1. science
2. physics
3. optics

Operation and Functionality of Fixed Beam Moving Stage
(FBMS) Exposure
A. Principles of Operation:
In FBMS mode curved lines of any shape and length can be exposed with a
stationary or expanded electron beam by controlled continuous stage motion. The
way of exposing pattern avoids fractioning of pattern at writefield boundaries
(stitching)
The lines to be fabricated in this way are handled and stored in a special way, called
UserDefs, integrated into a GDSII data file. The lines are assigned and identified with
the type “curved paths”, which is a Raith proprietary GDSII format extension. “Curved
Paths” can be defined and exposed with zero or finite line widths – and there is a
separate dose factor assignment for each curved path.
Lines and pattern of this type can be created directly within the GDSII editor by
“Add//FBMS/Path” resp. “Add//FBMS/Circle” or can be converted retrospectively
into the type “Curved paths” when generated with standard GDSII functions, such as
“Add/Open Path”, “Add/Circle”, or “Add/Polygon” etc. – see also chapter on Data
Conversion Functionality.
If the respective files, structures and layers have been selected in the usual way the
operator can define the general exposure speeds (parameters under
“Exposure/Details)” in mm/sec and then start the exposure in all standard ways. Max
exposure speed is 1 mm/sec, more typical speeds are in the range of 50 - 300
µm/sec. Based on the dose and linewith assignments of the individual pattern the
stage speeds are adjusted automatically to reach the predefined doses.
The general exposure sequence is then as follows: Normal beam scanning scan type
exposure is executed for selected layer as usual, whenever standard pattern are
found in that layer. After this, the stage drives to the start point of the first “Curved
Path” included in the selected structure. This is performed in the normal digital driving
mode and at full digital speed. The beam blanker is switched off.
After the first point has been reached the FBMS mode is enabled, the beam is
switched on and the controlled path motion starts based on the calculated exposure
speed. The stage is controlled to follow at almost constant speed as close as
possible the defined path by mechanical motion and the remaining difference
between the true position and the calculated ideal position is fed as an analogue
signal to the (calibrated) column scan coils via dedicated input channels of the
RAITH150 high speed digital pattern generator. The motor driving regulation loop has
a time constant of about 50 msec whereas the beam tracking operation has an
update time of 10 µsec. (details see specification sheets).
At the end of the first path the beam is switched off again and the stage is driven to
the next starting point again in normal digital address mode, and so on. At the end of
the entire exposure the system is stopped in normal mode and available for next
commands.
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B. Generation of Curved Paths
“Curved paths” are fully integrated functionalities within the standard Raith GDSII
data base, editor and viewer. Within this they can be handled like “normal” GDSII
elements. However, please note, that they cannot be directly exported to third party
GDSII editors. Curved paths may be assigned with a finite linewidth “Width” and a
dose factor, as known form standard GDSII elements.
Please note that the width of “Curved Paths” will not be graphically displayed in the
GDSII editor/viewer, but can be assigned and inspected only digitally by “Select/Edit”.
1. Direct Editing Function
Curved paths can be created manually with the GDSII editor by the Commands
“Add/FBMS/Path”. This is mainly intended for evaluation and testing purposes.
Individually data points can be set by mouse click and edited retrospectively (double
click on pattern) in the usual manner – see the example window below.
These paths may consist of combinations of polygons with circles and circular arcs.
Please try yourself and get some experience with the definition of arcs as part of the
direct editing function.
A range of curved path editing and modification functions are provided. Each curved
path may consist of any combination of polygons and arcs. Max number of nodes
(=definition points) for each curved path is around 1000. For each path the dose and
the width can be defined. Width=0 means that the path will be exposed in spot mode,
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a finite width (defined in µm) will be generated by applying the beam shape mode
during exposure.
1.1 Creation of Polylines
Nodes for a polygon line can be defined by sequential left mouse button clicks within
the editor window and completed by right mouse button operation. Whenever the
definition has been completed, a digital editing within the “Edit Curved Path” window
is possible by double mouse click on the selected structure – all as known from
standard editor operation. Here, node addition, insertion, deletion etc. operations are
possible – and nodes can be typed in manually.
1.2 Creation of Full Circles
The command “Add/FBMS/Circle” allows to generate full circles with parameters
specified by the mouse cursor: First click defines the center coordinate, second click
defines the diameter. A numerical editing – also with respect to dose and line width
ca be started by double mouse click on the circle.
1.3 Creation of Polylines and Arcs
Same start as described in 1.1. After switching to the digital input window any kind of
arc can be added to the leading polyline by activating the attribute “Arc” and defining
a radius. Please note that radius may be used with a sign, which defines the direction
of curvature. Fine tuning of the edited results can be done by comparing with the
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simultaneous display in the editor window. A screen update will be given whenever
an input window has been left.
GDSII Data Conversion Functionality
This is the routine way of defining and creating curved paths. Pattern which consist
either of standard GDSII types Box, Polygon, Open Paths Circles or Arcs can be
copied & converted into exposable curved paths. The procedure is as follows:
1. Open the file containing the structures to be
exposed within the GDSII editor.
2. Select all patterns you want to convert into curved
paths by applying the respective “Select”
functionality with the GDSII editor.
3. Start the conversion by mouse click on the
“Modify” function of the editor and select the
function “Copy & Convert to curved paths”
4. Select the layer in which you want to copy the
selected collection of converted pattern within the
appearing layer selection box.
Figure: Data conversion from GDSII format into “Curved Paths” (example)
Please note:
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a. The conversion routine leaves the selected start pattern as they are in the original
layers and copies the converted pattern into another or the same layer as defined
by the user. If you only want to expose the pattern in the FBMS mode please take
care for a correct layer selection!
b. If area type pattern like boxes, polygons etc. are converted, the results will be
curved paths consisting of the boundaries (frames) with zero width (spot mode).If
required, they can be assigned with a finite widths later on in a normal
“Select/Edit” step.
c. If open paths with non zero widths are converted into “curved paths”, the widths
will be preserved in the converted data type
d. Special GDSII Userdefs, such as ellipses and elliptical arcs will be ignored in the
data conversion routine. If these structures shall be converted please convert
them into standard GDSII elements such as polygons, or open paths beforehand.
C. Exposure Control and Parameter Calculation
The ways to control the FBMS exposure parameters and start exposure are fully
integrated into the standard exposure control procedures.
1. Select the Correct Writefield
Within the RAITH 150 lithography
systems please select a Magnification
of 1530 in “Column Control” as the
standard size for FBMS operation; the
corresponding size of the scan field
will be +/- 32.768 µm. For calibration
of the FBMS writefield see chapter D.
“Beam Tracking Calibration”, below. If
there is a need to expose curved
paths in the higher speed range (i.e. above 500 µm/sec), please select a lower
Magnification, see table below as a recommendation:
General recommendation of writefield sizes as function of exposure /stage speed:
Stage Speed
10-500 µm/sec
100-1000 µm/sec
or: 500-1000 µm/sec
Magnification
1530
1000
765
Writefield Size
65.536 µm
100.000 µm
131.072 µm
Please note: Whenever a new writefield size is selected for exposure in FBMS mode
a precise calibration field size is required – same as for standard exposure runs; for
details see section D.
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2. Exposure Parameter Calculation
In order to find the best suited parameters you can select the “Calculator” within the
Exposure window and go to one of the “FBMS” buttons. Here the correct speed
/beam current combination can be evaluated and selected for a given resist
sensitivity
FBMS Line Mode
The calculator scheme shall be used to control curved paths with zero width to be
exposed in the beam spot mode. Important input parameters are the beam current
and the “Line Dose” as known from the normal line exposure mode. Typically this
sheet is used to calculate the optimum stage speed for spot exposure. The results
are transferred automatically to the parameter settings in the exposure control, to be
inspected /edited under “Details”
FBMS Area Mode
The “Area Mode” calculator allows to propose a best suited stage speed for all
curved paths which have a non zero width and thus shall be exposed in the beam
shape mode. Input parameter again are the “Beam Current “and the “Area Dose” as
known from standard exposure.
The “Reference Width” is a parameter which shall the typical or “average” linewidth
for curved paths in the design; it is used to have a standard for the parameter
evaluation, i.e. the stage speed calculation process. Differences in the individual
linewidths found in the design with respect to the “Reference Width” will be used to
adjust the stage speed accordingly in an automated way: Double width results half of
the speed.
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3. FBMS Exposure Parameter Control
The latest data, generated form the “Exposure Parameter Calculation” are
automatically transferred and used as “Exposure Details” and can be controlled,
edited and confirmed within the “Details” window of the “Exposure” window, see
below:
The exposure of FBMS and standard GDSII elements can be separately activated by
two ticks. The above window shows the standard setting where both FBMS and
normal elements are exposed.
The entry data of area and line stage speed are typically preset from the exposure
parameter calculator. The speed settings for stage motion during exposure are
independent from the normal digital motion speed. As obvious from the section
above, the stage speeds are directly related to the exposure dose at a given beam
current.
In particular for “Curved Paths” with zero width, where the “Beam Shape Mode” is not
active, the exposure parameters are very similar to the case of single pixel line
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exposure – and there is a strong correlation between beam current, speed and the
resulting line widths.
Please note that the real stage speed during the exposure will automatically be
adjusted (and thus will differ from the specified stage speeds in the window above),
whenever the dose factor of the selected pattern differs from 1 and the of FBMS area
elements differs form the “Reference width” that has been chosen in the exposure
parameter calculator window.
The parameters entered as “Loops” will be also valid for the number of sequential
exposure runs in FBMS mode (normally, the setting is “Loops”=1). If multiple loops
have been defined, first all pattern in a selected layer will be exposed in one
sequence and then the complete sequence will be repeated until the number of
specified loops are reached.
4. Beam Shape Mode
The exposure can be performed in spot or in beam shape mode. The latter allows to
activate an additional exposure pattern that is continuously repeated by the pattern
generator during writing of FBMS elements. The pattern can either be defined
individually by the operator (see section 4.2) or be set automatically by the system in
the so-called circle generator mode. The circle generator is used during the exposure
of FBMS area elements. It generates a circle symmetric pattern with an outer
diameter that corresponds to the width of the FBMS area element. The inner part of
the pattern is exposed in way that gives a constant dose profile perpendicular to the
stage movement. Since the pattern is also circle symmetric, FBMS path elements
with a constant width can be exposed in any writing direction of the FBMS path with
this mode.
Both user defined and circle generator patterns are basically addressed by the digital
pattern generator as a series of single pixels. The maximum number of addressable
pixels is 3400. The pixels are exposed without settling time but with a fixed dwell time
per pixel. The dwell time is called FBMS Circle Dwelltime and can be specified in the
Module Status in the topic Digital Pattern Generator. The recommended standard
setting for the R150 is 0.002 ms with respect to electromagnetic deflection system.
The minimum value that can be set with respect to the digital pattern generator is
0.0002 ms.
4.1 Circle generator
The circle generator allows to write a line as a “Curved Path” with a predefined width
with larger process latitude than in spot exposure mode. The line width can be
specified directly in the GDSII design by the “Curved Path” parameter “Width”.
If the parameter width has a non zero value the pattern generator creates a high
speed circulating shape while the stage is moving along a path. Due to the high
speed beam “spinning” (single pixel addressing is typically done in 0.5 – 1 MHz
regime) the resulting beam dose profile is almost stationary in relation to the moving
stage.
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The used circle symmetric pixel pattern is automatically calculated by the system.
The parameter that can be set by the operator for this mode is the so-called shape
step size. The shape step size gives the average step size perpendicular to the stage
movement. Therefore it corresponds to the area step size for conventional GDSII
elements and should be set with respect to the chosen aperture as in normal area
exposure mode.
Due to the limited number of addressable pixels the chosen shape step size results
in a maximum width for FBMS path elements. For exposing elements with a larger
width the operator has to choose a larger shape step size and correspondingly a
larger aperture or use a defocused beam.
Like in the exposure parameter calculator window, a reference width can be set for
calculation of the resulting cycle time. Thereby the operator has the possibility to
check if the cycle time is reasonable with respect to the area stage speed.
4.2 User generated pattern
Furthermore, the operator has the possibility to define his own beam shape patterns
for FBMS writing. The pattern could be either a different circle generator pattern
defined by the user for dedicated exposures tasks, or a special geometric pixel
pattern for further sophisticated applications.
The pixel pattern is defined in a text file with a series of dots that are defined in
Raith’s ASCII format. Thereby Raith’s GDSII editor can be used to create the pattern
as GDSII file. The required ASCII file is easily generated by applying the ASCII
export filter on the GDSII dot file. The only relevant dot data in the ASCII file for the
FBMS shape mode are the pixel xy-coordinates, which define for this mode the offset
to the center position 0/0. Further parameters of the dots in the ASCII file like dose,
layer, and width setting are simply ignored. The next figure shows a typical ASCII file.
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xy-coordinates
(offset to center)
dose, layer, and
width ignored
In order to use a certain ASCII file for the beam shape mode, its directory location
and file name has to be set in the FBMS parameter window (see next figure).
The number of pixels in the ASCII file and the maximum coordinate offset in x and y
is automatically displayed after defining the filename and folder location. Please note
that the maximum number of pixels in the beam shape mode is 3400. If the file
contains more than 3400 pixel, pixel number 3401 and all following pixels will simply
be ignored. Moreover, the maximum offset in x and y has to fit to the scan field
range. In order to have sufficient additional scan range for beam tracking during
FBMS writing, a reasonable maximum offset is half of the maximum scan deflection.
The user defined beam shape mode is activated by the tick Use user pattern. In case
the tick has been set, all FBMS single pixel lines (elements with a width of 0) will be
exposed with the user defined pattern. The result is a mixed exposure of a fast,
continuously repeated deflection by the patter generator, where the center position of
the deflection follows the path of the FBMS single pixel line. Please note that
correspondingly only FBMS elements in either spot mode or in user defined beam
shape mode can be exposed in a single exposure.
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5. Exposure Time Calculation
The time required for the exposure of the curved paths
can be roughly estimated by hitting the “Times” button
within the exposure control window. The calculation and
simulation initiated by this function will display the
respective exposure and overhead times for all selected
layers and includes the time required for the execution of
the FBMS elements in the last but one line.
If a selected layer contains only curved paths, only
“Calculation time” and “Curved path time” (=net exposure
time) will show a contribution.
6. Exposure Control via Position List
The Exposure Control in the FBMS Mode via Position List is similar to the operation
in the normal mode – and thus allows to mix and combine both modes in a common
task.
The GDSII structure to be exposed can be placed per drag and drop into the position
list. The result is that the actual stage position will be taken as the center of the
lower left writing field within the working area. Note that in FBMS mode the writing
field has the meaning of the “beam tracking field”:
As known for normal exposure mode, in particular the layers to be exposed and the
dose can be edited as “Properties” in the position list. Please note, that the
parameter “dose” will be used in a multiplicative way and changes the actual stage
speeds accordingly.
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D. Beam Tracking Calibration
The operation of the FBMS exposure mode requires that the scan field of the e-beam
column has been calibrated carefully. This has to be done at a fixed selected
magnification. Based on the RAITH150 column and pattern generator hardware
magnification shall be set to a fixed value of MAG=1530. (ideal for calibration is a
65.536 µm writefield; for selection of other writefield sizes, see note below). Assign /
“Edit” the corresponding writefield size as 65.536 within the Microscope Control
window and “Save” the setting.
Please note that the scaling and alignment parameters of the pattern generator are
not relevant for this writefield calibration. The calibration will be executed only by the
digital motor control unit.
In general, the calibration procedure requires the following steps and sequence:
1. Perform a standard “Adjust UV” procedure to accommodate for the sample
orientation
2. Perfom a standard “Align Writefield Procedure” (i.e. Manual or Automatic..) for
the pattern generator at magnification setting of 1530
3. Start the “Align FBMS Control Procedure” as described below:
The FBMS calibration procedure is very similar to the standard alignment procedures
as organized by the scan manager in RAITH150 software version 4 and higher. A
new scan type is provided, which is named “Align FBMS Control Procedures”. The
definition and operation of the scan parameters for this calibration task are to a great
extend identical to those of the manual match write field procedure for the pattern
generator calibration (see screenshots below).
A useful set of alignment procedures is provide in a preconfigured way within the
software setup, see example above:
a) “Align FBMS 20 micron … 02 micron” for the 65.536 micrometer writefield and
b) “LF_Align FBMS 20 micron … 02 micron” for the 131.072 micrometer writefield.
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The physical FBMS beam tracking calibration procedure is as follows:
At a pre selected magnification a marker (particle) has to be placed into the center of
the image field.
The calibration procedure now drives the marker by normal stage motion into the
specified position in one corner of the scan field. This stage motion is now
“compensated” by a reverse addressing of the Motor Control DACs driving the scan
coils via the external scan interface (connected via the “sense” inputs of the pattern
generator). This means that the markers should be roughly centered again. Now the
Pattern generator captures an image of the mark (at center position for the pg!) and
deviations between the real and the ideal marker position are assigned as known
from the normal mark recognition and alignment procedures.
This step is repeated for a number of selected mark locations (min 2 locations, max 4
locations). When the mark assignments have been completed a macro/script is
executed which sends the scaling and rotation corrections (which can be temporarily
inspected in the Align Writefield window) to the motor control unit and sends the shift
parameters to the pattern generator.
In order to enhance the accuracy this procedure can be repeated i.e. with larger
marker displacements and/or smaller marker windows, as seen for example in the
Scan Manager screenshot above.
The position list will automatically guide the user through the individual alignment
steps very similar to all Raith150 alignment procedures.
Use standard 2D mark
capture parameters as
known from manual write
field alignment with laser
stage
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Continue with „Scan“ list item … and the following task list will be automatically
generated and executed in an interactive way:
… GetListWFMarks
… Halt()
… SendFBMSCorrection
Whenever this procedure is finished the correction data are stored in the
configuration file (User.vdb). The alignment parameters will be automatically
reloaded and send to the motor control unit whenever the FBMS mode is activated
i.e. the curved path motion is started.
Note 1: Required accuracies in FBMS alignment
The accuracy of the alignment procedure will directly influence the quality of the
beam tracking and consequently the fidelity of the pattern in FBMS exposure mode.
A good estimate of the actual calibration accuracy is displayed temporarily in the
“Align Writefield Window” after the mark assignment have been completed. The
factors shown under “ZoomU/V” shall not differ from 1 by more than +/- 0.0002. The
difference in “RotationU/V” shall not be larger than 0.01 degrees. Whenever the
corrections are higher than the values above another alignment cycle, typically with a
smaller mark capture field, is required.
Note 2: Larger speeds and writefields
For higher speed exposures you may follow the same procedure with larger
writefields. Magnification of 765X for a writefield of 131.072 is one recommendation
for exposure speeds above 500 µm/sec, however, any magnification and writefield
can be selected and calibrated in the manner described before. Please have in mind
that the accuracy may be limited at large writefield sizes. As a typical rule, the
smallest feature size should not be less than 1/5000 of the writefield.
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Appendix I-A: Control Functions
The Alignment window of the FBMS
Control module allows to send out
directly commands to the beam
shifting DACs (16 bit /1 bit = 1 nm at
Mag 1530X) within the motor control
unit (fields: “DAC Output”). It further
displays the actual calibration values
applied within the motor control unit
(grey numbers).
In addition, it allows to send a set of
“Correction parameters” the motor
control
unit.
These
correction
parameters (ScaleX, ScaleY and
RotationX, RotationY) control the correct values and orientation of beam the shifting
DACs operating within the Motor control unit.
Please note, that the entry values for “Zoom” will be multiplied to the actually stored
values and that the “Rotation” entry values will be added to the existing ones. In more
detail:
Alignment: Zoom and Rotation
“Send”
Allows to sent values entered in the respective fields directly
to the motor control unit; note that the zoom values are
multiplied with the active values and that the rotation values
are added to the active values.
“Preset”
Copies the alignment parameters of the pattern generator as
excellent start values for the beam tracking alignment. After
having performed an “Align Writefield Procedure” for the
pattern generator typically the Preset function allows to get
the correction values with a maximum error of 0.5%.
“Reset”
Resets the alignment parameters of the beam tracking
calibration (sets scale factors to 1 and rotation values to
zero)
DAC Output Control
“Set”
“Reset”
Transfers the voltage entered into the respective window
directly into the motor control unit as shift voltages for beam
tracking – for testing purposes only!
Please note: A non zero value of the DAC output
deactivates the beam tracking for that axis, until a Reset
(see below) is entered or exposure is started
Resets the DAC Output voltages to zero values
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Appendix I-B: Control Functions - for Testing and Service, only.
This menu only appears for users logged in as and authorized as “Supervisor” or
“Service”
Please note that for testing and general setup, it is useful to open and activate the
“Logger” functionality within the program
− allows to toggle the motor control operation between normal and FBMS control
mode: “FBMS Control Mode”. When the FBMS Mode is active, the beam tracking
can be enabled – based on the actual alignment parameters for beam tracking.
− enables a read out of the motor control status according to the motor control
protocol: “Status request” – actually not used
− can activate the beam tracking function and the FBMS motion (the latter requires
that the stage is already at the start point of the path and that a curved path has
been loaded): “Start”.
Note: This command may be used for testing/verification of the beam tracking by
continuous fast image acquisition with the pattern generator while being stationary
on a point
− can stop the FBMS operation: “Stop”
Please note: After any operation sequence within this module please switch off the
FBMS control by means of the button “FBMS Mode”; this will be confirmed by a grey
status indication of the lower three buttons on the left side.
“Beam On”: Controls the beam blanker signal level within the motor control unit for
test purposes.
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Appendix II:
1. Required Hardware and Connections based on standard R150
configuration
− Digital Pattern Generator, 10 MHz, version 6 is mandatory
− New FBMS Motor Control Unit
− 2 BNC type cables to connect MC with XY sense beam tracking input of the
pattern generator
− Special cable to connect feedthroughs for piezoelectric fine shift transducers at
chamber wall with digital Motor Control Unit
− BNC cable to connect with input signal at BB-electronics
− For details, see connection diagram in appendix III
Please note:
1. Within the Motor Control unit there is a logical “And” function on the “Beam On”
input from the Pattern Generator and the internal “Beam On” for the curved path
exposure. (“Beam On” means “active low” in TTL signals level)
This means: The beam is “ON” if the Pattern Generator AND the Motor Control
Unit are in an “ON” status. If one or both units are in “OFF “status, the beam is
“OFF”.
2. All output signals for scan and tracking are set to +/-10V full DAC range.
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