**Alignment, tolerances, commissioning: PEEM-3**

![](media/image1.jpeg){width="6.275in" height="4.575in"}

Hi-res.: Assume 2 um FOV, Magnification 12000 (M4=M5=M6=10) (CCD:
24x24mm)

Lo-res.: Assume 20 um FOV, Magnification 1200 (M4=M5=10, M6=0)

Aux.: Assume 17 um FOV, Magnification: 1440 (MA1+6, MA2=20) (CCD: 24x24
mm)

  ----------- -------------------------- ------------------- -------------------
  Component   Description                Beam alignment      Physical tolerance
                                         (Hi-res./Lo-res.)   relative to
                                                             relevant magnetic
                                                             axis of SM

  S           Sample                     \-                  \<1 mrad (tilt),
                                                             200 nm rel. O

  O           Objective lens (M=12)      \-                  25 um

  D1          El. dodecapole             \-                  250 um
              deflector/stigmator                            

  D2          El. dodecapole             \-                  250 um
              deflector/stigmator                            

  F1          Field lens (M=1)           2.5 um / 25 um      25 um (or better)

  SM          Separator magnet           2.5 um / 25 um      \-

  F2          Field lens (M=1)           2.5 um / 25 um      25 um (or better)

  D3          El. Dodecapole / Mag.      \-                  250 um
              Octupole                                       

  D4          El. Dodecapole / Mag.      \-                  250 um
              Octupole                                       

  M           Mirror (M=1)               2.5 um / 25 um      25 um

  A1          Aux. projector lens (M=6)  25 um               25 um (to mirror)

  A2          Aux. projector lens (M=20) 150 um              100 um (to mirror)

  CCD1        Aux. CCD                   \-                  1000 um

  D5          El. Dodecapole             \-                  250 um

  I1          Intermediate lens (M=1)    2.5 um / 25 um      25 um

  AP1         Contrast Aperture          2.5 um / 25 um      1 um rel. I1

  I2          Intermediate lens (M=10)   2.5 um / 25 um      25 um

  D6          El. Octupole               \-                  250 um

  P1          Projector lens (M=10)      25 um / 250 um      250 um

  D7          El. Octupole               \-                  1000 um

  P2          Projector lens (M=10 /     250 um / -          250 um
              off)                                           

  CCD2        CCD                                            1000 um
  ----------- -------------------------- ------------------- -------------------

Beam alignment will be achieved by either using the upstream deflector
and mechanical pre-alignment. The criteria for the above numbers are:

Beam alignment: 10% of local beam size at chosen magnification.

Physical tolerance: 10% of beam size at intermediate magnification.
Deflector elements not used

Sample alignment: approximate number chosen from experience with PEEM-2.

Optical alignment is based on mechanical pre-alignment of PEEM. The
mechanical tolerances were chosen such that a high-quality image can be
seen on the detector at moderate magnification without use of deflector
elements. PEEM-2 has shown that this is possible in a straight PEEM. The
major unknown is the function of the separator magnet. The 90° auxiliary
PEEM will help adjusting the trim coils of the 1^st^ sector and will
give hints about the alignment of the 2^nd^ sector.

The final electron optical alignment has to be based on image quality,
image distortion, stigmatism, coma, and movement while wobbling lens
voltages, deflector voltages/currents and trim coil currents. Beam
monitors will be useful to identify problems, which lead to a complete
loss of transmission. Whether the precision and the amount of the
information that they will provide are sufficient to assist with the
final alignment of PEEM-3 at high magnification is questionable. The
accepted electron beam size before the contrast aperture is on the order
of 1 mm and its "center" would have to be determined with an accuracy of
2.5 um. 2.5 um is 10% of the accepted beam size for a FOV of 2 um and
12x magnification of the objective lens. The beam intensity is
furthermore laterally modulated by the profile of the illumination. The
"center point of the object" that has to be transferred through the
optical center of all components of the microscope is actually not
defined as the center of gravity of the electron beam but by the
symmetry axis of the sample/objective lens group. In the following I
therefore have to assume that the final alignment will not require any
additional beam monitors in the microscope.

For the PEEM alignment a monochromatic electron source of high
brightness is required. Possible sources are an electron gun in a LEEM
configuration or a monochromatic UV source operating at a wavelength
close to the workfunction threshold of the sample. An energy bandwidth
of \<0.5 eV would be ideal. Our PEEM design does not allow using an
electron gun. We therefore either use a well-focused UV lamp or a UV
laser. The illumination intensity should be comparable to the intensity
of the x-ray source: \~100 W/cm^2^ (10^12^ ph/s at 1keV in 10um x 10um).
A frequency-tripled, focused Ti:Sa at 4.5 eV (270 nm) has a CW averaged
intensity of 100 W/cm^2^ (1mW in 30um x 30um). If necessary, tighter
focusing is possible. (Will inquire about Hg arc lamps)

The microscope alignment consists of two stages: beam alignment and
aberration correction. During the 1^st^ stage the "beam" will be
centered and aligned in optical components using trim coils, and the
dodecapole and octupole elements. During the 2^nd^ stage residual
aberrations will be corrected using trim coils, dodecapoles and the
mirror. In general, alignment will be an iterative process starting at
low magnification and step-by-step optimization of all elements while
ramping up the magnification until the ultimate resolution is reached.
The initial alignment will be carried out using a monochromatic electron
source such that only spherical aberrations will contribute. Dispersion
and chromatic defocus can be studied by adding an offset to the sample
voltage. Initial alignment will also be carried out without a contrast
aperture to maximize the effect of misalignment and "mis-correction". A
PEEM behind the mirror arm is essential for the initial alignment and
would be very useful later because it allows a) to independently test
and optimize the first sector of the separator and b) to independently
optimize ingoing and outgoing beam at the mirror. Samples with
high-contrast structures with structure sizes of down to 30 nm
(lithography) and down to 5 nm (islands, multilayer?) will be required.
Below the 11 major stages of alignment are described:

  --------- -------------------------------------- -----------------------
  Stage     Description                            Settings

  0         Choose default voltage, currents       

  1         Beam alignment, sample, objective,     No aperture, UV, 90°
            1^st^ sector                           

  2         Correction 2,3 fold stigmatism,        No aperture, UV, 90°
            objective, 1^st^ sector                

  3         Focus mirror                           No aperture, UV,

  4         Beam alignment 2^nd^ sector, projector No aperture, UV,

  5         Correction 2,3 fold stigmatism, 2^nd^  No aperture, UV,
            sector                                 

  6         Correction spherical defocus, mirror   No aperture, UV,

            =\> maximum resolution using UV        
            illumination                           

  7         Dispersion correction 1^st^ sector,    No aperture, UV, sample
            readjust D1,2                          bias, 90°

  8         Dispersion correction 2^nd^ sector,    No aperture, UV, sample
            readjust DM3,4                         bias

  9         Correction chromatic defocus, mirror   No aperture, UV, sample
                                                   bias

  10        Insert aperture, repeat 1-9            Aperture, UV

  11        Switch to x rays, repeat 1-9           Aperture, X-rays

            =\> maximum resolution using x ray     
            illumination                           
  --------- -------------------------------------- -----------------------

**0: Choose default voltages, currents**

Goal: Focused image on CCD1, CCD2. Optimize distance by focusing with
z-travel at fixed O.

**1: Beam alignment, sample, objective, 1^st^ sector**

Settings: No aperture, UV, 90°

Condition: Focused image on CCD1

Goal: Align the sample axis with the objective axis and transfer the
point on the sample, which is thus defined through the center of the
following optical elements: F1, SM(1), F2, M. The beam is steered using
the D1, D2, D3, D4 electric and magnetic deflectors.

a)  Align sample using xy tilt:\
    - Maximize intensity for objective voltage well below focus (assumes
    PEEM alignment is perfect).\
    - Wobble O and minimize drift.

b)  Pick sample feature in the center of image expansion while
    wobbling O. This is the object that needs to be transferred through
    the center of all following lenses. In the following it will be
    called SC (sample center)

c)  Wobble F1 and minimize drift of SC with D1.

d)  Wobble F2 and minimize drift of SC with SM(1) trim (xy-deflect.).

e)  (Wobble F2 and minimize image breathing with SM(1) trim (focus))

f)  Steer SC through center of mirror aperture with DE3 (low
    magnification)

g)  Wobble M and minimize drift with DE3.

**2: Correction 2,3 fold stigmatism, objective, 1^st^ sector**

Settings: No aperture, UV, 90°

Condition: Focused image on CCD1

Goal: Correct 2,3-fold stigmatism

a)  Wobble O and use D1 dodecapole to correct 2,3-fold stigmatism

b)  Wobble SM(1) focus and use D3 dodecapole to correct 2,3-fold
    stigmatism

**3: Focus mirror**

Settings: No aperture, UV

Condition: Sector 1 aligned, focused image on CCD2

Goal: Mirror operates at magnification 1

a)  Wobble F2 and minimize image breathing using M focus

**4: Beam alignment 2^nd^ sector, projector**

Settings: No aperture, UV

Condition: Sector 1 aligned, focused image on CCD2

Goal: Steer SC through F2, SM(2), I1,2, P1,2 using D3,D4,D5,D6,D7

a)  Wobble F2 and minimize drift of SC using D4

b)  Wobble I1 and minimize drift of SC using SM(2) trim

c)  Wobble I2 and minimize drift of SC using D5, readjust SM(2) trim

d)  Wobble P1 and minimize drift of SC using D6

e)  Wobble P2 and minimize drift of SC using D7

**5: Correction 2,3 fold stigmatism, 2^nd^ sector**

Settings: No aperture, UV

Condition: Sector 1 aligned, focused image on CCD2

Goal: Correct 2,3-fold stigmatism

a)  Wobble SM(1) focus and use D5 dodecapole to correct 2,3-fold
    stigmatism

**6: Correction spherical defocus, mirror**

Settings: No aperture, UV

Condition: Sector 1,2 aligned, focused image on CCD2, high magnification

Goal: Optimize spatial resolution by correcting spherical aberrations
with mirror. Determine curve of optimal Cs correction in Cs vs. Cc
diagram in absence of significant chromatic errors.

**7: Dispersion correction 1^st^ sector, readjust D1,2**

Settings: No aperture, UV, 90°

Condition: Sector 1,2 aligned, no drift of SC while wobbling lens
voltages, focused image on CCD1

Goal: Minimize dispersion in 1^st^ sector of separator.

a)  Wobble S and study drift. This drift should be only due to due to
    chromatic dispersion in the separator since all other elements have
    cylinder symmetry and SC passes through their optical center (step
    1)

b)  Adjust D1, D2, and SM trim coils to find "sweet spot" of separator
    while keeping SC in the center of F1, F2

c)  Finally the image should only breath with SC as the center. This
    breathing is due to defocus and has to be finally corrected using M

**8: Dispersion correction 2^nd^ sector, readjust DM3,4**

Settings: No aperture, UV

Condition: Sector 1,2 aligned, no drift of SC while wobbling lens
voltages and no dispersion in SM(1), focused image on CCD2

Goal: Minimize dispersion in 2^nd^ sector of separator.

a)  Wobble S and study drift. This drift should be only due to due to
    chromatic dispersion in the separator since all other elements have
    cylinder symmetry and SC passes through their optical center (step
    1)

b)  Adjust D3, D4, and SM trim coils to find "sweet spot" of separator
    while keeping SC in the center of F2, I1

c)  Finally the image should only breath with SC as the center. This
    breathing is due to defocus and has to be finally corrected using M

**9: Correction chromatic defocus, mirror**

Settings: No aperture, UV

Condition: Sector 1,2 aligned, no drift of SC while wobbling lens
voltages and no dispersion in SM, focused image on CCD2

Goal: Optimize spatial resolution by correcting chromatic aberrations
with mirror.

a)  Wobble S and minimize defocus with M. Stay on curve determined in
    step 6 which guarantees optimum correction of spherical aberrations.

**10: Insert aperture, repeat 1-9**

**11: Switch to x rays, repeat 1-9**