Joerg Bischoff

LIGHT DIFFRACTION BASED OVERLAY MEASUREMENT

Optical overlay measurement methods are very effective since they are rapid and non-destructive. Imaging techniques need sophisticated image processing and suffer from the wave- optical resolution drawback. Presently, leading edge devices are offered with 5 though 10 nm measuring accuracy. In this paper a method is proposed that relies on the diffraction of a probing laser beam at a periodic reference pattern. This special pattern is implemented in the circuit layout. After the resist patterning of the second of two consecutive layers, the diffraction at the resulting net grating is measured. If an appropriate grating design is chosen, the misalignment error can be directly extracted from the diffraction efficiency. In order to obtain a strong diffraction signal and thus a sufficient signal-to- noise ratio optimum grating designs have been computed by means of rigorous diffraction modeling. Experimental results supported by rigorous modeling suggest that this technique could have the potential to meet next generation overlay accuracy requirements.

Optical scatterometry of quarter-micron patterns using neural regression

With shrinking dimensions and increasing chip areas, a rapid and non-destructive full wafer characterization after every patterning cycle is an inevitable necessity. In former publications it was shown that Optical Scatterometry (OS) has the potential to push the attainable feature limits of optical techniques from 0.8 . . . 0.5 microns for imaging methods down to 0.1 micron and below. Thus the demands of future metrology can be met. Basically being a nonimaging method, OS combines light scatter (or diffraction) measurements with modern data analysis schemes to solve the inverse scatter issue. For very fine patterns with lambda-to-pitch ratios grater than one, the specular reflected light versus the incidence angle is recorded. Usually, the data analysis comprises two steps -- a training cycle connected the a rigorous forward modeling and the prediction itself. Until now, two data analysis schemes are usually applied -- the multivariate regression based Partial Least Squares method (PLS) and a look-up-table technique which is also referred to as Minimum Mean Square Error approach (MMSE). Both methods are afflicted with serious drawbacks. On the one hand, the prediction accuracy of multivariate regression schemes degrades with larger parameter ranges due to the linearization properties of the method. On the other hand, look-up-table methods are rather time consuming during prediction thus prolonging the processing time and reducing the throughput. An alternate method is an Artificial Neural Network (ANN) based regression which combines the advantages of multivariate regression and MMSE. Due to the versatility of a neural network, not only can its structure be adapted more properly to the scatter problem, but also the nonlinearity of the neuronal transfer functions mimic the nonlinear behavior of optical diffraction processes more adequately. In spite of these pleasant properties, the prediction speed of ANN regression is comparable with that of the PLS-method. In this paper, the viability and performance of ANN-regression will be demonstrated with the example of sub-quarter-micron resist metrology. To this end, 0.25 micrometer line/space patterns have been printed in positive photoresist by means of DUV projection lithography. In order to evaluate the total metrology chain from light scatter measurement through data analysis, a thorough modeling has been performed. Assuming a trapezoidal shape of the developed resist profile, a training data set was generated by means of the Rigorous Coupled Wave Approach (RCWA). After training the model, a second data set was computed and deteriorated by Gaussian noise to imitate real measuring conditions. Then, these data have been fed into the models established before resulting in a Standard Error of Prediction (SEP) which corresponds to the measuring accuracy. Even with putting only little effort in the design of a back-propagation network, the ANN is clearly superior to the PLS-method. Depending on whether a network with one or two hidden layers was used, accuracy gains between 2 and 5 can be achieved compared with PLS regression. Furthermore, the ANN is less noise sensitive, for there is only a doubling of the SEP at 5% noise for ANN whereas for PLS the accuracy degrades rapidly with increasing noise. The accuracy gain also depends on the light polarization and on the measured parameters. Finally, these results have been proven experimentally, where the OS-results are in good accordance with the profiles obtained from cross- sectioning micrographs.

Photoresist metrology based on light scattering

Angle resolved light scatterometry along with advanced data analysis is a promising new metrology technique to meet the challenges of todays and tomorrows submicron technology. The measurement accuracy strongly depends on the performance capabilities of the algorithms utilized for data exploration and analysis. Presently, multivariate regression methods such as inverse least squares and principal component approaches are preferred. Substantial accuracy gains may be achieved by applying quasi-nonlinear methods, i.e. nonlinear data pretreatment followed by the usual linear regression. In this way, not only were the linewidth prediction errors in measuring developed resist lines pushed to below 20 nm, but likewise more complicated tasks such as silylation profile evaluation and latent image measurement could be addressed satisfactorily.

New aspects of optical scatterometry applied to microtechnology

During the last years, optical scatterometry (OS) has become a competitive technique in micrometrology. Not only is it a very rapid and non-destructive method, but it also meets the accuracy needs imposed by nowaday technology. Relative rms- values of one percent and below have been reported for the multi-parametric characterization of submicron profiles. Although being basically a far field approach, the resolution and accuracy limits of imaging optical methods may be overcome to a certain degree. The goal of this paper is to discuss some new aspects of this technique. Particularly, there are two main topics--the improvement of the 2(theta) -principle for the characterization of sublambda gratings and the extension of the optical scatterometry to the measurement of real scene features instead of periodic ones. As for 2(theta) -scatterometry, first the ability of this method for quantitative measurements is shown with a 0.8 micron pitch grating etched in silicon oxide. Second, the polarization sensitivity is investigated with a 512 nm chromium grating on quartz. While the TE polarization is useful for the coarse characterization of the basic profile in terms of linewidth and height, TM polarized light might be the better choice for sensing sidewall variations and other profile subtleties. And third, a new scatterometer design is presented, which enables a simultaneous 2(theta) - measurement providing for an increased measuring throughput. Here, the application as a resist development monitor is outlined. The second main topic is the extension of the OS to single features in a more common sense, i.e., comprising also small groups of lines or spaces. In a former paper, the authors presented the basic principle and first modeling results. Here, further investigations are discussed aiming at the light scatter dependence on the sidewall angle and the characterization of double lines. The simulations confirm the enhanced sensitivity of TM polarized light to variations in the sidewall steepness. Besides, some nearfield calculations reveal how the light interacts with the scatterer.

Characterization of 3D resist patterns by means of optical scatterometry

We report about our first attempts to apply optical scatterometry to the characterization of 3D resist patterns. Particularly, nominal quarter micron dots and holes are investigated having a pitch of a half micron in both directions. The specular reflected light versus the incidence angle shows significant variance with changing exposure dose and thus with changing dot or hole diameter, respectively. Scanning electron micrographs serve as an assisting tool for the characterization of the developed resist pattern. Additionally, an algorithm for the rigorous modeling of diffraction from regular 3D-patterns is sketched. In some cases, the match between experiment and theory is already quite satisfying. This might be improved by more refined profile adaption and enhanced computer power.

Numerical investigation of the resolution in solid immersion lens systems

Solid immersion microscopy, an optical method with the capability for super-resolution has received a considerable amount of attention in the literature in the past few years. The main targets of the technique are lithography, pattern inspection (including critical dimension measurement) and data storage. The classical theory predicts a resolution gain proportional to the refraction index of the solid immersion lens. The intent of the paper is to prove this prediction by means of simulations and to find optimum measuring conditions. To this end, we present a very efficient, rigorous modeling method. By means of this method, we show that the inclusion of evanescent waves is crucial for the resolution gain. This is detailed with different excitation and detection schemes. Further more, we investigate the impact of polarization and different sample types.

Diffraction-analysis-based characterization of very fine gratings

Fine gratings with spatial periods below one micron, either ruled mechanically or patterned holographically, play a key role as encoders in high precision translational or rotational coordinate or measuring machines. Besides, the fast in-line characterization of submicron patterns is a stringent demand in recent microelectronic technology. Thus, a rapid, destruction free and highly accurate measuring technique is required to ensure the quality during manufacturing and for final testing. We propose an optical method which was already successfully introduced in semiconductor industry. Here, the inverse scatter problem inherent in this diffraction based approach is overcome by sophisticated data analysis such as multivariate regression or neural networks. Shortly sketched, the procedure is as follows: certain diffraction efficiencies are measured with an optical angle resolved scatterometer and assigned to a number of profile parameters via data analysis (prediction). Before, the specific measuring model has to be calibrated. If the wavelength-to-period rate is well below unity, it is quite easy to gather enough diffraction orders. However, for gratings with spatial periods being smaller than the probing wavelength, merely the specular reflex will propagate for perpendicular incidence (zero order grating). Consequently, it is virtually impossible to perform a regression analysis. A proper mean to tackle this bottleneck is to record the zero-order reflex as a function of the incident angle. In this paper, the measurement of submicron gratings is discussed with the examples of 0.8, 1.0 and 1.4 micron period resist gratings on silicon, etched silicon oxide on silicon (same periods) and a 512 nm pitch chromium grating on quartz. Using a He-Ne laser with 633 nm wavelength and measuring the direct reflex in both linear polarizations, it is shown that even submicron patterning processes can be monitored and the resulting profiles with linewidths below a half micron can be characterized reliably with 2(theta) - scatterometry.

Light-scattering-based micrometrology

Optical scatterometry, defined as the characterization of surfaces via diffracted light analysis, has been shown to be an attractive tool for the metrology of microlithographic structures. To tackle the inverse scattering problem, advanced data analysis schemes have been developed. This paper illustrates the application of light scattering to characterize developed resist lines in terms of multi- parameter measurements. Additionally, the depth prediction and the width prediction of special silicon concentration profiles, embedded in a plane resist layer, are reported. Substantial accuracy gains have been achieved by using partial least squares (PLS) regression along with quasi- nonlinear data preparation techniques, including range splitting or enhanced quadratic and cubic approaches. Moreover, a combination of PLS and minimum mean square error methods enables rapid and nearly arbitrarily accurate measurements.

Diffractive solid immersion lenses: characterization and manufacturing

Solid Immersion Lenses (SILs) have an outstanding potential for applications in future generations of optical data storage systems. We report the realization of a diffractive Solid Immersion Lens (dSIL) which is the diffractive analog of the refractive hemispherical SIL. Here, inside the medium the propagation angles of the first order diffracted waves point in the same direction as the incident angles from outside the SIL. We realized two types of dSILs: binary phase elements were fabricated in a highly refracting glass (LaSF35) by direct 3-beam writing and successive reactive ion etching, and dSILs with a blazed profile were manufactured in photoresist by holographic lithography. The minimum distance between adjacent zones in the diffracting structure is in the range of one wavelength. Polarization dependencies and phase impacts have to be consideration in the design of an optical element with features this small. In comparison to the lithographically realized binary phase grating, the holographic elements have the advantage of high diffraction efficiency.