Nanometer Pattern Generation System(NPGS) for Direct Write Lithography
The Nanometer Pattern Generation System(NPGS) is the top selling SEM lithography system at research institutions in North America and its use has become widespread around the world. The objective for NPGS is to provide a powerful, versatile, and easy to use system for doing advanced electron beam lithography or ion beam lithography using a commercial SEM (Scanning Electron Microscope), STEM (Scanning Transmission Electron Microscope), FIB (Focused Ion Beam), dual beam (SEM/FIB), or Helium Ion microscope. SEM lithography can be used for the fabrication of a wide variety of devices. Research areas include: quantum structures, such as single electron transistors; optical structures, such as binary holograms and linear/circular gratings; electro mechanical structures, such as Surface Acoustic Wave (SAW) and MEMS devices; as well as the testing of novel resists and ultra small sensor fabrication. NPGS is designed to be extremely flexible, yet easy to use. There are three basic steps to the pattern generation process: pattern design, parameter run file creation, and pattern writing with alignment for multilevel lithography.
Nanometer Pattern Generation System(NPGS V9.1)
<for Direct Write Lithography using a commercial Electron Beam or Ion Beam Microscope>
The Nanometer Pattern Generation System is the top selling SEM lithography system at research institutions in North America and its use has become widespread around the world. The objective for NPGS is to provide a powerful, versatile, and easy to use system for doing advanced electron beam lithography or ion beam lithography using a commercial SEM (Scanning Electron Microscope), STEM (Scanning Transmission Electron Microscope), FIB (Focused Ion Beam), dual beam (SEM/FIB), or Helium Ion microscope.
The Nanometer Pattern Generation System (NPGS) provides a user-friendly environment for the delineation of complex structures using a commercial electron microscope. Virtually any SEM, STEM, or FIB can be used with NPGS as a powerful lithography tool for basic research and R&D applications. While no SEM lithography system can provide the speed and stitching accuracy of a dedicated beam writer, the advanced features of NPGS make it an ideal choice when e-beam lithography is needed, but the cost of a dedicated beam writer (or a modified SEM that is sold as a dedicated system) is prohibitive. Also, for many basic research applications, the capabilities of NPGS make it the preferred instrument, as shown by the fact that several customers have purchased NPGS even though they already have access to expensive beam writers.
SEM lithography can be used for the fabrication of a wide variety of devices. Research areas include: quantum structures, such as single electron transistors; optical structures, such as binary holograms and linear/circular gratings; electro-mechanical structures, such as Surface Acoustic Wave (SAW) and MEMS devices; as well as the testing of novel resists and ultra-small sensor fabrication. Pattern sizes may range from the nanometer scale up to the maximum field of view of the microscope, which can be as large as 10 mm. However, as on any SEM lithography system, the writing resolution will decrease as the field size is increased.
NPGS is designed to be extremely flexible, yet easy to use. There are three basic steps to the pattern generation process: pattern design, parameter run file creation, and pattern writing with alignment for multilevel lithography. The fundamental aspects of each step are described below.
Patterns are created using DesignCAD, which is a commercial computer-aided-design program. The many powerful construction and editing features of DesignCAD simplify all aspects of pattern design. Several enhancements have also been added to DesignCAD specifically to facilitate the design of lithography patterns. All of the following drawing elements may be used in pattern design: lines of arbitrary slope, circles, circular arcs, and arbitrary filled polygons. Text, Bezier curves, cubic spline curves, and elliptical arcs can also be easily generated and written as series of short lines. Pattern elements that are to have different exposure parameters (such as dose, exposure point spacing, microscope beam current, microscope magnification, etc.) are designed in different drawing layers and/or different colors. This gives an almost unlimited number of exposure conditions within a single pattern. Patterns can also be imported from DWG, DXF, GDSII, CIF, and IGES file formats.
Run File Creation
Once a pattern is designed, the exposure conditions for the different drawing elements in the pattern are entered into a "Run File". This approach offers the advantage that the details of the exposure are separated from the pattern design, therefore, to vary the exposure conditions only the run file needs to be changed. Parameter entry and modification are also extremely easy. For example, a dose may be entered as an area dose (uC/cm2), a line dose (nC/cm), or a point dose (fC) and the correct point exposure time will be calculated automatically. A single run file may contain parameters for a nearly unlimited number of different patterns that will be written sequentially and each pattern may be repeated an almost unlimited number of times. In addition, advanced features include Global Stage Corrections, Pattern Arrays, X-Y-Focus, External Commands, and Fracturing of large patterns. A single run file may instruct NPGS to control the SEM in an automated mode over many hours as it aligns and writes thousands of exposures on a wafer.
Pattern Writing and Alignment
Once a run file has been created, the pattern(s) it describes may be written using the program NPGS. This program reads the run file and automatically calls the writing (PG) and alignment (AL) programs described below. NPGS can be used in a turnkey fashion with minimal user interaction, or the user can have complete control over the pattern writing - even writing patterns that were not called for in the original run file. Alternately, PG and AL are completely modular and can execute run files directly.
PG writes a pattern by simultaneously controlling the x-y scan coils and beam blanking of the microscope. The scan coils are stepped with 16-bit voltage resolution within the field of view of the microscope and the beam is blanked as needed. Patterns can be written as a series of point exposures where the beam is blanked between exposure points, or the beam can be stepped without blanking. In either case, the dwell times are controlled with better than 0.25% timing resolution at all writing speeds. To maximize throughput, the software automatically calculates the exposure points as fast as possible, independent of the actual writing speed.
Patterns may be aligned to existing alignment marks without exposing the writing area by using the alignment program AL. An alignment pattern may have several sets of windows. For example, large windows for coarse alignment followed by smaller windows for fine alignment are very useful. Each set may have up to four user defined windows anywhere on the sample. The images of the sample areas within the windows are simultaneously displayed on the PC screen. User defined overlays are also displayed superimposed on the sample images. In the Auto-Alignment mode the overlays are controlled by the computer and are individually positioned to align with the marks on the sample using a very versatile and robust cross-correlation technique, while in the semi-automatic mode the overlays are positioned manually. Once they are aligned, the program calculates a general transformation matrix that corrects for x and y magnification errors of the microscope, as well as sample rotation and offset. Signal averaging is used to optimize the image of the alignment marks for maximum alignment accuracy. This transformation matrix is subsequently used by PG when writing the pattern to give accurate registration between lithography levels.
After the samples are processed, it is convenient to use the digital imaging feature of NPGS to save images of the devices. This feature has also proven to be very valuable in providing support to NPGS users around the world. For example, during the early stages when learning to do SEM lithography, developing the skill to optimization the SEM is critical when trying to obtain the finest linewidths. By using the NPGS Digital Imaging feature, users can easily e-mail JPEG images so that detailed, expert analysis of their results and suggestions on which aspects of the writing procedure they should be more careful with can be provided.All NPGS v9 software is 32 bit and runs under Win2000, WinXP, and Win7.
Required Connections (bold arrows):
- Analog XY Inputs; +/-3v to +/-10v range typical, other ranges can be supported; >5k ohms impedance typical, >2k ohm is supported.
- Picoammeter; A picoammeter that can read the beam current hitting the sample is required for lithography. Typically, an external picoammeter is connected to the specimen current output from the microscope stage, or less often, a picoammeter will be built into the microscope. In either case, the picoammeter is usually not directly connect to NPGS, although it can be through a optional interface.
Typical Connections (thin arrows):
- Image Signal; within +/-10v; used for NPGS Alignment feature.
- Blanker; within +/-5v, <200 mA; fast blankers, slow shutters, and systems with no blankers are all supported.
Optional Connections (dashed arrows):
- Automated Stage; interface programs for several common automated stages are available at no charge. Any automated stage can be supported, if the serial interface protocol is available.
- Digital Microscope Control; interface programs for several common digital microscopes are available at no charge. Any digital microscope can be supported, if the serial interface protocol is available.
- Faraday Cup & Picoammeter; NPGS can optionally be connected to control a Faraday cup and/or read from a picoammeter, but most systems will have a manually controlled Faraday cup and picoammeter.
- NPGS - Version 9.1
- DesignCAD Express v21.2 (five copies included).
- 64 bit Windows 7 Pro (when PC is included).
- High speed (5 MHz), 16 Bit, high resolution (0.25%) PCIe516 lithography board.
- Cables to connect NPGS to microscope.
- Optional relay or switch to share microscope input with another accessory, typically an EDX system.
- Workstation PC with (or better):
- 4 GB RAM, 250 Gb Hard Disk, DVDRW Drive.
- 19" LCD Monitor.
- Wheel Mouse, LAN, USB, Serial Port.
The user must supply:
- SEM, STEM, FIB, or dual* SEM/FIB with:
- XY External Scan Control Input (within +/- 3 to +/- 10 volts, >2k ohms).
- Image Signal Output (within +/- 10 volts).
- Picoammeter for Measuring Beam Current (0.1 pA resolution or better; typically Keithley 6485).
- *A dual SEM/FIB can have both beams controlled by NPGS, but only one at a time where either a software or hardware switch (depending on microscope model) will select the writing mode.
- Beam Blanker with rise/fall times <1 usec (digital input within +/- 5 v and <200 mA).
- (Slow Beam Shutter or no blanker will also work).
- Fine Z stage control to ~1um.
- Faraday Cup (apertures to make cup are included with NPGS).
- Scan Rotation Option.
- Gold on Carbon SEM standard mounted on sample holder.
- Stray AC less than 3x10-7 Tesla(p-p); Magnetic shielding for chamber or active field cancelling system can reduce interference.
- Vibration less than 2x10-6 meters(p-p) over 5 Hz.
NPGS software updates are available at no charge to current* NPGS users. (*This includes all original owners of NPGS who have purchased a full version of the NPGS software.)