Projected evolution of semiconductor nanotechnology

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1 REU Scatterometry Project REU Student: David Mckee Graduate Mentor: Ruichao Zhu Faculty Mentor: S.R.J Brueck Projected evolution of semiconductor nanotechnology Nanotechnology involves studying and working with matter under 100 nm scale. Amazing progress has been made in the last decade in the field of Nanoscience. Below the Graph shows the expected progression of the Lithography Technology alone. Year Figure 1 The current drive for smaller and smaller nanostructures is causing a need for new nondestructive metrology techniques. They must be accurate, reliable, and reproducible measurements. The semiconductor industries need to have a reliable quality control method. A nondestructive and an inline process are extremely desirable characteristics for any new methodology used to measure periodic nanostructures. Current technologies being used are capable of accurately measuring such nanostructures. There are also limitations to such technologies in use today. This paper will discuss such limitations in a condensed way to give validity to the pursuit of a technology with such characteristics as mentioned previously. Scatterometry is one such technology which is believed

2 to possess these characteristics, but as with all technologies which are in the developmental stage much research is needed to perfect the technique. There is however one other characteristic that is an advantage with the methodology of scatterometry and that is the fact it does not need vacuum to control atmospheric conditions around a sample being characterized by the technique of scatterometry. Therefore vacuum pumps not needed to remove air from a chamber which contains the sample in testing. The expense of providing vacuum equipment is very expensive addition to any nanometrology tool. The fact that scatterometry does not require such support equipment will be an advantage to the overall expense of the technology and will drive maintenance cost down. This paper will attempt to break down the technique of scatterometry to a measurement side and a data processing side. Scatterometry exhibits the characteristics of being nondestructive and an inline process. For clarification some background information is required. Some details of SEM and AFM technologies will be discussed to make the point more clearly. Scanning electron Microscope advantages and disadvantages: Figure 2 Optical scatterometry has received much attention because of its repeatability and capability to measure side wall shapes. A similar method has been developed for the Scanning Electron Microscope. The method is much like scatterometry in that it measures shape parameters such as CD (critical dimension) and wall angles. This method also uses a library of known measured values and compares it with a library of modeled shapes (MBL). The scanning electron Microscope uses electrons; whereas scatterometry uses photons. Therefore the images can be more localized then scatterometry. Because of this property of the electron beam SEM can measure isolated structures more easily but risks contaminating the image from secondary electron emissions. Another difference between SEM and scatterometry is that SEM produces an image rather than a scattering pattern. Accuracy is critical. The ITRS requires precision of 1nm and bias less that of 10%.These requirements are being tightened continuously. A model based library can be used as another

3 method of measuring which may provide the accuracy needed. This has advantages of higher resolution and disadvantages of contamination. Figure 3: Errors due to threshold edge assignment: the black lines depict an edge shape and the associated modeled secondary electron image with peaks scaled to the same value. The edge on the left is 90ᵒ and the one on the right is 84ᵒ. The 84ᵒ image on the right is exaggerated due to the expanded X scale. Edge position assignments are shown by the dashed line where the image intensity crosses a threshold set half way between an intensity peak and a baseline. In this example the error is shown by the difference in the estimated positions of the edge. In the figure above the images of two different edge geometries are shown where the peaks are representative of the line edges. Without the peaks there may be no difference between the line width and the silicon substrate. The sidewall shape introduces some miscalculation of the width at the top of the structure. The peak on the right is not centered on the edge of the side wall while the peak on the left is. This means that the SEM image is affected by more than the position of the edge of the side wall. Figure 3 (above) Demonstrates 2 aspects of the given measurement: (1) For any given edge shape such simple edge definitions are subject to bias (i. e., systematic error that does not approach zero with increased averaging). (2) The size of the bias depends upon the shape of the edge. If bias were the only issue, it could be corrected by adding an appropriate offset to the measurement results. The significance of (2) is that there is no offset that works for all edge shapes.

4 If edge shapes introduce the standard deviation that occurs during the manufacturing process, then there will be some random deviation in the error. This could affect repeatability possibly in a negative way. This will affect tool-to-tool matching if different CD s are used to create the criteria that a tool uses to calculate error. The difference will have great consequences on repeatability especially if measured CD s are used to compare with computerized modeled images stored in a MBL(Model Based Library). Before moving on to the discussion of a molded based Library one more subject AFM needs some discussion. Atomic force Microscope Advantages and disadvantages of atomic force microscopy Atomic force microscope has advantages and disadvantages. To determine whether or not to use this method, advantages and disadvantages must be considered. One advantage is that they can give three dimensional images. The tool can be used without the need for any special conditions. There is no need to create a vacuum under normal conditions. However, for the type periodic structures being measured today the technology is definitely being pushed into a new dimension. For the type of measurements that we are doing, a vacuum is certainly needed. SEM gives a two-dimensional projection of a sample, and the AFM provides a threedimensional surface profile. AFM does not require any special treatments such as metal/carbon coatings that would irreversibly change or damage the sample. SEM needs an expensive vacuum environment for proper operation. AFM modes can work perfectly well in ambient air or even a liquid environment. This makes it possible to study biological macromolecules and even living organisms. In principle, AFM can provide higher resolution than SEM. It has been shown to give true atomic resolution in ultra-high vacuum (UHV) and, more recently, in liquid environments. High resolution AFM is comparable in resolution to scanning tunneling microscopy and transmission electron microscopy. Disadvantages A disadvantage of AFM compared with the scanning electron microscope (SEM) is the single scan image size. In one pass, the SEM can image an area on the order of square millimeters with a depth of field on the order of millimeters. Whereas the AFM can only image a maximum height on the order of micrometers and a maximum scanning area of about micrometers. AFM s are limited by the probes they use during scan.

5 Figure 4: above shows the inability of AFM to measure a sidewall <90 deg. The figure below shows how wear affects the capability of the tip to scan optimally. Figure 5: Lines pattern measurements preformed with the same AFM on the same sample, with a single standard measurement. Difference are due to differently worn tips.

6 Scanning speed of an AFM is also a limitation while Scatterometry is limited by the speed of the system to match entities from the MBL. AFM cannot scan images as fast as a SEM, requiring several minutes for a typical scan, while a SEM is capable of scanning at near realtime, although at relatively low quality. The relatively slow rate of scanning during AFM imaging often leads to thermal drift in the image making the AFM microscope less suited for measuring accurate distances between topographical features on the image. Figure 6. Schematic illustration of molded based library matching approach. Blue thin lines are parameterized by wall shapes and upper corner radius. The library consist of the sampling of edge shapes and their computed images (thicker black lines. A sample is measured by it left and right edge separately to the entities in the library. Model Based Library The schematic is oversimplified only Sidewall angle and the radius of the upper corner are accounted for this simulation. It is worth mentioning the Model Based Library at this point because of the relationship to SEM method and Scatterometry method. In optical scatterometry light is scattered from a group of similar line patterns. Scatterometry is dependent on line width, wavelength, angle and shape. Having to model the resultant outcomes of all previously mentioned parameters would be a tremendous undertaking. There are methods of calculating the outcomes of reflected scattered patterns for given samples establishing an expected model. Each sample pattern will now have expected outcomes of a definite range, angle versus power, for the given sample. This only needs to be done once for a given sample type storing the calculated out comes into a library of pre-

7 calculated solutions. Therefore enabling us to compare expected results in the stored library with measured results unknown. SEM uses an electron beam which can use various methods of calculating expected electron beam image simulations. SEM is also dependent on the same parameters as Scatterometry although the electron beam requires the use of different calculations methods for expected outcome. Simulations are time consuming as well. Scatterometry Haven given some background and very limited discussion on the competing technologies disadvantages and advantages we now come to the motivation for the research being done here at University of New Mexico s Center for High Tech Materials (CHTM). Scatterometry has an advantage non-contact, non-destructive and needs no special environment such as vacuum. The Research can be broken down to a Measurement side and a Model side (RCWA). First a discussion of the measurement side and technique used. The following diagram will give some idea how the measurement side works and some of the problems that need to be overcome. Figure 7 The above diagram depicts an Incident ray striking a sample and multiple orders of light being diffracted as a result. The zero order is usually the strongest even when the grating is extremely small this order may be the only one available. Photons are used to produce this

8 phenomenon so there is no interaction with the sample such as an electron beam to contaminate the sample as SEM uses. AFM uses a tip which rides over topology and is limited to certain surface topology and orientation of the sample. Sample Detector 1 st rotational stage 2nd rotational stage Figure 8 As in research directed by Professor John McNeil at the CHTM, University of New Mexico we have attempted to construct a computerized variable angel scanner. We have created the machine in such a way as to let us measure power being detected at the sensor and correlate measured power with the angle. Allowing us to graph the scan and relate power versus angle of rotation. Using a computer to drive the rotational stages via Labview and proprietary software provided by Thor Labs. Alignment of the two rotational stages and the sample in relationship to the incident beam are critical issues. The incident ray must hit the same spot on the sample so that the non-uniformity of the sample grating will not affect the measurement.

9 Figure 9 A single rotation stage from Thor Labs. Labview controller RCWA software Sample Mode 0 Locking Amplifier chopper sensor 244nm Polarizer Figure 10

10 Power Power A simplified schematic of the existing setup here at CHTM. Using two different orientations that are referenced to line direction and polarization also a polarized 244nm laser. TM with vertical lines perfect condition 98 nm pitch 102nm pitch (TM with vertical lines) 0.7 TE with horizontal lines perfect condition 98nm pitch 102nm pitch TE with horizontal lines Figure 11 The figures above show the two most productive laser to line grating orientations. TE with vertical lines and TM with horizontal lines seem to stay constant (compared with the modeled) even though the angle was increasing. So for the moment the orientations shown above produced the best variations between model and measure. The black lines represent the modeled pitch data, blue 102nm pitch and the red lines represent the 98 nm pitch. The two graphs above are graphed using power as a function of the angle.

11 Reflection Reflection There are five parameters that will be central to the measuring technique being developed here at as well as UNM s Center for High Tech Materials Al height Sidewall angle Fused silica height Line width pitch Figure Al thickness for TE 5 0 Al thickness for TM 5 200nm 205nm 210nm 215nm 220nm 225nm nm 205nm 210nm 215nm 220nm 225nm Figure 13 Above tis the representative Graphs for several different heights.

12 Reflection Reflection Reflection Reflection Reflection Reflection Below are the representative Graphs for several side wall angles. 0.7 Sidewall angle for TE 5 0 Sidewall angle for TM 5 50nm-46nm 50nm-48nm 50nm-50nm 50nm-52nm 50nm-54nm nm-46nm 50nm-48nm 50nm-50nm 50nm-52nm 50nm-54nm 0 5 Figure 14 FigurewFFig ure 4 Below is the representative Graphs for several different line widths. 0.7 Linewidth for TM Linewidth for TE 44nm 46nm 48nm 50nm 52nm 54nm 56nm 44nm 46nm 48nm 50nm 52nm 54nm 56nm - Figure 15 FigurewFFig Below is the representative Graphs for several different Pitch. ure 4 5 Pitch for TM 0.8 Pitch for TE nm 98nm 99nm 100nm 101nm 102nm 103nm nm 98nm 99nm 100nm 101nm 102nm 103nm

13 Reflection Reflection Figure 16 FigurewFFig Below are the representative Graphs for several different fused silica height. ure 4 Fused silica undercut for TM 0.7 Fused silica undercut for TE 6nm 8nm 10nm 12nm 14nm 6nm 8nm 10nm 12nm 14nm Figure 17 FigurewFFig Previously mentioned was the concept that there are two sides to the research here at CHTM: a measured side and a modeled or calculated ure 4 side. All the above graphs belong to the Modeled side. One of the goals of this research is to create an accurate method of modeling and comparison, a process to use as a template for given structure with a given set of attributes; we are using 5 attributes for the time being. Later on, many different attributes will have to be modeled in order for scatterometry to be a viable method for measuring line grating. The modeled side will be an immense data base which will need to be fast in ordered for scatterometry to be a inline process. It should have a fast method of finding the correct modeled template to do comparisons with the measured graph being produced by the mechanical and photonics equipment. A Model based Library will be employed to make the metrology method useful. Much work has been done towards this goal but much more research is needed. The following are goals as stated on the poster for nanometrology: a. design a better code which could include the metal roughness to improve the accuracy of the simulation; b. design an auto-fitting system to improve the speed and robustness of the fitting process.

14 Reflection Summary 37.6nm 22nm 89.74º Fused Silica 12nm 39.6nm 97.2nm pitch Figure 18 Above are the best matching parameters used for the following graphs, simulation and measured for the respective polarizations, TE first with horizontal grating and then TM with vertical grating TE with horizontal grating Measurement Simulation

15 Power Figure 19 The Graph above is a representation for the values displayed in the preceding schematic. The orientation TE with horizontal grating show the minimum power between 60 and 65 degrees for both simulated and measured plots to be about the same. There is some discrepancy between the 0 degree and 55 degree angles as far as power being measured. This is accounted for in the fact that the simulation is assumed to have perfectly smooth surface on the top of the grating. In the measured (black graph) the surface is rough and causes light to reflect in many more directions then a perfectly smooth surface might. The overall trend is the same of the simulation between 55 and 0 degrees. TM with vertical lines Measurement at 244nm laser beam Simulation Figure 20 The graph above is simulation and measured plots of power versus angle for the given polarization and line grating (vertical). The overall trend is good but here again the measured value shows a difference in power because of the diffuse surface, whereas the simulation again assumes a perfectly smooth surface. As the modeled simulation approaches the measured values with more testing and adjusting of the modeling process it should become easier to generate simulations that are closer to real measured values. Of course these are not images but graphs of trends related to a given periodic. This is a very time consuming task and many more graphs will need to be simulated using different values for the five given attributes mentioned earlier in the paper. The simulations will also have to be adjusted for different processes accounting for more or less diffuse roughness of the surfaces. CONCLUSION These results show promise for what can be done in the future with more research and fine tuning the technology. Scatterometry for measuring periodic structures on a substrate may be the future of nano metrology. As mentioned before scatterometry can be an inline quality control mechanism providing a more instantaneous measure of the state of manufacturing parameters. The

16 accuracy and non-destructive nature of scatterometry gives the technology a bright future. The fact that it does not use expensive vacuum technology is another plus. In conclusion with further study and time I suspect that the following could be shown to be true. In both of preceding graphs the overall trend and minimum point are the important criteria for further studies. It will be possible to simulate many grating values and measure actual sample with the same attributes. This will create the need for a data base representing a number of different structures. The process of fitting, matching measured values to the simulations, will need to be done quickly and accurately for the concept of scatterometry to be successful. One may consider using a structured data base. It may be possible to shorten the fitting process using the LUN (logical unit number) employed in some data bases. Using structured data base will allow a given server to use LUNs for reference to point to data for faster retrieval and storing. It may also be possible to match LUNs to a set of given values for a given set of attributes facilitating the lookup process. EMC which produces a product in which the flexibility and the speed needed may be achievable though their concept of data storage. It may also lower cost. Continuing research will prove scatterometry to be a viable method of quality control. Of course there is still much research needed to be done on the measured side and the simulation side. References 1.)McNeil, J. R., Coulombe, S. A., Logofatu, P. C., Raymond, C. J., Naqvi, S. S. H., & Collins, G. J. (1998, October). Application of optical scatterometry to microelectronics and flat panel display processing. In SPIE's International Symposium on Optical Science, Engineering, and Instrumentation (pp ). International Society for Optics and Photonics. 2.) Villarrubia, John S., et al. "Scanning electron microscope analog of scatterometry." SPIE's 27th Annual International Symposium on Microlithography. International Society for Optics and Photonics, ) Allgair, John, and Benjamin Bunday. "A review of scatterometry for three-dimensional semiconductor feature analysis." Future Fab Intl 19 (2005): ) Lensing, Kevin, et al. "Lithography process control using scatterometry metrology and semi-physical modeling." Advanced Lithography. International Society for Optics and Photonics, ) Husu, H., et al. "Scatterometer for characterization of diffractive optical elements." Measurement Science and Technology 25.4 (2014): ) Bai, Chunli. Scanning tunneling microscopy and its application. Vol. 32. Springer, ) Frank Goodwin, Vibhu Jindal, Patrick Kearney, Ranganath Teki, Jenah Harris-Jones, Andy Ma, Arun John Kadaksham, Stefan Wurm: Roadmap in Mask Fab for Particles/Component Performance Sematech 2009