-wtO/o measurements reported can be assigned a relative error of ks%. 4 I 4 3 -- E m 2 OJ 0 1 2Pwt% 3 4 Fiq. 2 Relative Error in Phosphorus Fin. 3 Relative Error in Boron
A quantitative study of precision of the XRF method for dopant measurements was made by a statistical analysis of results obtained from the repeatability and reproducibility tests. The data collected from these tests is treated as a sample of the population and can be fit to a Students-t distribution having 9 degrees of freedom. The standard deviation (s) for the sample is calculated taking into account the constraint of X(X- X )=O. This gives the sample (n- 1) degrees of freedom, where n is the sample size. The same data was also plotted with v= 00, which corresponds to a normal distribution. These distributions are shown for P-wt h and B-wtO/o in Fig. 4 & Fig. 5, respectively. The experimental data gave a good fit to the Students-t distribution (with v=9), without any outliers. Hence, it represents a true sample and is a valid representation of the population. A normal distribution was assumed for subsequent analysis and the values of sample statistics were used for the population statistics. L 3.78 3.83 3.88 P-w% Fig. 4 Students-t Distribution Phosphorus -I 3.93 3.98 for 3.77 3.8i 3.87 3.92 3.97 B-w% Fia. 5 Students-t Distribution for Boron Upper and lower control limits @JCL & LCL respectively) for the distribution curves were selected as fo.15 wto/o. These are the acceptable limits for quality control in an IC device fabrication environment with limited effect on device yields. Dopant concentration variations inherent to the APCVD deposition process are also within these limits. The capability of the XRF measurement technique was quantified by defining two statistical indices; 1) machine potential C,, and 2) machine capability Cpk. The C, index for machine potential is calculated as a ratio of the allowable spread to the actual spread, and can be represented as c = UCL-LCL P 60 PI C, values greater than unity are necessary to place 100% confidence levels that the measurement can be within the control limits. Precision of the measurement is shown by higher values of the
C, index. Although a value of C, greater than unity is a necessary condition for measurements to lie within the control limits, this is not a sufficient condition due to the fact that all processes have a tendency to shift from the target. At times, this process shift can be large enough to create a high probability for the measurement to lie outside the control limits. This phenomenon is accounted for by defining a machine capability index, C,. The CPk index quantifies the degree of centering of the actual distribution with respect to the allowable distribution, and can be represented as cpk = smaller of &JCL - mean) or (mean - LCL) 30 PI CPk >l is the necessary and sufficient condition for 100% confidence that the measurement will be within the specified control limits. Fig. 6 & Fig. 7 show the probability curves and their position with respect to the target as obtained from the repeatability and reproducibility tests. Values for C, and Cpk obtained from this data are reported in Table 1. Target LCL Target 3.57 3.62 3.67 3.72 3.77 3.82 3.87 3.92 3.97 P-w% 3.57 3.62 3.67 3.72 3.77 3.82 3.87 3.92 3.97 P-wr% L Fin. 6(a) Repeatability of Phosphorus Fin. 6(b) Reproducibility of Phosphorus
Target Target 3.64 3.69 3.74 3.79 3.84 3.89 3.94 3.99 4.04 B-w% 3.64 3.69 3.74 3.79 3.84 3.89 3.94 3.99 4.04 B-wt% Fin. 7(a) Repeatability of Boron Fig. 7(b) Reproducibility of Boron Table 1. Statistical Indices for Precision & Accuracy of Dopant Based on Repeatability Testing Based on Reproducibility Testing The C, and Cpk indices reported above show that the dopant measurements are within the selected process control limits, and are precise. The C, values indicate that the precision of P- wt% measurements is at least 3 times the precision of B-w-to/o measurements. The industry s required lower constraint on the precision of a process is a 6-sigma capability. This implies the probability density distribution of the measurements must have a standard deviation such that the condition 60 I l/2 of the total specification range is satisfied. Fig. 8 illustrates the normal distribution necessary for the phosphorus measurement to be 6-sigma capable in comparison to the experimentally obtained distribution. Fig. 9 illustrates the same data for boron. As observed from these plots, the XRF technique well exceeds precision requirements for phosphorus. In the case of boron, it is capable of achieving a 5.3-sigma distribution, where the B-sigma control limits (+O. 169 wt%) are greater than the required limits (ko.15 wt%).
40 experimental 30 -. 3.73 3.78 3.83 3.88 3.93 3.98 4.03 3.72 3.77 3.82 3.87 3.92 3.97 4.02 P-w-t% B-w% Fin. 8 8dgma Capability of Phosphorus Fig. 9 84gma Capability of Boron Conclusion The x-ray fluorescence technique was evaluated for its precision and accuracy on dopant measurements in BPSG films. Its ability to measure phosphorus meets the future 6-sigma capability requirements of the semiconductor industry. Measurement of P was found to have better than a 3% relative error for the full range of the desired calibration. The technique has a high capability for the precision and accuracy of P measurements in BPSG films (C,=5.34, C&=1.29). Worst case analysis also yields a Cpk value greater than unity thus meeting our requirements for accuracy. Boron measurements made by the XRF technique are not as accurate and precise as those reported for phosphorus (C,=l.77). Relative errors in this case were approximately 5% of the measured WCA dopant concentrations. Precision for these measurements also falls short of the 6-sigma capability requirement. These results are attributed to limitations inherent to the XRF technique which produces low intensity signals for light weight elements like boron. As the semiconductor industry moves towards 6-sigma capable processes, the same requirement is becoming necessary for measurement techniques. The x-ray fluorescence technique used for quantification of dopants in BPSG films is currently a viable method for measuring dopant levels. However, improvements in boron measurements will be necessary to keep pace with industry standards.
. s List of Symbols V degrees of freedom n sample size S standard deviation of sample 0 standard deviation of population x measurement variable x sample mean Acknowledgments The authors would like to thank Neil Fernandes and the Watkins-Johnson Group for their support of our work. Company Applications References 1) Techniques of Analysis of Thin Films, Balazs Analytical Laboratory, 1992 2) Box G.E., Hunter W.G., Hunter J. S., Statistics for Experimenters - An Introduction to Design, Data Analysis, and Model Building, John Wiley & Sons, New York, 1978 3) Hogg R.V., Ledolter J., Applied Statistics for Engineers and Physical Scientists, Macmillan Publishing Company, New York, 1992 4) Horrell K., Introduction to Measurement Capability Analysis, Sematech Inc., Sept. 1991 5) Statistical Control Handbook, AT&T Technologies, 1984