Focused Ion Beam Metrology

Scatterometry is gaining acceptance as a technique for critical dimension (CD) metrology that complements the more established scanning electron microscopy (SEM) techniques. Scatterometry determines the dimensions of the submicron structures by inverse diffraction calculations. SEM and scatterometry are complementary in many respects. Therefore, they are likely to coexist in the foreseeable future. Scatterometry and CD-SEM instruments can be integrated to take advantage of the complementary nature of the two techniques. To explore the joint use of scatterometry and SEM measurements, we measured a set of photoresist grating samples with CDs ranging from 240 nm through 40 nm by scatterometer (Sensys CD-i) and cross-section SEM (Hitachi S-4700). Although a cross-section SEM was used as an absolute standard for comparison of profiles and CDs, our conclusions range to include CD-SEM techniques. It was found that for measurements of profiles that were patterned with high uniformity within the measurement area, scatterometry was very effective, and correlated best with SEM measurements. However, in cases of substantial line-to-line profile variations, or for isolated or non-periodic lines, SEM is the more appropriate measurement method.

In semiconductor device manufacturing, critical dimension (CD) metrology provides a measurement for precise line-width control during the lithographic process. Currently scanning electron microscope (SEM) tools are typically used for this measurement, because the resolution requirements for the CD measurements are outside the range of optical microscopes. While CD has been a good feedback control for the lithographic process, line-widths continue to shrink and a more precise measurement of the printed lines is needed. With decreasing line widths, the entire sidewall structure must be monitored for precise process control. Sidewall structure is typically acquired by performing a destructive cross sectioning of the device, which is then imaged with a SEM tool. Since cross sectioning is destructive and slow, this is an undesirable method for testing product wafers and only a small sampling of the wafers can be tested. We have developed a technique in which historical cross section/top down image pairs are used to predict sidewall shape from top down SEM images. Features extracted from a new top down SEM image are used to locate similar top downs within the historical database and the corresponding cross sections in the database are combined to create a sidewall estimate for the new top down. Testing with field test data has shown the feasibility of this approach and that it will allow CD SEM tools to provide cross section estimates with no change in hardware or complex modeling.

Semiconductor technology is advancing below 50 nm critical dimensions bringing unprecedented challenges to process engineering, control and metrology. Traditionally, interconnect metrology is put behind high-priority gate metrology; however, considering metrology, process and yield control challenges this decision is not always justified. Optical scatterometry is working its way to interconnect manufacturing process control, but scanning electron microscopy (SEM) remains the number one critical dimension (CD) metrology for interconnect process engineering and optical proximity correction (OPC) modeling. Recently, several publications have described secondary electron (SE) trapping within narrow high-aspect ratio interconnect structures. In these papers, pre-dosing of the sample helped to extract SE from the bottom of the hole and measure its diameter. Based on current understanding of the phenomenon, one should expect that high-aspect ratio interconnect structures (holes and trenches) with critical dimensions below 100 nm may show signs of SE trapping of various degree. As a result, there may be an uncontrolled effect on SE waveform and, therefore, bias of CDSEM measurement. CD atomic force microscopy (AFM) was employed in this work as a reference metrology for evaluation of uncertainty of trench and hole measurements by CDSEM. As the data indicates, CDSEM bias shows a strong dependence on pitch of periodic interconnect structure starting from drawn CD of 50 nm. CDSEM bias variation for the evaluated set of samples is about 19 nm. A typical OPC sample consists of both photoresist and etched interlayer materials. As the AFM data for photoresist material indicates, the hole diameter changes quite significantly with depth and the hole profile varies from one OPC structure to another. Abe et al. [1] have used a clever way to correlate physical bottom diameter of holes with CDSEM measurements and demonstrated that for their process and dimensions the SEM top diameter and physical bottom diameter correlate well. Unfortunately, this conclusion can't be generalized and measurement uncertainty of CDSEM must be evaluated on an individual technology/process basis. A more general approach to improve CDSEM accuracy is necessary which is based upon SI-traceable CDSEM bias measurements, modeling and correction.

The mask pattern edge profile's impact on critical dimension (CD) metrology and printing performance was investigated. Three 193nm attenuating PSMs with intentionally changed pattern edge profiles were made. The three masks had patterns with different CD data width, so that we could chose isolated lines and holes from each mask that gave almost the same CD when measured by a mask CD-SEM. We chose patterns that were measured approximately 400nm and 800nm for isolated lines, and approximately 500nm for isolated holes, and simulated their wafer CD by an AIMS tool and found not only that the AIMS results were different from the mask CD-SEM, but also that the difference was not constant for the three masks. Though in this work, the pattern edge profiles were intentionally changed and were far beyond the actual deviation range in current photomask manufacturing, this meant that the mask CD-SEM might face a difficulty in accurate wafer CD prediction if we should take the pattern edge profile into account. In this work we employed an atomic force microscope (AFM) based mask CD metrology tool, and by comparing the output profile with cross sectional SEM observation, we confirmed that the tool can observe the pattern edge profile nondestructively. If, at a certain slice, this tool can output the same CD as the AIMS for the three masks, we thought that this tool might be a promising candidate in wafer CD prediction. We found that the optimal slice level for the CD-AFM depended on the pattern category. For 400nm isolated lines, a 40% slice was optimal, while an 80% slice was optimal for 800nm isolated lines. For 500nm isolated holes, the range of 30 to 40% show good match between mask metrology and AIMS. Though the cause of this difference among pattern categories should be investigated further, from the fact that a certain slice can match the mask CD and AIMS, we concluded that the CD-SEM was a promising tool for wafer CD prediction.

The measurement of resist image size using a finely focused ion beam (FIB) is described. Comparisons of FIB and scanning electron microscope (SEM) measurements of resist features are presented. The short penetration range of an ion relative to an electron is shown to offer fundamental advantages for critical dimension metrology. Ion beams can induce less dimensional change (damage) than SEMs during the measurement process, potentially yielding improved measurement precision. Resist cross sectioning using FIB is demonstrated, and the factors contributing to quality of the cross section are presented. An H2O gas assisted etching process for polymer resists has been developed and is shown to significantly improve the quality of resist cross sections.

Introduction of new material stacks, more sophisticated design rules and complex 3D architectures in semiconductor technology has led to major metrology challenges by posing stringent measurement precision and accuracy requirements for various critical dimensions (CD), feature shape and profile. Current CD metrology techniques being used in R&D and production such as CD-SEM, Scatterometry, CD-AFM, TEM have their inherent limitations that must be overcome to fulfil advanced roadmap requirements. The approach of hybrid automated CD metrology seems necessary. Using multiple tools in unison is an adequate solution when adding their respective strengths to overcome individual limitations. Such solution should give the industry a better metrology solution than the conventional approach. In this work, we will present and discuss a new methodology of CD metrology so-called hybrid CD metrology that mixes CD data coming from different techniques. In parallel to this hybrid metrology approach, we must address individual technique enhancement. Subsequently, scanning techniques enhancement will be presented (CD-SEM and CD-AFM) through contour metrology parameter which should become a pedestal feature for 1x node production. Finally, we will discuss the potential directions of a hybrid metrology engine as a generic tool compatible with any kind of CD metrology techniques.

Automated critical dimension (CD) metrology has long been known to have certain limitations. As a top down imaging technique, retrograde profiles, resist thickness loss and other process issues are difficult to detect with the standard production CD SEMs used throughout the industry. Tilting capability has recently added much needed degrees of freedom to CD SEMs, potentially opening the door to three dimensional metrology. Various methodologies can be used to interpret tilt image information. This paper investigates one particular technique used to extract three dimensional information. Using cross section and/or atomic force microscopy (AFM), one can evaluate the potential benefits and validity of CD SEM tilt capability. Sidewall angle, film thickness and possibly other parameters could possibly become standard calculations the top down CD SEM could perform on a routine basis. Data is reported comparing 3D information generated from top down CD SEM to AFM. Potentially, a couple of different approaches to evaluating 3D information will be compared. Lastly, recommendations for future applications will be discussed.

New ellipsometric approach to critical dimension metrology utilizing form birefringence inherent in a submicron line-and-space pattern

A procedure for calibrating a resist model iteratively adjusts appropriate parameters until the simulations of the model match the experimental data. The tunable parameters may include the shape of the illuminator, the geometry and transmittance/phase of the mask, light source and scanner-related parameters that affect imaging quality, resist process control and most importantly the physical/chemical factors in the resist model. The resist model can be accurately calibrated by measuring critical dimensions (CD) of a focus-exposure matrix (FEM) and the technique has been demonstrated to be very successful in predicting lithographic performance. However, resist model calibration is more challenging in the low k1 (<0.3) regime because numerous uncertainties, such as mask and resist CD metrology errors, are becoming too large to be ignored. This study demonstrates a resist model calibration procedure for a 0.29 k1 process using a 6% halftone mask containing 2D brickwall patterns. The influence of different scanning electron microscopes (SEM) and their wafer metrology signal analysis algorithms on the accuracy of the resist model is evaluated. As an example of the metrology issue of the resist pattern, the treatment of a sidewall angle is demonstrated for the resist line ends where the contrast is relatively low. Additionally, the mask optical proximity correction (OPC) and corner rounding are considered in the calibration procedure that is based on captured SEM images. Accordingly, the average root-mean-square (RMS) error, which is the difference between simulated and experimental CDs, can be improved by considering the metrological issues. Moreover, a weighting method and a measured CD tolerance are proposed to handle the different CD variations of the various edge points of the wafer resist pattern. After the weighting method is implemented and the CD selection criteria applied, the RMS error can be further suppressed. Therefore, the resist CD and process window can be confidently evaluated using the accurately calibrated resist model. One of the examples simulates the sensitivity of the mask pattern error, which is helpful to specify the mask CD control.

A hexagonal honeycomb magnetic tunneling junction array with 72 nm pitch and 30 nm MgO critical dimension was successfully fabricated on a 1× nm dynamic random access memory platform by using a mature dynamic random access memory patterning solution and ion beam etch. To our knowledge, both pitch size and critical dimension size are the world’s smallest ones for industrial magnetic tunneling junction arrays. To obtain such a high density and small sized magnetic tunneling junction array, a cross self-aligned double patterning technique, a triple layer hard mask scheme, and an optimized ion beam etch condition were adopted. During the optimization of the ion beam etch process, the dependence of a magnetic tunneling junction pillar profile on ion beam etch parameters for a high density hexagonal magnetic tunneling junction array has also been systematically studied. The depth of oxide recess and the magnetic tunneling junction sidewall angle increased with the ion beam etch amount, while magnetic tunneling junction critical dimension and hard mask remainder thickness decreased with increasing ion beam etch amount.

To accurately control critical dimension (CD) metrology in a standard real-time solution across a multi-site operation there is a need to collect measure-to-measure and day-to-day variation across all sites. Each individual site's needs, technologies, and resources can affect the final solution. A preferred statistical process control (SPC) solution for testing measure-to-measure and day-to-day variation is the traditional Mean and Range chart. However, replicating the full measurement process needed for the Mean and Range chart in real-time can strain resources. To solve this problem, an initially proposed measurement methodology was to isolate a point of interest, measure the CD feature <i>n</i> number of times, and continue to the next feature; however, the interdependencies in measure-to-measure variation caused by this methodology resulted in exceedingly narrow control limits. This paper explains how traditional solutions to narrow control limits are statistically problematic and explores the approach of computing control limits for the Mean chart utilizing the moving range of sample means to estimate sigma instead of the traditional range method. Tool monitoring data from multiple CD metrology tools are reported and compared against control limits calculated by the traditional approach, engineering limits, and the suggested approach. The data indicate that the suggested approach is the most accurate of the three solutions.

As device size is going under the sub-60 nanometer scale, lithography is facing its resolution limit. To solve this resolution limit it has been suggested that one critical device layer could be made from multi photo/etch process, which needs tighter overlay and critical dimension (CD) controls than normal single exposure process. For CD control in this multi photo/etch process, the problem is that we do not know which pattern is made from which photo/etch process after final process. This makes it impossible to measure the specific process pattern's CD independently. In this case the conventional CD measuring method, which measures multi CDs in FOV(Field Of View) of SEM(Scanning Electronic Microscope) and control their average and distribution, couldn't know the average difference between both patterns because the variance from simply measured multi CD is just sum of each variance and their cross terms not including the average difference. In this paper it is pointed out the statistical problem of classical multi CD measurement in the indistinguishable multi-process pattern and a compensative method is suggested with a new statistical formula.

Fano lineshapes associatedwith quasi bound-state-in-the-continuumresonances, that are supported by dielectric metasurfaces, have theadvantageous properties of being extremely sensitive to minute geometricalchanges in the meta-atoms. We show an approach to determine deep subwavelengthfeature sizes, comparable to semiconductor critical dimension metrology,by structurally infilling a void of a dielectric disk-hole metasurfacedesign. Our simulated results show a sensitivity of 40.5 nm resonantwavelength shift for a 1 nm feature width (i.e., critical dimension)change, at an optical line width of 1.8 nm. We present both experimentaland simulated results of different void infillings and attribute thespectral change of the resonance to the sensitivity to an effectiveindex in the void of the meta-atom, which arises from the filled volumefraction and the material boundaries orientation relative to the localpolarization. Treating our metasurface as an effective index sensor,the sensitivity is 262 nm·RIU–1 and the figureof merit is 146 RIU–1, which underlies the pronouncedresonant wavelength shift driven by similarly large changes in theeffective index caused by extremely tiny critical dimension variations.This approach could impact critical dimension measurements in semiconductormetrology, as it works at the high throughput of optical measurementswhile performing at the high resolution of scanning electron microscopy.

The objective of this research is to present novel methods for fabricating patterns with micro and nano-scale features on homo-polymer substrates and polymer blend films. Top–down (irradiations through patterned openings over large areas) and bottom–up (local modifications of substrate properties) approaches were combined to introduce nano/micrometer length scale features spanning over large areas (few mm2) of the substrates which can be used for products like environmental sensors, micro-reactors etc. Ion projection lithography technique was used to create specific local interactions in different substrates viz. cross-linking in the case of uniaxially stretched homo-polymer substrates and pre-patterned substrates with varying surface energies for thin polymer blend films. The irradiation of polystyrene (PS) surfaces with ion beams led to pronounced chemical and physical modifications, when the ions were scattered at the polymer chains. The local mechanical properties of the PS surface layers could be tailored (as measured by Young´s modulus) by changing the ion doses and ion species (with different masses). By annealing pre-stretched irradiated PS near the glass transition temperature, surface rippling occurred only in the irradiated areas. The moduli obtained from rippling periodicities and elastic model assumptions were in the range between 8 to 800 MPa and characterized the irradiated PS as rubber-like. From that, the network density and the molar mass of entanglement were quantified. Results confirm that even for non-discrete layered systems, a consistent elastic description can be applied and relevant polymer parameters can be derived from the well-known surface rippling. In the case of guided polymer blend phase separation, the gold layer coated silicon substrates were bombarded with focused ion beams (FIB) to sputter away Au grains in the irradiated regions and expose silicon oxide surface underneath, thereby creating pre-patterns of relatively hydrophobic (Au) / hydrophilic (silicon oxide) regions. In the pre-patterned regions, the spin-coated films consisting of immisible PS and PtBA (poly-tert-butyl acrylate) blend exhibited phase separation induced by the underlying pre-patterns and formed two distinct ordered morphologies with periodicities much smaller than that of the pre-patterns. The effects of varying periodicities of the pre-patterned structures, pre-pattern geometries (viz. 1-D arrays of stripes and spaces and 2-D arrays of squares), blending ratios and spin-coating parameters on the resulting morphologies were investigated. Observed affinities of PS and PtBA towards Au and silicon oxide surfaces respectively were explained in terms of work of adhesions from the wetting analysis.

Gauge Repeatability and Reproducibility (Gauge R&R , G.R.R) is of great importance for checking measurement system precision, traditional average-range G.R.R test involves equipment variation (EV) and appraiser variation (AV), which is not appropriate for current modern automatic measurement tools. In this paper, we designed different test experiments on film thickness (THK) and critical dimension (CD) metrology and statistically analyzed different variances impact on G.R.R. From THK G.R.R experiment ANOVA analysis, it can be seen that time interval and measurement process have no significant effect on G.R.R result, and the equipment repeatability variation is the major variation source. From CD G.R.R experiment ANOVA analysis, it can be checked that, G.R.R has significant time dependence of data collection due to test wafer changed. As a conclusion, not only appraiser’s variation but also time interval has less impact on metrology G.R.R of semiconductor wafer manufacturing process.