2D Dopant Profiling Research Articles

High-resolution two-dimensional dopant characterization using secondary ion mass spectrometry

A quantitative two-dimensional (2D) boron dopant profiling for submicron technology has been demonstrated using secondary ion mass spectrometry (SIMS). A novel test structure incorporating a moiré pattern has been developed to improve SIMS lateral resolution. This approach significantly simplifies 2D SIMS test chip manufacturing, data acquisition, and analysis. As a result, 2D dopant profiling with a lateral resolution limited by the photomask pixel size (10 nm) and sensitivity of 3×1017 cm−3 has been realized on commercial equipment. 2D dopant profiles are reproducible within 10 nm. The measured profiles were compared with 2D Monte-Carlo and calibrated TSUPREM4 simulations and showed good agreement.

Two-dimensional dopant profile of 0.2 μm metal–oxide–semiconductor field effect transistors

In this article, we report our progress in two-dimensional (2D) dopant profiling of metal–oxide–semiconductor field effect transistor devices fabricated with 0.2 μm technology by using dopant selective etching followed with atomic force microscope imaging (DSE/AFM). By comparing device simulation results based on our measurement with electrical measurement results, we have found that in these small devices the depletion region can have a significant effect on the results of dopant concentration conversions from the selective etching data, and the necessary correction can be found by a proper estimation of the depletion width. Here we discuss the factors that affect the etching reproducibility and reliability and show our approaches to these challenges. Our results show that DSE/AFM is a useful technique for 2D dopant profiling with certain advantages if it is dealt with properly.

Two-step dopant diffusion study performed in two dimensions by scanning capacitance microscopy and TSUPREM IV

We report the results of a two-step two-dimensional (2D) diffusion study by scanning capacitance microscopy (SCM) and 2D SUPREM IV process simulation. A quantitative 2D dopant profile of a gate-like structure is measured with the SCM on a cross-sectioned polished silicon wafer. The gate-like structures consist of heavily implanted n+ regions separated by a lighter doped n-type region underneath 0.56 μm gates. The SCM is operated in the constant-change-in-capacitance mode. The 2D SCM data are converted to dopant density through a physical model of the SCM/silicon interaction. This profile has been directly compared with 2D SUPREM IV process simulation and used to calibrate the simulation parameters. The sample is then further subjected to an additional diffusion in a furnace for 80 min at 1000 °C. The SCM measurement is repeated on the diffused sample. This final 2D dopant profile is compared with a SUPREM IV process simulation tuned to fit the earlier profile with no change in the parameters except the temperature and time for the additional diffusion. Our results indicate that there is still a significant disagreement between the two profiles in the lateral direction. SUPREM IV simulation considerably underestimates the diffusion under the gate region.

Direct comparison of two-dimensional dopant profiles by scanning capacitance microscopy with TSUPREM4 process simulation

Quantitative two-dimensional (2D) dopant profiling of a gatelike structure is achieved by scanning capacitance microscopy (SCM) on a cross-sectioned polished silicon wafer. The gatelike structures consist of heavily implanted n+ regions separated by a lighter doped n region underneath a 0.56 μm gate. The SCM is operated in the constant change capacitance mode while scanning with a 37 nm radius tip. The 2D SCM data are converted to dopant density through a physical model of the SCM/silicon interaction. The model parameters are adjusted so that the SCM dopant profile far from the gate edge fits the vertical secondary ion mass spectrometry (SIMS) profile. A 15% error in average accuracy is found between SCM and SIMS profiles evaluated over the dopant range of 1020–1017 cm−3. The same model parameters are used for all points in converting the 2D SCM data, indicating that the accuracy of the full 2D result should be comparable to that of the vertical profile. A direct comparison of the 2D SCM and 2D TSUPREM4 results is made for the first time. The agreement is generally good, but there are some notable differences.

A Study of Transistor Optimization in A 0.25 Micron CMOS Flow Using S/D and Silicide Process Modules and Their Interactions

ABSTRACTA modular approach to CMOS process development requires an understanding of individual process modules (channel, gate, etc.) and their interactions. The reverse short channel effect, (RSCE) in NMOS devices is one such interaction between the channel and source/drain (S/D) modules. Similarly the interaction between the S/D and silicide modules affects the contact and gate sheet resistance. This paper presents (a) an investigation of the effects of S/D processing (As and P implant conditions) on the RSCE, (b) effects of S/D and silicide processing on the contact and gate sheet resistance, (c) the use of an integrated system to optimize process modules and their interactions and (d) the validity of the modules and system in process development by obtaining a 4% improvement in drive current of a 0.25 micron NMOS device. The outputs of the S/D module include the doping profiles as a function of implant and anneal conditions. The interaction between the channel and S/D is caused by the damage created during the S/D implants, leading to channel dopant redistribution during subsequent thermal anneals. This interaction causes the RSCE in NMOS devices and has to be taken into account before the modules can be used for process optimization. The interaction is studied by varying the dose and energy of the As and P S/D implants and observing their effect of the RSCE. Further, to model this interaction, parameters of a 2D dopant profile model are extracted from device data and form a part of the S/D module in addition to the doping profile information. The system integrates the outputs from the process modules and their interactions and allows for a rapid search of the process space. The search criteria can be varied to include performance (e.g. drive and off current) and manufacturability criteria.

Defects and Diffusion issues for the Manufacturing of Semiconductors in the 21st Century

ABSTRACTWithin the past decade, process simulation has become an essential part of new technology development in the silicon IC industry. The use of TCAD (technology computer aided design) tools has been driven by the enormous cost of purely experimental approaches to technology development. Yet the power of these tools and their predictive capability are still greatly limited by the models they use. TCAD models for doping processes are universally based today on point defects. These models have evolved considerably in the past decade to incorporate additional understanding. The state-of-the-art today includes concentration dependent diffusion through Fermi level effects on defect concentrations, full coupling between defects and dopants which allows prediction of non-local diffusion effects, basic models for the effects of ion implantation damage (the +1 model), surface and interface effects (through effective recombination velocities and segregation), and full 2D and 3D simulations.As devices continue to shrink, better models will certainly be required. Challenges for the future include more detailed information about damage resulting from ion implantation, better understanding of point defect properties (equilibrium populations, diffusivities, transient response to temperatures changes), better models for point defect behavior at interfaces, and finally, development of accurate methods to actually measure 2D and 3D dopant profiles. This paper will attempt to describe where we are and where we need to be in the future.

Direct comparison of cross-sectional scanning capacitance microscope dopant profile and vertical secondary ion-mass spectroscopy profile

The scanning capacitance microscope (SCM) has been shown to be useful for quantitative 2D dopant profiling near the surface of silicon. An atomic force microscope is used to position a nanometer scale tip at a silicon surface, and local capacitance change is measured as a function of sample bias. A new feedback method has been recently demonstrated in which the magnitude of the ac bias voltage applied to the sample is adjusted to maintain a constant capacitance change as the tip is scanned across the sample surface. The applied ac bias voltage as a function of position is then input into an inversion algorithm to extract the dopant density profile. The new feedback approach allows for the use of a quasi-1D model in the inversion algorithm. Since there are no alternative 2D dopant profiling techniques which are well established at present, evaluation of the quantitative character of 2D SCM measured profiles has been a challenge. To avoid this obstacle, we have developed sample preparation methods which allow direct comparison of lateral SCM measured profiles on cleaved wafers (cross-sectional plane) with vertical secondary ion-mass spectroscopy (SIMS) profiles. The direct comparison of inverted SCM data and SIMS profiles indicates that quantitative 2D dopant profiling can be achieved by the SCM on a nanometer scale.

Two-dimensional dopant profiling of submicron metal–oxide–semiconductor field-effect transistor using nonlinear least squares inverse modeling

We present an inverse modeling technique to determine the two-dimensional (2D) dopant profile of a metal–oxide–semiconductor field-effect transistor from electrical measurements. In our method, the profile is formulated using two tensor product splines (TPSs). This analytical representation is general, compact, and flexible. It simplifies the profile determination problem to the extraction of the TPS coefficients from experimental data. We show the results of applying the new technique on data collected from a sub-0.5 μm complementary metal–oxide–semiconductor technology with various source/drain implants. We also compare the measured and simulated I–V and C–V characteristics. The results illustrate the importance of accurate 2D dopant profiles for short-channel device simulation and modeling.

Characterization of two-dimensional dopant profiles: Status and review

The National Technology Roadmap for Semiconductors calls for development of two- and three-dimensional dopant profiling methods for calibration of technology computer-aided design process simulators. We have previously reviewed 2D dopant profiling methods. In this article, we briefly review methods used to characterize etched transistor cross sections by expanding our previous discussion of scanned probe microscopy methods. We also mention the need to participate in our ongoing comparison of analysis results for test structures that we have provided the community.

Lateral dopant profiling of submicron VLSI device structures using scanned-probe microscopy

Present day very large scale integrated circuit (VLSI) technology requires accurate knowledge of the spatial extent in three dimensions (3D) of active impurity dopants which have been purposely incorporated into the device structures. The active region of a typical device, where most electrical conduction is confined, is engineered to contain dopants such as As, B, or P in a concentration range of 1015 to 1020 cm−3. It is necessary to control the variation of the dopant "profiles" to a resolution of 100 nm for device reliability. The devices themselves occupy a footprint of, typically, 10 um2.Except in 1D, it has been impossible to achieve a high degree of precision in the metrology of dopant profiles in actual device structures. Existing methods which have been developed for 2D dopant profiling, such as junction-staining or TEM, generally yield only qualitative information and are destructive measurements. Secondary ion mass spectrometry (SIMS) yields quantitative data but the technique is time and labor intensive, but it, too, is destructive.

Methods for the measurement of two-dimensional doping profiles

The problem of measuring two-dimensional (2D) diffusion profiles in silicon is a critical one. Several very different methods for 2D dopant profiling have been proposed recently, but the methods are often applied only by specialists to selected problems and wide acceptance of these methods is yet to take place. This article presents a comprehensive review of methods for the measurement of 2D dopant profiles, along with a complete bibliography. The goal is to compare the various methods on the basis of some common factors, and to begin to establish some criteria for selecting 2D dopant profiling methods. First, some issues are raised which, in addition to criteria such as resolution and sensitivity, form a basis for differentiating between various methods. The methods described in the literature to date are then classified and discussed briefly. All reported methods and their variations are at least mentioned with appropriate references. Finally, a comparison of various methods is presented, focusing on specific issues such as resolution, accuracy, complexity of sample preparation, and the complexity of equipment required for the measurement.