Metrologia Application Note

By Stephen Hsu, National Semiconductor Corp.

Subject: Phase Grating Simulation and analysis for Stepper/Scanner Alignment signal

Objective: The purpose of this application is to give user a step-by-step guide on how to use Metrologia diffraction grating analysis tool to analyze and optimize alignment signals from grating type stepper alignment targets.

Introduction:

In the past ten year optical lithography has made steady progress in improving resolution from 1 um, 0.5 um, 0.35 um, 0.25 um to 0.18 um. To keep pace with the improvement in resolution, the overlay accuracy has to be improved to meet the design rule requirement. It has been shown that when the overlay design rule is not met, the impact on yield can be very significant, up to 20 % yield loss has been reported [1]. In order to improve the depth of focus of stepper/scanner lithography tool, Chemical Mechanical Polishing (CMP) is now widely use as a global planarization technique. As a result, the topology on top of the alignment mark is drastically altered compared with the process architecture without the CMP. Meanwhile, as the process is becoming more and more complex, the film stacks over the alignment marks are also getting more complicated. The SEM cross-section below shows the complex film stacks on an alignment phase grating for an advanced 0.18um logic device.

In order to meet the tighter alignment/overlay needs, equipment vendors design more sophisticated alignment system with both on-axis and off-axis alignment subsystems with the capability to collect more diffraction orders in order to try to improve the alignment/overlay performance of the exposure tool. Experimental results have shown that once the alignment signal drops below a certain level (0.1% of the normalized intensity in some systems), the global alignment error increases significantly. The overlay data collected from these experiments showed large wafer to wafer variation in the alignment error with very high residual value. Currently, most of the fab process development engineers have to work with the film stacks which come from the integration group, without knowing whether these film stacks can provide robust alignment signal for the stepper/scanner. The overlay data will eventually discover if the alignment signal is too weak and causing an overlay problem. Then, the process engineers must go through many try and error attempts to come up with a good alignment scheme for the entire process. The phase grating module in Metrologia provides a process engineer a powerful and flexible tool to perform the phase grating analysis that enables the engineer to simulate the alignment signal with on axis and off-axis capability.

As the semiconductor industry continues pushing smaller design rules, the overlay budget is decreasing. In order to meet the overlay challenge for next generation devices, the stepper/scanner alignment scheme and alignment signal analysis is becoming more and more important. To provide a robust alignment/overlay control through out the entire device fabrication process, it is important to carefully design the film stacks on top of the phase grating to produce good alignment signal for all layers. In this application note the ASML zero mark is selected as an example. The procedure can be applied to front-end-of-the-line (FEOL) process (STI) or backend of the line process (WCMP) for alignment signal analysis.

Outline of Metrologia Simulation for Phase Grating:

There are seven steps in running the Phase Grating simulation:

1. Draw the grating including entire film stacks on top of the grating

2. Collect the refractive indices (n and k) for all the film stacks on top of the alignment marks.
The best way to obtain this information is to use the real measurement data for each film. However, if the measurement data is not available, or the alignment scheme is still in the initial development stage. The Metrologia package provides an nkReader, which allows user to easily retrieve the reflection data from the Sopra Database.

3. Input the film stacks including all the reflection index information into a script input file which can be accepted by the Diffraction Grating Analysis module in Metrologia

4. Type the alignment wavelength and load the scrip input file into the Diffraction Grating Analysis GUI interface

5. Launch the simulation

6. Verify the simulation is correct by checking the displayed reciprocity numbers. Display the result.

7. Save the output file in ASCII format. User can use MSExcel to plot the alignment result for different diffraction order

Procedure

Step 1: Draw the grating including the entire file stack on the phase grating into block.
It will be easier if the SEM cross-section is available when drawing the picture. The examples shown here are ASM zero mark (8 um) with two different grating depths. (800 Å and 1200 Å) with 4500 Å Shipley UV5 on top on the mark.

Step 2: Find out the reflection indexes
Click on the nkReader.exe icon(figure 1), the n&k reader windows will open (as shown below)

Figure 1

Input the wavelength of interest. Click on the space underneath the file name field and select the SopraDatabase folder and scroll to find the specific film for which the n and k values are needed for grating analysis. Click on the Plot (n,k) data button to view the graph. The n and k values for the specific wavelength will show up underneath the Wavelength box

Step 3: Input the film stack and substrate indexes information into a script file.

The following example shows how to create a script file to describe the grating.

Figure 2

First, in the first line of the file specify the grating size, 16 um(figure 2) for this example, and the meshsize for the grating. The smaller the meshsize, the more accurate the results, but it take longer to run the simulation. Line 10 is where the user specifies the total layers in the grating. Line 10, 11 and 12 are for complex refractive index information. From left to right, the first material is air. The second material is silicon substrate and the third material is UV5. The user can add more materials for later use. It is better start with simple film stacks and build up the film stacks. Line 13 is the refractive index of the substrate, which in this case is Silicon.. After inputting all the refractive index information, the user starts to build layer from line 15 by specifying the building blocks to form a layer or slice. In the current example, the first layer consists of three building blocks, from left to right they are: 3 2 3. The second layer also has three building block: 3 3 3. For complicated film stack, repeat the above processes until the entire film stacks are converted into script file.

Step 4: Input the simulation parameters
Click on the Diffraction Grating icon to open the diffraction grating analysis GUI. Key in the wavelength of interest and the incoming beam angle and order is applicable.

Figure 3

Step 5: Launch the simulation.

Select file run and the mode (reflection or transmission). A file selection window will open. Select the file that contains the input script. Click on open button to launch the grating simulation.

Figure 4

Figure 5

Step 6: Display the simulation result and verify the correctness of the simulation.

To display the simulation result, click on the Display button in figure 4. Two small windows will pop up figure 6 and figure 7.

Figure 6

Figure 7

Metrologia allows user to select different output format from the property box in figure 4. For stepper alignment signal analysis, the diffraction efficiency is selected. The maximum diffraction efficiency is the ratio between the output and the input beam. It is important to validate the result of the simulation by checking maximum reciprocity error. The error reciprocity should be always less than 0.001. The user can change the zoom factor (figure 4) and the incoming Bragg angle (order) to see the relationship between the input and the output beam in real time

Step 7: Save the result in ASCII format.

Metrologia has another useful feature that allows users to export/save the output file in ASCII format. User can import the ASCII file into excel and plot out the specific results that fit user’s need. To save the simulation result in ASCII format click on the File/Save As menu item, a dialog window will pop up prompting the user for the file name to save the output window’s data in.

Figure 8

After the output table is saved, the user can use Microsoft Excel or other spreadsheets ofr graphics programs to import the file. The first row in the file shows the user the path of the file. In the case of ASML alignment phase grating simulation results, the user can delete all the unwanted columns (all the orders incoming) leaving just the zero order incoming angle. For the output row, the user can leave the 0 and +/- 1 orders for the standard model /100, /200, /300 steppers and /500 scanner that has the standard phase grating system. For newer tools like the model /700 that has the ATHENA (Advanced Technology using High order Enhancement Alignment), the user can change the wavelength (532 nm) and the incident angle in Metrologia phase grating parameters windows (figure 4) and run the simulation.

Figure 9: Simulation results after imports into MS excel.

Figure 10 Metrologia simulation result for ASM Phase Grating Alignment (PGA)

Reload the simulation result

Click on file and select the Load Smatrix, a window will open. Select the simulation result that needs to be displayed. The default simulation output file is sm.dat. Click on the display button, the simulation result will show up.

Figure 11

Metrologia File Management

After every simulation run, the scattering Smatrix result automatically gets stored in the sm.dat file in the same directory as your input file. The easiest way to keep track and manage the simulation results is to rename the sm.dat to a specific name so that the user can identify the specific simulation result without rerunning the entire simulation. Metrologia allows the user to reload and display the simulation result without going through the entire simulation.

Summary

Metrologia Diffraction Grating analysis is a powerful tool for the process engineer to analyze stepper/scanner alignment signals to ensure that the stepper/scanner has sufficient signal to give good alignment/overlay result.

1. William Arnold, James Greeneich “Impact of Stepper Overlay on Advanced Design Rule” OCG interface 98.