Chapter 8.41
JEOL 6400
SEM and
Nanometer
Pattern Generation System
(jeol107)
1.0
Title
JEOL107 – JEOL series 6400 Scanning Electron Microscope
(SEM), and Nanometer Pattern Generation System (NPGS)
2.0
Purpose
JEOL107 is a scanning electron microscope that can be used for imaging of samples up to 4” in size, with up to 300,000X magnification. It can also be used for electron beam lithography, using the Nanometer Pattern Generation System (NPGS).
3.0
Scope
This manual will describe the basic operation of the JEOL 6400 microscope, both for imaging and for its use with the NPGS E-beam lithography system.
4.0
Applicable Documents
4.1 JEOL 6400 manual (located in Cory 107, on the shelf above the microscope).
4.2 Nabity NPGS manual (located in Cory 107, on the shelf above the microscope).
4.3 Further information on NPGS is available on the JC Nabity website at: http://www.jcnabity.com.
5.0
Definitions & Process
Terminology
5.1
SEM: Scanning
Electron Microscopes are used for imaging with very fine (nm) resolution. This
equipment is also configured for Electron Beam Lithography (EBL) using the NPGS
software and hardware.
5.2
NPGS: The Nanometer
Pattern Generating System allows the user to pattern a sample from a computer
layout design (Design Cad). The NPGS controls the beam position and blanking
during the writing phase.
6.0
Safety
The SEM operates
under high vacuum. Care must be taken during loading/removing of the sample,
and during adjustments being made on the column, to avoid loss of vacuum to the
column and damage to the filament. Samples must be properly prepared for
minimal outgassing and chamber contamination, and handled with gloved hands
only.
7.0
Statistical/Process
Data
Enable messages for
JEOL107.
8.0
Available
Process, Gasses, Process Notes
8.1
Standard Imaging
The JEOL 6400 scanning electron microscope (SEM) is a
high-resolution SEM. It can provide beam voltages ranging from 0.2kV to 40 kV
and beam currents from 10 picoamps to 10 microamps. It offers high performance
and low noise at low accelerating voltages, making it useful for IC and
photoresist evaluation. Resolution of 3.5 nm is attainable, and available
magnifications range from 10 X to 300,000 X. The cathode is a high-brightness
lanthanum hexaboride (LaB6) source. The SEM is equipped with two-inch and four-inch
airlocks and a Faraday cup for beam current measurements. The sample stage is
computer-driven.
The JEOL 6400 is equipped for electron-beam lithography
(EBL). A beam blanker with a rise time of 3 microseconds is installed. The beam
blanker and scan coils are linked to a computer with a commercial pattern
generation system, the Nanometer Pattern Generation System (NPGS) from J. C.
Nabity Lithography Systems. This system is designed to generate patterns
ranging in scale from nanometers to millimeters. Patterns are designed using
commercial CAD software (DesignCAD) and then assigned exposure parameters using
the Make Run File (mrf) command. The resulting run files are then written by
calling the NPGS program. NPGS makes calls to Pattern Generation (PG) and
Alignment (AL) programs as necessary. Both PG and AL can be run separately if
desired. PG writes the pattern by controlling the X-Y scan coils and beam-
blanking of the SEM. Resolution is 16-bit for the beam position and 12-bit for
the beam blanking. By using AL, patterns may be aligned to existing alignment
marks without exposing the pattern area. The user confines the beam raster to
designated windows. The image from the windows is displayed both on the CRT of
the SEM and at the monitor of the pattern generation computer. Overlay marks
are simultaneously displayed on the computer screen, and can be aligned with
marks on the sample. Rotational and scaling information from the alignment
process is used to generate a transformation matrix. PG can use this matrix
subsequently when generating patterns.
9.0
Operating
Procedure
9.1
Daily Maintenance/Pre-Use Checks
9.1.1
When not in use, the JEOL 6400 is normally left with the
high voltage turned on but set to 0 kV. This leaves the LaB6 filament
pre-heated with ~ 1.0 A running through it. Keeping the filament warm maintains
its cleanliness, and allows for faster start-up when using the SEM. Note that
all the controls required for standard sample observation and EBL are described
below. Under no circumstances should users operate any other controls without
first consulting staff. This includes the entirety of the vacuum system, with
the exception of the sample airlocks.
9.1.2
Prior to using the SEM you should check that all is in
order. First, check that the gun pressure is acceptable. The gun is pumped by
an ion pump whose power supply is located to the right of the SEM control
cabinet. The power lamp should be illuminated. The meter range switch should be
on the 500-microamp scale (change scales if necessary) and the meter should
read toward the low end of the scale, typically about 70 microamps. This meter
reads the ion pump current, which is proportional to the gun pressure. If the
meter reads above 250 microamps, the pressure is too high for the SEM to be
used. Report the problem on FAULTS.
9.1.3
Verify that the filament knob is turned fully
counterclockwise and that the filament meter light is on. The filament button
FIL toggles the meter between filament and emission current measurement. If FIL
is not lit, you are looking at the
emission current, and should toggle to FIL; the meter should then read ~1.0 A.
9.1.4
Verify that Vib is closed: Vib LED is off, Vib button is in.
9.1.5
If the meter light is not on, the gun pressure is not
acceptable. If both FIL and the meter light are lit but there is no filament
current, the SEM is not usable. Report the problem on FAULTS.
9.1.6 Check that the accelerating voltage is set to 0 kV and the secondary electron detector is off using the EOS menu.
9.2
Keyboard and On-screen Menus
Most of the functions of the JEOL 6400 can be accessed through keyboard and menu operations. This includes most of the knob and button controls, with a few exceptions (e.g., the filament current knob). In addition, much of the fine-tuning required for successful lithography can only be accomplished using keyboard control. The various menus through which control takes place are displayed only on the right-hand (RH) CRT. The left-hand (LH) CRT is used primarily for viewing. A cursor is displayed on one of the two CRTs at a time; the CRT in which the cursor appears is active. The cursor can be toggled back and forth between the two CRTs by pressing the escape (ESC) key.
9.2.1
Computer Keyboard
The most useful keys are escape (ESC) mentioned above,
BREAK, the return key, and some of the pre-defined function (PF) keys. Also
useful are insert (INS), and the plus (+) and minus (-) keys. Uses of these
keys will be described below.
9.2.2
Basic Screen
The basic screen is always displayed on the LH CRT; it
is also displayed on the RH CRT when
the menus are hidden. This screen consists of two information lines customarily
located at the bottom of the screen. When a given CRT is active (i.e. the
cursor is in it) and is displaying the basic screen, its information lines can
be alternately shown or hidden by
pressing the BREAK key. The information displayed in the lines includes a
micron bar and size, a title, the accelerating voltage, the magnification and
the working distance. The pre-defined function key PF1 brings up the basic
screen on the RH CRT regardless of which screen is active.
9.2.3
Electron Operating System (EOS) Menu
PF2 brings up the EOS menu on the RH CRT regardless of which
screen is active. The EOS menu consists of three screens: EOS-1 (optics), EOS-2
(mode) and EOS-3 (image). When the RH CRT is active and EOS is displayed,
pressing the 1, 2, or 3 keys will call up the corresponding screen. Most of the
SEM functions required for viewing or lithography can be controlled though the
EOS menu. To change a parameter, make the RH CRT active using the ESC key. If
the item you want to change displays a set of possible choices (e.g. the raster
scan speed), position the cursor over your choice using the arrow keys. Then
press the return key to select the item. If instead the item is numeric (e.g.
the accelerating voltage), position the cursor over the number and press the
INS key. A white command window will appear on the RH CRT. Enter the new
setting and press return to make a change. You can also increment or decrement
a numeric setting by positioning the cursor over it and pressing the + or -
keys.
9.2.4
EOS Screens
Settings available in the EOS screens are indicated in the
list found in appendix 1 (Section 12.1). Ranges are shown for numeric items
when appropriate. When control over an item is not needed for basic use, only
the standard setting of that function is listed. Settings not needed for basic
use are not listed, and should not be used without consulting staff.
Standard settings of the other items are indicated by asterisks.
9.2.5
MEMO Menu
The MEMO menu is called up by pressing the PF5 key. This
menu lists ten sets of pre-defined operating conditions; the chosen settings
for various SEM parameters are listed in the lower half of the screen. To load
the selected set of operating conditions, select the title of the set in the
upper half of the screen using the arrow keys. The press "l" for load. The listed set of
conditions will immediately be loaded into the SEM.
Note: Loading a set of operating conditions will result
in immediate application of an accelerating voltage! DO NOT load a set unless
you are ready for the listed conditions to come into effect immediately. See
Section 9.3: System Operation below.
Do not change the contents of any of the pre-defined states;
they are provided for the convenience of all users.
9.3
System Operation
9.3.1
Sample Preparation
Samples can be most sizes or shapes up to a whole 4"
wafer. Samples must be clean and dry, i.e., free from moisture, oils, grease,
and particles that could contaminate the vacuum system. If the chamber becomes
contaminated, the contaminants can be "ironed on" to your sample by
the beam. Once this happens, it is usually impossible to remove the
contaminants from your sample.
9.3.1.1 Gloves
must be worn when handling samples and sample holders. Baked photoresist can be
inspected in the jeol107. Note that nonconductive materials are subject to
"charging" and for best results should be coated with a thin layer of
gold. For cross-sections, cleave the sample through the area of interest.
9.3.1.2 The sample
should be as small as possible, but large enough that it can be securely held
by the sample holder. If the sample is to be coated with gold, it should be
coated after cleaving. For cross sections, use the slotted sample holder; these
holders have a set screw for securing the sample. Be careful not to over
tighten the screw, or you may break your sample. The "Z" axis knob on
the stage is calculated from the sample holder top. If the sample must be
secured to the holder, use SEM-compatible (vacuum compatible and conductive)
carbon paint.
9.3.1.3 After
attaching or securing your sample to the sample holder, the sample holder is
mounted into the larger dovetail holder. This dovetail holder will slide or
"dovetail" over the stage in the SEM chamber. For cleaved
cross-sections, it is recommended that you mount the sample so that the cleaved
edge is parallel to the longer side of the dovetail holder.
9.3.2
Loading the Sample and Pumpdown (2 Inch Airlock)
The sample is loaded using a threaded rod; this rod is
mounted through the center of a thick, circular viewport. The viewport allows
you to visually guide the dovetail holder onto the stage and visually assess
the orientation of the sample for tilt and rotation before viewing on-screen,
where orientation is not as obvious.
9.3.2.1
Thread the screw at the end of the sample-loading rod into
the dovetail holder, and hook the clip attached to the viewport over the screw.
The clip secures the sample holder to the viewport until you are ready to move
it onto the stage; if you don't use the clip, the sample and rod will move
forward into the airlock door, possibly damaging your sample or the door! If
the rod does not easily slide through the viewport vacuum seal, apply grease to
the rod sparingly.
9.3.2.2
Verify that the working distance (Z height) is set to the
specimen exchange distance of 39mm; the red light on the front of the airlock
exchange panel will come on to indicate that the working distance is correctly
set to 39mm. Also check that the fine Z micrometer is set to 2.0 mm; it is
located between the X and Y micrometers.
9.3.2.3
Set the stage to its home position by typing "h"
at the keyboard of the stage control computer; this should automatically set
the X and Y at the home or center position (25 mm on the X and 35 mm on the Y
travel counter). Manually adjust the rotation (angular and circular) counters
to zero. Warning: Any operations on the optics table must be done gently,
without disturbing the table. Bouncing the table even a little will crash the
turbo-pump - it is magnetically levitated.
9.3.2.4
Check that the white toggle switch on the four-inch airlock
is set to OTHER. The four-inch airlock is located to the left of the SEM and the
two-inch airlock is in front. When the toggle is set to OTHER, the white light
on the two-inch airlock should be illuminated, indicating it is the active
airlock.
9.3.2.5
Check the viewport and airlock o-ring for dust, remove dust
to make a good seal. Hold the viewport with the sample facing towards the SEM
chamber against the o-ring seal at the two-inch airlock entrance. Press the
illuminated white button on the airlock chamber and wait for the light to go
out. The light is on a timer and should allow enough time for the chamber to
evacuate. Improper evacuation of the airlock can cause the turbo to
"dump" when you open the airlock door resulting in down time!
9.3.2.6
When the light goes out, open the airlock door by rotating the airlock knob towards you 1/4
of a turn. Slide the door open by pulling the knob to the right, about 3".
Be gentle. There is a clip in the airlock to hold the door open. Do not lock it
with the knob as it will get debris on the door o-ring. The light in the SEM
chamber should come on when you open this door. If the light is off, then the
secondary electron detector is on. To turn off the detector, select OFF under
DETECTOR in the EOS-3 menu. Improper evacuation of the airlock can cause the
turbo to "dump" when you open the airlock door resulting in down
time!
9.3.2.7
Carefully push the sample rod into the chamber and slide the
dovetail holder onto the stage. You may have to slightly raise the sample
holder in order to get it onto the stage. The sample holder should NEVER have
to be forced onto the stage.
9.3.2.8
Unscrew the threaded rod from the dovetail sample holder.
Make sure that the rod is completely unscrewed before retracting or you may
pull your sample off the stage.
9.3.2.9
Gently retract the rod until the screw is secured in the
clip attached to the viewport (you will hear a click when this happens).
9.3.2.10 Slide the
door closed and turn the knob 1/4 turn away from you until the flat area on the
knob is parallel to the floor.
9.3.2.11 Holding
the rod near the viewport, press the white airlock button and wait for the
light to go on. Make sure that you hold the rod during the venting of the
airlock or the rod could fall to the floor!
9.3.2.12 Remove
loading rod and place it in its holder.
9.3.2.13 Verify
that the vacuum system is functioning properly. The filament meter light should
still be on; this indicates the pressure is low enough to apply high voltage.
Also, ensure that the gun pressure is still acceptable by checking the ion pump
current. If the filament light is off or there has been a significant rise in
column pressure, there may be a problem in the vacuum system. Report the
problem on FAULTS. If the filament light has gone out or the gun pressure has
risen above the acceptable limit, the SEM SHOULD NOT BE USED, AND YOU SHOULD
LEAVE YOUR SAMPLE INSIDE. Contact staff to have your sample removed.
9.3.2.14 Never open
the airlock door to the specimen chamber unless you have just pumped out the
airlock. It will not preserve the vacuum and unless pumped immediately before
opening, will cause the turbopump to crash, shutting down the SEM!
9.3.3
Turning on the Beam
Note: The area swept by the beam either on the screens or on the sample is called the raster.
9.3.3.1
Check that control of the raster is given to the SEM. The
small metal switch in the Raster Control auxiliary panel to the right of the
CRTs should be set to SEM (down). If it is set to NPGS (up) control of the
raster is given to the lithography computer, and a single spot in the center of the LH CRT will be
lit. IT IS EASY TO BURN THE PHOSPHORS AT THAT SPOT IF THE CONTRAST AND
BRIGHTNESS ARE TURNED UP WHILE IN NPGS MODE. Switching the raster control to
SEM protects the CRT from burn-out.
9.3.3.2
Bring up the information bar in the basic screen (using ESC
and BREAK), or display EOS-1 in the RH CRT. Increase contrast on screens so
that display information is visible. Verify that the machine is in SEI mode
(SEI button will be lit). If not, press the SEI button or select SEI under IMS
CH1 in EOS-3.
9.3.3.3
Check that the Beam Blanking toggle switch is set to OFF.
The Beam Blanker control is located in one of the auxiliary panels to the right
of the CRTs. OFF leaves the beam unblanked, ON blanks the beam, and EXT turns
control of the blanking over to the lithography computer. Normally EXT leaves
the beam blanked except during an exposure.
9.3.3.4
Check that the probe current detector, which collects the
beam current for measurement, is not in the beam path. The blue PCD button in
the auxiliary panels should be out (unlit).
9.3.3.5
On the JEOL keyboard, set SCAN options to FAST and single
line scan (LSP) by pressing the FAST and MODE buttons (buttons will
illuminate). This may also be accomplished using EOS-2. The MODE button toggles
between line, cursor and point options. Continuing to press MODE will
eventually bring up the line scan.
9.3.3.6
Use the BRIGHTNESS and CONTRAST knobs (SEI IMAGE) to adjust
the signal so that the line is positioned just above the bottom of the screen.
9.3.3.7
Apply an accelerating voltage. You can set the accelerating
voltage by turning the ACCEL VOLTAGE knob, by changing ACC VOLTAGE in EOS-1, or
by or using the MEMO menu to load a predefined state. 20kV is a good setting to
begin with. The voltage is displayed in the information lines of the basic
screen, and also in EOS-1. Note that higher voltages will give increased
resolution, but also may damage your sample.
9.3.3.8
Check that the objective aperture you desire is in place.
One of four apertures (1, 2, 3, or 4) can be chosen. Aperture 1 is the largest
and 4 the smallest. Generally 1 will be used for low voltages or high probe
currents whereas 4 will be used for high voltages or low probe currents. When
you have applied an accelerator voltage and chosen a condenser lens setting,
the optimum aperture will be displayed under OPT APERT in EOS-1. If you know in
advance what aperture you wish to use, you can select it before applying an
accelerator voltage.
9.3.3.9
To change from aperture 4 to 3 for instance, gently pull the
aperture selection knob on the SEM column straight out. When the number 3 is
visible, rotate the control toward you; it will lock in place. To change from
aperture 3 to 4, simply rotate the knob away from you and it will automatically
move inward. Remember that the turbo pump is very sensitive to jarring of the
SEM, so be careful when changing the aperture!
9.3.3.10 Rotate FILA (filament current) knob clockwise slowly while watching both the screen and the filament current meter. BE CERTAIN YOU ARE LOOKING AT THE FILAMENT CURRENT (FIL BUTTON LIT); IF YOU ARE ACCIDENTALLY LOOKING AT THE EMISSION CURRENT, YOU RISK BURNING THE FILAMENT OUT! You should see the signal on the screen move upward, peak at a maximum near 1.9 A, and then decrease sharply. Center the filament knob on this peak - this is the highest brightness and will give you the best resolution or current-vs.-spot size for lithography. Running above this setting will damage the tip and increase the spot size for the given current into the sample. NEVER INCREASE THE FILAMENT CURRENT ABOVE 2.0A; DOING SO WILL DRASTICALLY SHORTEN THE LIFE OF THE FILAMENT. If the line scan is flat and shows no response to filament current, there are four likely culprits. The secondary electron detector may be off; the Probe Current Detector may be intercepting the beam (PCD in); the beam blanking may be set to ON or EXT; or the gun tilt and shift settings may be far off (see Section 4.4 below). You should also check the SEI brightness and contrast. The Emission Current Meter should read about 40 microamps at 20kV with standard bias selected. If not, report this in Faults.
9.3.4 Aligning
the Beam
9.3.4.1
To align the beam straight down the column, press the SHIFT
button so that it is dimmed. The tilt/shift knobs will now change the gun tilt.
Adjust the PROBE CURRENT knob clockwise; this changes the coarse condenser lens
(CL COARSE) setting so as to minimize the probe size. Choose a high setting
(e.g. 16). Equivalently, you can also use EOS-1 to set CL COARSE. Adjust the X
and Y tilt/shift knobs to maximize the signal on the LH CRT. You may find it
easier to find the optimal tilt and shift settings by maximizing the current
detected by the PCD. This current is measured by a Keithley picoammeter located
on top of the SEM control cabinet.
9.3.4.2
Press the SHIFT button again to enable the shift feature
(SHIFT button light will go out). The tilt/shift knobs will now change the gun
shift. Adjust the PROBE CURRENT knob counterclockwise until the signal drops
near to zero. This increases the probe size and is equivalent to decreasing the
CL COARSE setting in EOS-2. Use the X and Y tilt/shift knobs to maximize the
current. Continue to reduce the CL COARSE setting and maximize the signal with
the tilt/shift knobs until you have done so at CL COARSE of 1.
9.3.4.3 Repeat the adjustment of gun tilt (TILT button lit) at high CL COARSE settings and gun shift (TILT button dimmed) at low CL COARSE settings until the settings remain unchanged as you go from high CL to low CL and back. Check to see that your filament current is still "sitting on top of" the high brightness peak by moving the knob up or down slightly while observing LSP.
9.3.5
Filament Temperature Stabilization
9.3.5.1
It will take about 1 hr. for the filament temperature to
stabilize. During this interval, the metal and ceramic parts in the gun heat to
equilibrium. The optimal gun tilt and shift settings also may require about one
hour to equilibrate.
9.3.5.2 If your process is sensitive to changes in the probe current, the best approach is to adjust the filament current and gun tilt/shift as described above.
9.3.6
Viewing the Sample
9.3.6.1
You may now adjust the working distance according to your
requirements. Smaller working distances give better resolution, but if you have
a large sample that needs to be tilted, consult the chart on the front of the
SEM chamber for the safe working distance for your particular sample. This
chart details the X, Y, and Z distances allowable for a given sample size and
tilt; the black zone indicates the safe operating regions. For small samples, a
working distance of 15 mm is a good operating distance. If the sample is not
flush with the top of the holder, focus on the high point, read the working
distance on the PNU printout or EOS.
9.3.6.2
On the FUNCTION keyboard, set the SCAN option to picture by
pressing the PIC button. There are three PIC settings: a full screen view (PIC
in EOS-2), and two smaller screen views (RD2 and RD4). Each time you press the
PIC button, you will toggle to one of these three views.
9.3.6.3
Confirm the scan rotation is on and that the tilt correct is
set to zero (fully counterclockwise). These controls are located in one of the
auxiliary panels in the SEM console. The scan rotation changes the orientation
of the image on the screen and is useful for aligning sample axes with the
screen axes. The tilt correct compensates for the foreshortening of the image
of a tilted sample by intentionally stretching the image along the X-axis. If
your sample is not in fact tilted, a non-zero tilt correction setting will lead
to incorrect magnification along the tilt axis.
9.3.6.4
Using the FUNCTION keyboard, verify that the FINE focus
button is off (not illuminated); this disables the fine focus, and allows for
coarse focusing of the sample. (The fine and coarse focus are used to adjust
the objective focus.) Adjust focus knob until you can see the sample holder.
You can toggle between coarse and fine focus by pressing the FINE focus button.
You may have to use the joystick to move the stage and sample holder directly
under the beam.
9.3.6.5
Move your sample using the joystick to the area of interest.
Optimize focus by watching the LH CRT while adjusting the fine focus.
Alternatively, you can use the single line scan mode (press the MODE button) to
optimize focus. At low magnifications and coarse focus, adjust the focus knob
until the signal exhibits the sharpest peaks possible. At low beam current
(high condenser lens settings) it may be easier to use the RH CRT to adjust the
focus. This screen shows an averaged image and is quite useful when dealing
with a noisy image.
9.3.6.6
You must make final alignment of the beam with the objective
apertures. Make certain the SEM is in SEM1 mode; do so by selecting SEM1 under
EOS MODE in EOS-2. SEM1 disables the image shift capability of the JEOL 6400.
If you remain in SEM mode, any image shift present will interfere with the
aperture alignment. In SEM mode, you may set shifts to zero, read on EOS page.
9.3.6.7
Find a small speck of bright material on your sample and
focus on it. Make this adjustment at a magnification of 5000X or higher. Press
the WOBBLE button and look at the LH CRT (not the RH CRT). The WOBBLE button
automatically over- and under-focuses the SEM. Misalignment of the beam with
the objective aperture will cause the image to shift from side to side (X) or
up and down (Y). Adjust the X and Y aperture centering knobs to minimize motion
of the image. The X and Y knobs are on the aperture selection control on the
SEM column. The Y adjust is on the end of the control, while the X adjust is
comes out of its side. When you have finished, return the JEOL 6400 to full SEM
mode (select SEM under SEM MODE).
9.3.6.8
Adjust the X and Y stigmator knobs until you get the most
"real" looking image (equivalent to changing STIGMA settings in
EOS-1). These knobs control the beam shape, changing it from an oval to a round
shape (best image is with a round beam). You will get better results if you
make this adjustment at a higher magnification (an increase of 10,000 if
possible) than you plan to use for viewing or picture taking. You should make
these adjustments while looking at a very small feature (about 100 nm in
lateral dimensions). To adjust the position of a feature in the screen while at
high magnification, it is useful to use the electrostatic image shift described
above. The image position can changed with the X and Y IMAGE SHIFT knobs. To
activate the image shift feature, press the POSITION button so that it is
dimmed.
9.3.6.9
At high magnifications, adjusting the focus and stigmators
may not be sufficient to bring the sample into good focus. In this case, you
will need to adjust the PROBE CURRENT; this changes the beam's diameter. You
will get greater resolution with a narrower beam. To narrow the beam, rotate
the PROBE CURRENT knob clockwise. You should now see more detail, but you will
also have more noise visible on the screen (if you take a photo, this noise
should not appear on the photo). You will have to compromise between the amount
of noise and the detail required. Higher accelerating voltages also give higher
resolution but less surface contrast due to the greater penetration of the
electron beam. Another compromise is required here.
Note: If the image becomes "fuzzy" after
viewing it for long periods of time, the area may be charging with electrons
that are not drained away to ground. When this happens, you should move to
another location for a few minutes. When you return to the area of interest if
it is still not in focus, then the area has been irreversibly contaminated.
9.3.7
Taking a Picture
9.3.7.1
There are 2 screens from which you can view the image and
take a picture. The left screen shows a "real-time" image while the
right is an FIS (frame integrated storage) image. You can use the FREZ button
on the JEOL keyboard (DISPLAY field) to freeze the image on the right screen
for taking more than one photo.
9.3.7.2
Press the SLOW button on the FUNCTION keyboard (SCAN field).
9.3.7.3
Optimize the signal contrast and brightness by pressing the
WFM button on the JEOL keyboard (DISPLAY field). 5 straight, horizontal lines will
appear on the left screen. Use the BRIGHTNESS and CONTRAST control knobs to
adjust the scan signal so that it falls within this field during the entire
scan. The height of the signal is controlled by the CONTRAST knob, while the
overall position of the signal is controlled by the BRIGHTNESS knob. The
highest peak of the scan signal should just hit the top line of the grid, while
the lowest point should be positioned at the bottom line. Alternatively, you
can try using the ACB (automatic contrast and brightness) feature to optimize
the signal for picture taking, but the success of this method depends on the
sample.
9.3.7.4
Press the WFM button again (light will go out) and the image
will return to the left screen.
9.3.7.5
Load the Polapan 400 film into the camera to the right of
the viewing screens. To do so, switch the lever to "L" (load), and
push in the film until it clicks in place. Then pull on the film casing, which
will slide partway out, exposing the film inside the camera.
9.3.7.6
Press either the LEFT or RIGHT button on the JEOL keyboard
(PHOTO field) to initiate taking the photo. The LEFT button takes the signal
from the real-time image on the left screen, while the RIGHT button will use
the signal from the FIS screen. Wait until the scan is complete. To process the
film, push the film casing back in, switch the lever to "P" (process)
and pull the film out firmly and evenly.
9.3.8
Turning Off the Beam
9.3.8.1
Load the 0kVWD39 state from the MEMO menu. This performs
several functions: it returns the SEM to 0 kV without turning the accelerating
voltage off; it changes the magnification to 300,000X; it turns the SEI CONT
and BRIGHT down; and it changes the working distance to 39mm. There are good
reasons for these settings. Leaving ACC VOLTAGE on but at 0 kV leaves some
current running through the filament, keeping it clean and preheated. Turning
up the magnification reduces heating of the scan coils because at high
magnifications, the beam scan area is small. Turning down the SEI CONT and
BRIGHT reduces the chance of accidentally burning a spot in the CRTs. Finally,
setting the working distance to 39 mm leaves the SEM ready to view a
just-loaded sample.
9.3.8.2
Rotate FILA current knob fully counterclockwise.
9.3.8.3
Press SLOW button on JEOL keyboard (SCAN field).
9.3.8.4
Turn off the secondary electron detector by selecting OFF
under DETECTOR in EOS-3.
9.3.8.5
Return to the basic screen (press PF1), hide the information
bar on both CRTs (use ESC and BREAK), and put the cursor on the left screen.
Turn down contrast and brightness of both CRTs. These steps are designed to
prevent burn-in of images in either CRT.
9.3.9
Removing the Sample and Pumpdown
9.3.9.1
Set the stage to its home position by typing “h” at the
keyboard of the stage control computer located to the right of the SEM; this
should automatically set the X and Y at the home or center position (25 mm on
the X and 35 mm on the Y travel counter). Manually adjust the tilt and rotation
counters to zero.
9.3.9.2
Return the working distance (Z height) to the specimen
exchange distance of 39mm; the red light on the front of the airlock exchange
panel will come on to indicate that the working distance is correctly set to
39mm. If you have adjusted the fine Z position, return it to 2.0 mm.
9.3.9.3
Hold the glass viewport with the clip facing towards the SEM
chamber against the o-ring seal at the airlock entrance. Press the illuminated
white button on the airlock chamber and wait for the light to go out. The light
is on a timer and should allow enough time for the airlock to evacuate. You
should also visually check the o-ring for wear or contamination. If this is not
the case, report the problem on FAULTS. Improper evacuation of the airlock can
cause the turbo to "dump" when you open the airlock door resulting in
down time!
9.3.9.4
When the light goes out, open the airlock door by rotating
the airlock knob towards you 1/4 of a turn. The vacuum gauge on top of the SEM
will briefly rise to the 10-2 torr range when the door is opened. Slide the
door open by pulling the knob to the right. The light in the SEM chamber should
come on when you open this door. If the light is not on, then the secondary
electron detector is on; turn it off by selecting OFF under detector in EOS-3.
9.3.9.5
Carefully push the sample rod into the chamber and thread
the rod into the dovetail holder on stage. Make sure that the rod is completely
screwed into the holder before retracting or your sample may fall off into the
chamber.
9.3.9.6
Gently retract the rod until the screw is secured in the
clip attached to the glass viewport (you will hear a click when this happens).
9.3.9.7
Slide the airlock door closed and turn the knob ¼ turn away
from you until the flat area on the knob is parallel to the floor.
9.3.9.8
Holding the rod near the glass viewport, press the white
airlock button and wait for the light to go on. Make sure that you hold the rod
during the venting of the airlock.
9.3.9.9
Remove loading rod, unscrew the sample holder and place the
rod in its holder.
9.4 Electron Beam Lithography with NPGS
The
Nanometer Pattern Generation System (NPGS) from Nabity Lithographic Systems is a
flexible system for performing electron beam lithography. The purpose of this
manual is to provide users with a brief overview of the operation of NPGS v7.6.
After reading this, you should still study the following sections of the NPGS
manual itself:
Introduction
Microscope
Considerations
Designing
Patterns
Making a
Run Parameter File
Writing
Patterns
Section 11 (Sample Preparation and SEM Setup) is also useful reading. If you need to align your patterns with structures already present on your samples, you should read Section 8 (Aligning Patterns). Finally, the program NPGS.exe (only a part of the whole NPGS lithography system), while not required for exposure, adds some flexibility and is described in Section 6. Copies of the NPGS v7.6 manual are available for reading in the Microlab lobby.
9.5
Drawing Patterns with DesignCAD
9.5.1
Before doing an exposure, you must make patterns, which can
be interpreted by NPGS. NPGS is designed to accept patterns drawn in DesignCAD,
a commercial CAD program. This manual does not attempt to describe in detail
how to make drawings in DesignCAD, but does list some of the most useful
commands. A copy of the DesignCAD manual is available in the Microlab lobby. It
is advised that you study the tutorials in order to familiarize yourself with
basic use of DesignCAD before trying to make any patterns.
9.5.2
A directory "pgxxx" will be created for each user,
where xxx are the user's initials. You should save you patterns and run files
in your directory, and run the NPGS software from it as well. To start
DesignCAD from the DOS prompt type "dcxxx" from any directory. This
loads a customized version of DesignCAD, which includes several useful macros
for use with NPGS. Design CAD will now automatically save your patterns as
*.dc2 files, an ASCII file type which can be interpreted by the Nabity
software. Your patterns will be saved to your directory by default. You can use
many of the drawing features of DesignCAD when making patterns, including
lines, polygons, arcs, and cubic splines. See the NPGS manual (available in the
Microlab lobby) for details. The most common DesignCAD commands can either be
selected from the menus using the mouse, or entered from the keyboard (usually
a single stroke).
9.5.3
Some Useful Commands:
9.5.3.1
0/Ins or left mouse button: Sets a new
point at the position of the cursor
9.5.3.2
. (period): nearest existing point. Known as
a "gravity point."
9.5.3.3
: (colon): Sets a point at absolute
coordinates entered by the user.
9.5.3.4
V: connects all existing points in
the order they were drawn by a series of straight line segments. Useful for
drawing lines; width of lines may be specified by user.
9.5.3.5
ALT F5: Uses a set of points chosen by
the user to define a polygonal region. Such regions are treated as filled by
NPGS. Known as "PolyFill."
9.5.3.6
Q: Among other things, changes the
current color and line width.
9.5.3.7
L: Changes the active layer.
9.5.3.8
ESC: Removes the most recent structure
(point, line, etc.) added to the drawing. Pressing ESC a second time will
delete the second most recent addition, etc.
9.5.3.9
!: The inverse of ESC.
9.5.3.10
Y: Clears the screen.
9.5.3.11
F10: Saves the current drawing.
9.5.3.12
F9: Retrieves a drawing and overlays
it on the current drawing.
9.5.3.13
F8: Exits DesignCAD.
9.5.3.14
Most patterns not requiring curved lines can be drawn using
the absolute coordinate (:), gravity point (.), vector (V)
and PolyFill (ALT F5) commands. IMPORTANT: When interpreting
DesignCAD files, NPGS assumes the drawing units are in microns.
9.5.4
Drawing Layers and Colors
9.5.4.1
NPGS differentiates between the various elements of a
pattern based on the drawing layer and color of each element. For all elements
in a given layer, you use the program MRF to assign the following exposure
parameters: "Origin Offset", "Magnification",
"Center-to-Center Distance", "Line Spacing", and
"Measured Beam Current". Drawing elements are restricted to layers 1
through 19. Within a given layer, you use MRF to assign a different exposure
time/dose to each color you have used. NPGS uses only colors 1 through 16. The
same color can be used for different doses in different layers. Layers are
exposed sequentially in numerical order.
9.5.4.2
A typical design strategy is to put the most sensitive (i.e.
smallest) elements in layer 1, which is to be drawn at high magnification.
Larger structures are placed in higher layers, and drawn at lower magnification
with higher probe current. Exposure can be paused between each layer, allowing
microscope settings (typically the magnification, probe current and/or
objective aperture) to be changed.
9.6
Run Files
9.6.1
After you have saved a DesignCAD file, use the MRF program
to assign values to the various exposure parameters. The proper syntax is
"mrf" or "mrf run_file"; be sure you are in your directory
and that copies of the patterns you want are present. If you name an old run
file you will be asked if you wish to read the old run file (default is yes)
and if you wish to reread the pattern data (default is no). A screen will then
appear in which you can select which patterns are to be exposed. You can expose
up to 16 distinct patterns in a single run file. Add or remove patterns with
the INS and DEL keys. If the pattern list takes up more than one page, the PgUp
and PgDn keys will move to previous and following pages.
9.6.2
Once all the patterns have been chosen, enter the various
exposure parameters for each. "Ctl PgDn" will call up the run parameter
table for the first pattern. This includes the following parameters, for each
layer in the pattern:
"Origin
Offset": Offsets the origin of the layer during exposure.
"Magnification":
Sets the magnification for the layer.
"Center-to-Center":
sets the spacing between adjacent points in a line.
"Line
Spacing": sets the spacing between adjacent lines in an area.
“Measured Beam Current": Used
to calculate exposure time if a dose is specified.
9.6.3
In the first line of the run parameter table you may choose among
four options: "w", "p", "c", and "s".
"p" pauses the exposure process before any given layer until you tell
it to continue. The default "w" causes the layer to be written
without a pause, using beam blanking between exposure points. The program will normally
pause before the first layer of each pattern in the run file, even if
"w" is chosen. This automatic pause can be overridden using the
"Pause only for 'p'" option; see the NPGS manual for details.
9.6.4
After the list of layer run parameters, all the colors found
in that layer are listed. There are three ways to specify the exposure level
for each color: 1) set the exposure time per point; 2) set a line dose in
nanocoulombs per centimeter; 3) set an area dose in microcoulombs per square
centimeter. The latter two methods are generally more useful; pressing the
space bar while the dose line is highlighted toggles between them. When a dose
is specified, the exposure time per point is calculated from the measured beam
current, the center-to-center spacing and the line spacing (see NPGS manual for
details).
9.6.5
If the parameter table for a pattern takes up more than one
page, "PgDn" and "PgUp" will move among the pages. After
all the parameters for a pattern are specified, subsequent patterns can be
reached by pressing "Ctl PgDn". "Ctl PgUp" will return you
to the previous pattern.
9.6.6
After specifying the run parameters for all patterns, save
the run file by pressing "Ctl Home". Pressing "Ctl End"
will exit MRF without saving. Run files are saved with the suffix
".rf6".
9.7
Exposing Run Files
9.7.1
Once the run parameters have been set using MRF, the program
PG is used to expose the pattern. The syntax is "pg run_file" where
run_file is a .rf6 file created by MRF. Be sure you are in your directory and
that a copy of the run file and all patterns in it are present. A screen will
appear which lists the pattern name, layer number, magnification and measured
beam current specified in the run file. Several options are now possible.
Pressing the space bar will expose the pattern; "!" will skip the
current layer and move to the next in the run file; "Esc" will skip
the current pattern; and "Enter" will cause PG to terminate without
exposing the pattern. PG can be terminated during an exposure by pressing
"Ctl Break".
9.7.2
You can also use the NPGS.exe program to perform exposures.
It gives added flexibility, including the option to perform DOS commands
between patterns. See the NPGS manual for details. If you wish to align your
exposure with previously existing structures, you will need to make use of the
alignment program AL. See the NPGS manual for details on the creation of
alignment patterns and the use of AL.
9.8
System Operation for Electron Beam Lithography
System operation for electron beam lithography parallels that for general use, with some additional steps and precautions. This section is not stand-alone. Liberal reference will be made to Sections 2 though 4 above. See also the NPGS v7.6 manual, Section 11 (Sample Preparation and SEM Setup). For a general introduction to electron beam lithography, see Introduction to Microlithography, L.F. Thompson et al., Eds. (1983).
9.8.1
Sample Preparation
9.8.1.1
Samples must be clean and free from contaminants. Samples
should be coated with an electron beam resist such as PMMA or P(MMA-MAA). If an
electron microscope resolution standard is mounted along with the sample you
may use the standard for focusing and astigmatism adjustment. No particles of
anything, silver, graphite or other will be used. A scratch made on the surface
is fine, so is the edge of the chip. Silver particles in methanol were once
used for focusing. This procedure resulted in migration of the silver particles
with downtime and maintenance needed for cleanup.
9.8.1.2 It is useful to put two focusing marks on the sample, one on each side of the exposure area. This allows an estimate of the correct fine focus setting at the exposure area without risking exposure of the sample. Also, since the resist is sensitive to electrons, the sample should be mounted on the stage so that you know roughly its stage coordinates. This allows you to move the field of view off th