Christopher J. Raymond

Metrology of subwavelength photoresist gratings using optical scatterometry

The widths and overall profiles of dielectric grating lines can be determined by measuring the intensity of diffracted laser light from the sample over a specified range of incident beam angles. This technique, known as 2‐Θ scatterometry, is able to accurately and precisely measure photoresist structures in the subhalf micron regime. Moreover, a 2‐Θ scatterometer is capable of making measurements in a rapid and nondestructive manner. To test this technique we measured five identically processed wafers with nominal 0.5 μm line/0.5 μm space grating patterns. Each wafer comprised gratings in a Shipley 89131 negative photoresist exposed in a matrix of incremental exposure doses and focus settings. The scatterometry results were consistent with cross‐sectional and top‐down scanning electron microscopy (SEM) measurements of the same structures. The average deviation of 11 scatterometer linewidth measurements from top‐down SEM measurements, over a broad exposure range, is 14.5 nm. In addition, the repeatability ...

Multiparameter grating metrology using optical scatterometry

Scatterometry, the analysis of light diffraction from periodic structures, is shown to be a versatile metrology technique applicable to a number of processes involved in the production of microelectronic devices. We have demonstrated that the scatterometer measurement technique is robust to changes in the thickness of underlying films. Indeed, there is sufficient information in one signature to determine four process parameters at once, namely the linewidth and thickness of the photoresist grating, and the thicknesses of two underlying film layers. Results from determining these dimensions on a 25 wafer study show excellent agreement between the scatterometry measurements and measurements made with other metrology instruments [top-down and cross-section scanning electron microscopy (SEM) and ellipsometer]. In particular, measurements of nominal 0.35 μm lines agree well with cross-section SEM measurements; the average bias is −1.7 nm. Similarly, for nominal 0.25 μm lines, the average bias is −7.3 nm. In ad...

Scatterometry for CD measurements of etched structures

Scatterometry, the characterization of periodic structures via diffracted light analysis, has been shown to be a versatile technique for measuring critical dimensions in photoresist as small as 0.160 micrometer. Rapid, non-destructive and inexpensive, scatterometry has the potential to be applied to other microlithographic features as well. This paper discusses applications of scatterometry in the measurement of etched sub-um poly-Si line/space patterns. Since etched features represent the final dimensions of a finished product, the characterization of such features is important. Initial attempts at measuring the etched linewidth and height using scatterometry assumed the sidewalls were perfectly vertical. Although results from these two parameter predictions were good, our measurement algorithms suggested that the etch profiles were not square. Thus, sidewall angle was left as an unknown in our model and three parameter predictions were made. These improved results from measuring the linewidth, height and sidewall angle are presented, and comparisons to SEM measurements of the same samples are made. Finally, experiments to determine the repeatability of the scatterometer for measuring etched features were performed. Results show that the repeatability of the instrument, for both static and dynamic measurements of nominal 0.25 micrometer structures, is sub-nanometer for all parameters measured; the 3(sigma) repeatability for static CD measurements is 0.63 nm, and for dynamic measurements is 0.78 nm.

Scatterometry measurement of sub-0.1 μm linewidth gratings

The effort discussed here addresses the use of shorter incident wavelengths for characterizing sub-0.1 μm linewidths and the corresponding influence on scatterometry measurement sensitivity to linewidth variations. A sensitivity metric, based on the variance statistic, was developed using well-characterized, large-pitch (0.80 μm) photoresist grating structures on Si illuminated at 633 and 442 nm. The same metric was applied to short-pitch (0.20 μm), etched gratings on InP, with the result that appreciable scatterometry sensitivity was measured, even at the 633 nm incident wavelength. Modeling was used to estimate scatterometry sensitivity at three wavelengths for photoresist critical dimensions of 100 and 70 nm on Si. A significant increase in sensitivity was not found until the incident wavelength was reduced to 325 nm. We are presently investigating techniques to improve measurement sensitivity for short-pitch structures using the 633 nm incident wavelength.

Multiparameter CD measurements using scatterometry

Scatterometry, the characterization of periodic structures via diffracted light analysis, is shown to be a versatile metrology technique applicable to several processes involved in microlithography. Unlike contemporary inspection technologies, such as scanning force microscopy (SFM) and scanning electron microscopy (SEM), scatterometry is rapid, non- destructive, inexpensive and has the potential for use in-situ. Furthermore, the flexibility of the technique allows it to be used for a number of different process measurements. In the production of a sub-micron microelectronic device, a typical series of process steps could involve the deposition of a poly-Si layer on oxide, followed by the application of an anti- reflection coating (ARC) and resist layer. Thus in total there are four parameters which will ultimately affect the overall quality of subsequent processing: the linewidth of the resist, the resist height, and the thicknesses of the ARC and poly-Si. We have demonstrated that the scatterometer measurement technique is robust to changes in the thickness of underlying films. Indeed, there is sufficient information in one signature to determine four parameters at once, even when the linewidth dimensions are as small as 0.16 micrometer and the poly-Si thickness is on the order of 2500 angstrom. Results from determining these dimensions on several wafers show excellent agreement between the scatterometry measurements and measurements made with other metrology instruments (top down and cross-section SEM, and ellipsometer). For example, the average bias between nine scatterometry and cross-section SEM measurements on nominal 0.35 micrometer lines is minus 1.7 nm; for 0.25 micrometer lines, the average difference is minus 7.3 nm. In addition, results from measuring the sidewall angle (a fifth parameter) from these same scatter signatures indicate that the resist profiles at optimum focus and exposure are near-vertical. Finally, the dynamic repeatability of this technique is shown to be excellent for all of the parameters measured (linewidth, resist height, ARC thickness and poly thickness). For example, the 3(sigma) repeatability of measurements on a 207 nm linewidth is 0.75 nm and the 3 sigma repeatability for measurements on a 311 nm linewidth is 1.08 nm.

Subwavelength photoresist grating metrology using scatterometry

A precise and accurate technique for the characterization of periodic line/space gratings is presented. The technique, known as scatterometry, derives its sensitivity and robustness from the wealth of information present in diffracted optical radiation. Scatterometry is capable of determining width, height, and overall shape of sub-half micron lines as well as the thickness of underlying thin films. The characterization process consists of three elements: a diffraction measurement apparatus, a model built on calibration data, and a statistical analysis routine that uses the model to correlate empirical data to the unknown parameters of the structure. The measurement technique was evaluated on twenty five wafers fabricated with deliberate deviation in focus, exposure dose, and underlying thin film thickness. Each wafer consisted of developed photoresist lines on an antireflecting layer, placed on layers of polycrystalline silicon on gate oxide on a silicon substrate. Scatterometry was used to simultaneously determine the width and height of the nominal 0.25 micrometers and 0.35 micrometers photoresist lines, as well as the thickness of underlying layers. Comparison of results obtained using reference methods (ellipsometry and scanning electron microscopy) are included.

Resist and etched line profile characterization using scatterometry

In previous applications scatterometry has shown promise as a metrology for several process measurements. The linewidths of both resist and etched features, and the thicknesses of several underlying film layers, have been accurately characterized using the technique1 . Up until recently these results have been obtained by assuming the features being measured possessed a nominally square profile. However, as metrology tolerances shrink in proportion to device dimensions, errors in the measurement technique due to non-square line profiles could become significant. To test the ability of the scatterometry technique to measure non-square profiles, two models have been developed. The first profile model assumes the top and bottom corners of a resist line can be approximated as a segment of some circle with a given radius. With the center of the circle fixed in space by the overall height of the resist and a nominal linewidth, the sidewall of the line is then modeled as the tangent line that connects the two circles. This particular model can accommodate both overhanging (<900) and trapezoidal sidewalls (<900) with just four parameters: the radius of the top and bottom corners, and the nominal top and bottom linewidths. Comparisons between cross-section SEM images and scatterometry profiles using this model will be presented. The second model, which we call the stovepipe model, is a modified version of a simple trapezoid model and has applications to etched features. In this model an etched line is parameterized by assuming the trapezoidal portion of the sidewall starts at some distance below the top of the line, with the top portion of the line remaining square. In this manner an etched profile can be modeled with four parameters: the overall height of the etched line, the nominal etched linewidth, and the overall height and sidewall angle of the trapezoid layer. Once again, scatterometry profile results in comparison to cross-section SEM images will be presented. The use of both of these models has reduced the difference between scatterometry and SEM CD measurements. For example, the average difference of twelve resist CD measurements, when compared to crosssection SEM measurements, improves from 19.3 to 10.1 urn when the full profile model is incorporated. nnKeywords: metrology, diffraction, optical metrology, scatterometry, process control

Developed photoresist metrology using scatterometry

Scatterometry is shown to be a viable alternative to current methods of post-developed line shape metrology. Five wafers with focus-exposure matrices of line-space grating patterns in chemically amplified resist were generated. The gratings were illuminated with a He-Ne laser and, utilizing only the specular reflected order measured as a function of incident angle, we were able to predict linewidth and top and bottom rounded features. The scatterometry results were verified with those obtained from scanning electron microscopy (SEM). A set of wafers having a SRAM device pattern was analyzed. These wafers contain columns of devices, each having received an incremental exposure dose. We present exposure predictions based on data taken with the dome scatterometer, a novel device which measures all diffraction orders simultaneously by projecting them onto a diffuse hemispherical `dome. A statistical calibration routine was used to train on the diffraction patterns from die locations with known exposure values.

Scatterometry for 0.24- to 0.70-μm developed photoresist metrology

Scatterometry, the characterization of periodic structures via diffracted light analysis, is shown to be a viable and versatile metrology for critical dimensions as small as 0.24 micrometers . Scatterometry is rapid, nondestructive, inexpensive, and potentially useful for on- line control during several microlithographic processing steps. This paper discusses two recent studies in which scatterometry was applied to the measurement of developed photoresist patterns. First, scatterometric measurements of developed resist lines in the 0.38 micrometers to 0.70 micrometers range will be presented. Results from four sample wafers are shown to be consistent with SEM measurements. For one wafer, the average deviation of scatterometry linewidth measurements form top-down SEM measurements, over a broad exposure range, is 14.5 nm. Moreover, our scatterometer is shown to be highly linear with the SEM; linearity coefficients have typically been above 0.99. The goal of our second project has been to determine whether scatterometry measurements are affected by variations in the integrated circuit production process. A set of twenty-five wafers was fabricated with deliberate variations in the exposure dose and the underlying film thicknesses. We are presently investigating the effects of the film thicknesses on the measurements of critical dimensions (CDs) as small as 0.24 micrometers . Preliminary results indicate that CDs and multiple thin films can be simultaneously measured by applying multi-parameter prediction algorithms to the scattered light data. Results from four different prediction algorithms are given. Finally, the repeatability of the scatterometer is shown to be excellent: 0.5 nm for consecutive measurements and 0.8 nm for day-to-day measurements. The results of an extensive repeatability/precision experiment are presented.

Application of optical scatterometry to microelectronics processing

We have shown the applicability of optical scatterometry to provide a metrology technique that satisfies many, and sometimes all, of the requirements needed. Scatterometry is a two-step process in which a sample having periodic structure is illuminated. The diffraction characteristics are extremely sensitive to the dimensional and optical structure of the sample. The diffraction is first characterized as a function of measurement parameters (e.g., angle of incidence). The second step is to analyze the diffraction characteristics to determine the structure characteristics. We have applied this technique to characterize samples from several phases of microelectronics and flat panel display processing.