Charging phenomena are investigated using a scanning electron microscope (the Applied Materials VeritySEM),
equipped with an energy filter. Three types of charging are studied: wafer charging, uniform charging of the field-of-view (FOV), and non-uniform charging of the FOV. Wafer charging occurs when the wafer is charged by some source other than the scanning electron beam. Uniform and non-uniform charging of the FOV occur when a wafer is
scanned with a primary electron beam.
On insulating materials, the primary electron density, in units of electrons per unit area, governs whether the
charging regime is uniform or non-uniform. At low electron density, the charging regime is uniform FOV charging.
In this regime, the surface potential increases linearly with FOV size and extraction field, in agreement with
calculations based on an electrostatic simulation. At high electron density, the charging regime changes to a non-uniform
local charging, varying over adjacent pixels within the FOV. The local field attracts the emitted SE's until a
steady state is reached having a local yield of one. In this regime, the FOV charging potential is weakly depend on
FOV size, and it can be either positive or negative, depending on the strength of the applied extraction field.
One of the well-known problems the industry faces concerning 193-nm resists is its shrinkage under scanning electron microscope (SEM) measurements. While different phenomena arising from electron-material interaction are assumed to take place (such as cross-linking and scission), the primary mechanism that brings about this shrinkage is still unclear. Three experiments were performed relating to three theories for the primary mechanism that brings about the shrinkage. The first experiment examined how the shrinkage is affected by 193-nm radiation and corresponds to a theory that the electron exposure induces an effect similar to that of 193-nm exposure. The second experiment deals with electron-beam curing, using parameters similar to those used in SEM measurements (curing entails a much lower power density). The third experiment addresses the theory of disassociation of carbonyl bonds in the resist, leading to cross-linking and eventual evaporation of CO2 molecules. The results from the exposure and curing experiments lead us to believe that an exposurelike effect and resist local heating are not good candidates for the primary mechanism. The last experiment shows that slimming is related to the release of carbonyl bonds.
Downscaling of semiconductor fabrication technology requires continuous improvements in production process control. To ensure tool-to-tool matching and compatibility of critical dimension-scanning electron microscopy (CD-SEM) measurements to measurements from other technologies, such as optical CD, or from other fabrication entities, accuracy has become a much more important factor than in the past. CD-SEM measurements have always yielded a bias, which can be quite significant, but also typically neglected since it does not vary much over a process window. However, the standard CD-SEM metrology approach to algorithm accuracy (which can be formulated "Accuracy= Precision + Calibration") does not work for small features; i.e., the measurement bias is not constant for small features. Limitations of the standard measurement algorithm, based on the treatment of the singular point of the waveform for CDs smaller than 30 nm and the new model library-based approach, were considered. The implementation of reliable measurement algorithms for features at the 45 nm node and beyond requires development of more sophisticated approaches to SEM signal treatment. A three-dimensional (3-D) physical model that takes into account physical processes related to the beam interaction with material is considered. Reliability of the new approach is verified using Monte-Carlo SEM simulation and real SEM images as compared to reference measurements; total measurement uncertainty (TMU) is improved with the better models. The relation of the developed method to the standard SEM measurement algorithm and model-based approach is also considered.
As the need to create smaller features increases, the industry is moving on to 193nm photoresist systems. It is well known that one problem with this resist is its shrinkage under secondary electron microscope (SEM) measurements. While different phenomena arising from electron-material interaction are assumed to take place (such as crosslinking and scission), the primary mechanism which brings about this shrinkage is still unclear.
This paper comprises four main experiments, relating to four theories for the primary mechanism which brings about the shrinkage. In the first experiment we wanted to examine how the shrinkage is affected by subjecting the resist to 193nm exposure (after patterning). This experiment examines the theory that the electron exposure induces an effect similar to that of 193nm exposure. The second experiment deals with e-beam curing in different doses, using working parameters similar to those used in SEM measurements (e-beam curing entails a much smaller power density than SEM measurements). The third experiment addresses the theory of disassociation of carbonyl bonds in the resist, leading to crosslinking and eventual evaporation of CO2 molecules. The last experiment tests the theory that the shrinkage is caused by the collapse of voids within the photoresist, generated during the resist coating or subsequent bakes.
From the results we conclude that an effect similar to radiation exposure, local heating and the collapse of voids are not likely candidates for the primary mechanism. We did, however, find a correlation between the carbonyl levels in the resist and the shrinkage.
Monitoring the critical dimension (CD) of integrated circuit features is important for the process control of wafer fabrication. To serve this purpose, a CD tool has to measure the CD precisely and accurately. Moreover, since in many cases there are a number of CD tools that perform the measurements, the CD result should be tool independent: the control limits are learned on one tool, and should be applicable to all. The shrink of the technology puts very tight limits on the total precision, which includes both the single tool precision and the tool-matching tolerance. In order to get the required performance, the image quality of the tool should be the best possible, yet the same on all tools. To maintain good image quality, it should be routinely tested. In this paper we present an automatic image quality utility (IQU) that allows the user to perform such tests and take corrective action without having prior image processing knowledge. Our IQU integrates three basic measures: Signal to Noise Ratio (SNR), Contrast to Noise Ratio (CNR) and Resolution. The SNR and CNR are calculated on images, grabbed from a calibration wafer in pre-defined areas. The resolution is calculated from an image of a specific resolution target, at high magnification.
To minimize the effect of noise, our resolution measurement is calculated in the spatial domain, using information from the edge areas only. The utility calculates the edge location and direction, extracts the waveforms in the x and y directions, and computes the spatial resolution. We discuss the capabilities of this utility, and its use in improving tool performance. We demonstrate that the IQU detects even very small image quality degradations. Using the IQU results, a corrective activity of resolution, astigmatism, probe-current change, loss of detection efficiency or other can be made.
The IQU was developed and tested and is currently embedded on the VeritySEM SW.
In this work we describe a simple and easy way to implement line edge roughness measurement using the Applied Materials VeraSEM 3D CD SEM. The outcomes of the measurement are the roughness amplitude and its main spatial wavelength components. In the first step we test this method on an e-beam resist wafer with known roughness and correlate the results with AFM measurements. In the second step, we measured the line roughness of a 193 nm resist FEM wafer. It was found that negative defocus is characterized by long wavelength components while positive defocus shows line roughness at all wavelengths.
We use CD-SEM side-wall imaging using the Applied Materials VeraSEM 3d system as a destruction free and quick method to determine side-wall profiles. The system allows the reconstruction of profiles by tilting the SEM beam up to 6 degrees. Using two different tilt angles the reconstruction of side-wall profiles is possible in a quick and destruction free way even for negatively sloped profiles. The use of the profile analysis utility is believed to reduce cycle time significantly especially for process development and troubleshooting in production. We compare profiles obtained from the profile analysis utility of the VeraSEM 3D to X-SEM measurements to qualify this method for use in development and high volume production. For selected examples containing resist lines we investigate process windows determined from topdown CD measurements, X-SEM measurements and the profile analysis utility and compare the best stepper focus and exposure dose values obtained from these methods. It is shown how the results from the profile analysis utility can be used for process monitoring by comparing the obtained data to reference data from FEM wafers.
Proc. SPIE. 4344, Metrology, Inspection, and Process Control for Microlithography XV
KEYWORDS: Scanning electron microscopy, Electron beams, Lithography, 3D metrology, Line edge roughness, Polymers, 3D image processing, Imaging technologies, Semiconducting wafers, Photoresist materials
In addition to stability and collapse issues facing 193 nm resists, a new concern is rising regarding line width decrease when exposed to an electron beam (e-beam) during CD measurements using scanning electron microscope (SEM). Such an interaction between the measurement system and sample materials poses a great challenge in process development for 193 nm lithography which is believed to be next lithography node. This paper reports the investigation results of 193 nm resist line width slimming under e-beam. We have observed vertical, as well as lateral 193 nm resist shrinkage under e-beam exposure using VeraSEM 3D's unique sidewall imaging technology. We have observed different CD changing behaviors for lines and spaces, as expected. Repeated SEM CD measurements on space magnify the CD changing effect due to 3-5 times more resist exposed to the d-beam than a line. Hence, the influence of other competing effects form line edge roughness, carbonization etc. are reduced. By measuring a space or an edge width at a tilted view, the severity of resist shrinkage of different resist types can be compared directly with a high level of confidence.