Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05B—CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS
  • G05B17/00—Systems involving the use of models or simulators of said systems
  • G05B17/02—Systems involving the use of models or simulators of said systems electric

Definitions

• This invention relates to analysis of simultaneous variation of one or more random variables, representing device structure or manufacturing process characteristics (parameters), and to applications of the results to statistical analysis of semiconductor device behavior and semiconductor fabrication processes.
• Background of the Invention Since the early 1960s, technology simulation has played a crucial role in the evolution of semiconductor integrated circuit (IC) technology.
• LSI large scale integration
• TCAD Technology Computer-Aided Design
• the device simulator's task is to analyze the electrical behavior of the most basic of IC building blocks, the individual transistors or even small sub-circuits, at a very fundamental level.
• the simulator uses as inputs the individual structural definitions (geometry, material definitions, impurity profiles, contact placement) generated by process simulation, together with appropriate operating conditions, such as ambient temperature, terminal currents and voltages, possibly as functions of time or frequency.
• a TCAD system's principal outputs are terminal currents and voltages, appropriate for generating more compact models to be used in circuit simulations.
• a circuit simulator typically defines the interface to higher-level, application-specific synthesis tools.
• TCAD In principle, given a recipe and a set of masks, the TCAD tools themselves can build virtual device structures and then predict performance of entire circuits to a very high degree of accuracy. Modern process and device simulators are capable of rapid characterization, on the order of many structures per hour, as compared to experiments where on is sometimes fortunate to obtain a new structure within a period of a couple months.
• TCAD is extensively used during all stages of semiconductor devices and systems manufacturing, from the design phase to process control, when the technology has matured into mass production. Examples of this are discussed by R.W. Dutton and M.R. Pinto, "The use of computer aids in IC technology evolution", Proc. I.E.E.E., vol. 74, no. 12 (December 1986) pp.
• fabrication processes may include substrate formation, ion implantation, thermal diffusion, etching of a trench or via, surface passivation, packaging and other similar processes.
• random variables or parameters associated with these processes include concentration of a semiconductor dopant deposited in a semiconductor substrate, temperature of a substrate, temperature used for a thermal drive process for diffusion of a selected dopant into a substrate, initial impurity concentration of the substrate used for the thermal drive process, thickness of a substrate used for the thermal drive process, concentration of a selected ion used for implantation of the ion into a substrate, average ion energy used for implantation of an ion into a substrate, areal concentration of an ion used for implantation of the ion into a substrate, thickness of a substrate used for an ion implantation process, initial impurity concentration of a substrate used for an ion implantation process, desired incidence angle for etch into a substrate, desired
• Two types of uncertainty affect one's confidence in modeling response of an electronic process, or of an electronic device produced by such a process: structural uncertainty, arising from use of inaccurate or incomplete models; and parametric uncertainty, arising from incomplete knowledge of the relevant variables and of the random distributions of these variables.
• Parametric uncertainty can be reduced by (1) refining measurements of the input variables that significantly affect the outcome of an electronic fabrication process and/or resulting device and (2) refining the knowledge of interactions between variations of two or more of the input variables and the effects of these joint variations on the process and/or resulting device.
• Sensitivity of a fabrication process or a resulting device to variation of one or more of these variables may depend strongly upon the particular values of the other variables.
• Each Monte Carlo sample or run to model one or more steps in a fabrication process may require use of a complex physical model in order to reflect the interactions of the variables, and each run may require many tens of seconds or minutes to complete. Further, the totality of runs may not adequately illuminate the joint effect of simultaneous variation of several variables because of the plethora of data generated.
• What is needed is an approach that allows modeling of simultaneous variation of N random variables, where N is at least 1, so that subsequent Monte Carlo sampling can be performed in less time.
• this approach should allow estimation of an N-variable model function and calculation of joint moments, such as means and variances, for any subset of the N variables.
• this approach should allow presentation of at least some of the results in analytic form as well as in numerical form. Summary of the Invention
• the invention provides an approach for determining a model function for an N-variable process (N ⁇ l) related to modeling of output responses for semiconductor fabrication and semiconductor device behavior.
• An output response for a process or an electronic device is approximated by a model function G(x ⁇ ,...,X ) that is mathematically tractable and that can be used to estimate a probability density function that applies to the N variables jointly.
• the model function can be used to calculate statistical moments (mean, variance, skew, kurtosis, etc.) for these variables jointly or individually and can be used to identify regions in (xi ,...,x j s ⁇ )-space where subsequent
• Monte Carlo sampling may be reduced or eliminated.
• a multidimensional model function G(x j ,...,x js r) of degree up to D in the variables is constructed as a selected sum of polynomial products H ⁇ r ⁇ ( ⁇ )H2 r 2(x2)— Hj f rN( ⁇ N)
• each orthogonal polynomial set ⁇ H j r i(xi) ⁇ r i may have its own maximum polynomial degree Dj (O ⁇ ri ⁇ D j ).
• the polynomial coefficients are determined by requiring that values of the model function G(x ⁇ ,...,x j s ⁇ ) be close to observed or simulated values of an output response function F(x ⁇ ,...,xj ), when the N variables are set equal to any of a set of collocation values.
• G(x ⁇ ,...,x j f) may be used in subsequent Monte Carlo sampling or other statistical sampling to characterize the output response of interest.
• the estimate G(x ,...,x jN f) may also be used to estimate statistical moments for the different variables, individually or jointly.
• FIG. 1 illustrates a suitable use of the invention.
• Figure 2 is a flow chart illustrating a procedure to practicing the invention.
• Figure 3 is a schematic view of computer apparatus arranged to practice the invention. Description of Best Modes of the Invention
• Figure 1 illustrates an environment in which the invention might be used.
• a fabrication process for a semiconductor or other electronic device depends in some way on N randomly varying variables x , X2 > ... , X
• each of which is preferably continuously variable over its own finite or infinite range, aj ⁇ XJ ⁇ bj.
• Each variable XJ has an associated non-negative probability density function PJ(XJ).
• the output response F may be a physical measurement, such as a threshold or breakdown voltage for a particular gate produced by one or more steps in a fabrication process.
• the output response F may be a predicted value for a variable produced by a measurement or a simulation used to model the process.
• One object of the invention is to estimate a suitable multidimensional model function G(x ⁇ , X2, ... , XN) that can be used for various numerical calculations in modeling, but is easier to calculate than the output response(s) for the process.
• Each variable XJ has an associated probability density function
• PJ(XJ) (aj ⁇ x ⁇ b j ) that can be used as a weighting function in an integrand to generate an associated sequence of polynomials ⁇ Hj r j(x) ⁇ n of degree ri ⁇ 0 that are orthogonal when integrated over the domain a j ⁇ x ⁇ b j .
• the sequence of orthogonal polynomials obtained will depend upon the variable range (aj ⁇ XJ ⁇ bj) and upon the nature of the weighting function pj(xj).
• a Gaussian distribution is one of the most useful distributions.
• a random variable is more likely to be distributed according to this distribution than to be distributed according to any other distribution.
• Use of a semi-infinite interval (0 ⁇ x ⁇ ⁇ ) and an exponential weighting function pj(x) exp(-x) (a low order Poisson process), leads to the Laguerre polynomials. This exponential distribution is useful where the variable has a semi-infinite range.
• ⁇ ( ⁇ ri ⁇ ,N,D) indicates a permutation sum of terms in which each of a set of N numbers rl, ... , rN assumes all possible non-negative integer values that collectively satisfy the following constraint:
• D is a selected positive integer representing the highest degree of approximation desired for the output variable G
• coefficients c rl ... rN are characteristic of the deterministic process studied ( Figure
• An estimated multidimensional model function for the random variable G in Eq. (10) is
• G(x ⁇ ,x 2 ) A 0,0 + A l,0 H l,l( ⁇ l) + A 0,l H 2,l( ⁇ 2) + A 2,0 H l,2( ⁇ l) + A U H U ( X1 ) H 2 j(x 2 ) + A 0 , 2 H 2 , 2 ( 2)> ( i2 ) where A ⁇ , AI Q. AQ I, A2 Q, AJ 1 , and AQ 2 are undetermined coefficients.
• Gaussian e.g., distributed according to an exponential or finite uniform or finite non-uniform distribution
• the variable G(x ⁇ , ..., XN) is evaluated at collocation points, which are solutions of the orthogonal polynomials of degree D+l, where D is the highest degree for the variables x ]_, ..., XN that appear in G(x ⁇ , ..., XN).
• a 0 2 6.00 (18F) for the undetermined coefficients in Eq. (12).
• the number Ml of undetermined coefficients will be less than the number M2 of collocation value N-tuples (xj c , ...,XN C ) that are calculated using an output response function, such as that shown in Eq. (11).
• Determination of the number Ml of undetermined coefficients is more complex but may be verified to be
• Eq. (12) can serve as an estimate for the actual two-variable output response function F(x ,X2) adopted in Eq. (11).
• Monte Carlo sampling can now be performed, using the two-variable model function G(x ,X2) to determine the relative number of sample runs to be made in each region of (xj,X2)- space. This approach will usually be less expensive, measured in computer time, effort and funds expended, than a conventional approach to Monte
• This approach may also identify a region, corresponding to a coefficient A r _ r N wi h very small magnitude, where little or no interaction of the variables is expected so that the number of Monte Carlo samplings in this region may be reduced or eliminated.
• the system may provide or compute one or more quality of approximation statistics Q(G) that provide a measure of how well the model function G(XJ,...,XN) fits the available data.
• Q(G) quality of approximation statistics
• Q(G) is the mean square error MSS, defined as the average of the residual sum of squares
• a low value for MSS is desired, preferably one that increases no faster than a constant multiplied by V(M2).
• Q(G) is the coefficient of determination R 2 , which measures the proportion of the variation in the actual process values that can be predicted by changes in the variables using the model function G
• weights w k are all set equal to 1. Ideally, the weights w k are all set equal to 1.
• R 2 statistic equals 1, or is very close to 1.
• the statistic 1- R 2 is preferably less than a threshold value, such as 0.1 or 0.3. As the number M2 of simulations is reduced, the R 2 statistic will increase monotonically, but not necessarily strictly monotonically.
• R 2 (adj) (1 - R 2 )(M2-1)/(M2-M1), (26)
• a first preferred condition here is that the quantity 1 - R 2 be no greater than a selected threshold value.
• a second preferred condition is that R (adj) and R 2 be reasonably close to each other. If R 2 and R 2 (adj) differ substantially, it is probable that the model is not sufficiently accurate.
• Figure 2 illustrates a procedure corresponding to one embodiment of the invention.
• a maximum polynomial degree of approximation Dj for each variable X j is specified.
• a single overall polynomial degree of approximation D can be specified here, as discussed in the preceding development.
• each probability density function pj(xj) is used as a weighting function to generate a sequence of orthogonal polynomials ⁇ Hj r j(xj) ⁇ r associated with the weighting function, where the polynomial Hj r j(xj) is of degree ri in the variable XJ, with O ⁇ ri ⁇ Dj, and the polynomials satisfy the orthogonality relations set forth in Eq. (2).
• the polynomial degrees ri associated with the polynomials Hj r j(x j ) collectively satisfy the constraint 0 ⁇ rl+r2+...+rN ⁇ D.
• step 19 an estimate G(XJ,...,XN) for the N-variable model function G(XJ,...,XN) is generated, using Eq. (10) or another suitable estimate. This estimate will have several coefficients whose values are to be determined.
• step 21 the collocation points X jc for the corresponding orthogonal polynomial of degree Dj+1 are determined, and collocation N- tuples (XJ C ,X2 C ,...,XN C ) are formed. This will provide a total of M2 collocation N-tuples.
• step 23 the process function F(X ⁇ ,...,XN) is evaluated at each of the collocation N-tuples.
• At least one quality of approximation parameter Q(G) is provided or computed for the model function G(XJ,...,XN), and the system inquires if Q(G) is no greater than a selected threshold number Q(G) mr ? If Q(G) ⁇ Q(G) tj ⁇ r , the system proceeds to step 31.
• step 29 the system increases at least one of the maximum polynomial degrees of approximation D j , or alternatively the overall polynomial degree of approximation D, and recycles to step 17.
• step 31 one or more regions of (x j ,...,XN)-space is identified where the magnitude of a corresponding coefficient A r _ ⁇ N is less than a specified coefficient threshold value.
• an estimate G(xj, ...,XN) for the N-variable response output F(xj, ...,XN) of interest is assembled and used to perform Monte Carlo sampling in the N variables XJ.
• the N-variable function G(XJ,...,XN) is preferably used to determine the relative number of sample runs to be made in each region of (x ,...,XN)-space. This sampling can be used to estimate the values of the coefficients Bj in Eq. (1).
• the number N of variables XJ is often limited to a relatively small number in the range l ⁇ N ⁇ lO but may be larger if desired.
• the invention will accommodate any reasonable maximum polynomial degree D j or D and any reasonable number N of random variables.
• FIG. 3 schematically illustrates an arrangement of computer apparatus 40 suitable for practicing the invention.
• the apparatus 40 includes: a CPU 41 and associated registers for numerical and logical processing; a data/command entry mechanism 43 (keyboard or similar device) for entering selected estimation parameters (N, D or ⁇ O , ...,
• a memory component 45A to receive the parameters entered using the data/command entry mechanism 43; a memory component 45B that stores the software commands used to perform the steps set forth in the flow chart in Figure 2; a visual display
• the computer apparatus 40 includes a memory component 45C that stores the software commands used (i) to identify one or more regions of (x j ,...,XN)-space where a coefficient A r ⁇ ⁇ N has a magnitude no larger than a specified coefficient threshold (e.g., 10 " 3 or
• the collocation method developed and applied here is analogous to the method of Gaussian integration that is applied to estimate the value of a definite integral using orthogonal polynomials.
• Gaussian integration is discussed by F.B. Hildebrand, Introduction to Numerical Analysis.
• Solution of polynomial equations to identify collocation points, solution of least squares problems and solution of systems of linear equations can be performed using routines available in a numerical routine software package such as LAPACK, which is available on the Internet at lapack@cs.utk.edu. Currently, Version 2 of LAPACK is available. A printed version of the LAPACK Users' Guide is available for purchase from the Society of Industrial and Applied Mathematics, 3600 University City Science Center, Philadelphia, PA 19104-2688.

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• 1998
  • 1998-11-02 US US09/184,611 patent/US6173240B1/en not_active Expired - Fee Related
• 1999
  • 1999-10-28 AU AU13290/00A patent/AU1329000A/en not_active Abandoned
  • 1999-10-28 WO PCT/US1999/025338 patent/WO2000026730A1/en active Application Filing

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