COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY

(1)

HAL Id: jpa-00225707

https://hal.archives-ouvertes.fr/jpa-00225707

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY

Th. Schiller, U. Weigmann, S. Jaenicke, J. Block

To cite this version:

Th. Schiller, U. Weigmann, S. Jaenicke, J. Block. COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-479-C2-484.

�10.1051/jphyscol:1986273�. �jpa-00225707�

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JOURNAL DE PHYSIQUE

Colloque C2, supplément au n°3. Tome 47, mars 1986 page C2-479

COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY

Th. SCHILLER, U. WEIGMANN, S. JAENICKE and J.H. BLOCK Fritz-Haber Institut der Wax-Planck.-Gesellscha.ft, Faradaywegr 4-6, D-1000 Berlin 33, F.R.G.

Résumé - Un détecteur qui détermine deux coordonnées d'impact d'un ion a été utilisé en microscopie des ions de champs. Les possibilités de ce système ont été démontrées par deux expéri- ences: 1) 1'évaporation de champs 2) l'enregistrement d'une image des ions de champs characterisés par un temps de vol sé- lectionné, la plage de détection étant réduite a quelques nano- secondes .

I - INTRODUCTION

In field ion microscopy (FIM) the registration of micrographs is usu- ally done by photographing the phosphor screen. Optical loss and the properties of phosphor screens and film materials are disadvantageous at low luminous intensity. Video intensifier tubes like SIT make it possible to record very low intensity distributions yet with loss of dynamic range, additional noise, and geometric distortion. In con- trast the direct recording of a FIM-micrograph by a position sensitive detector (PSD) at low input rates gives higher quantum efficiency than a comparable video system. The coordinates of any impinging ion are counted into a two dimensional multichannel analyser (the image memo- ry). A complete description of the imaging system can be found in /ref. 1/.

II - INSTRUMENTATION

The image processing system shown in figure I is divided into the com- ponents: resistive anode PSD /ref.2/, analog computer /ref.3/, digital image memory and host computer. The ion distribution of the FIM-image is converted and intensified by a chevron plate. Each incomming ion

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986273

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JOURNAL DE

PHYSIQUE

FIM r e a l t i m e

image display

a n a l o g d i g i t a l dual

computer p o s i t i o n - ^, ^PSD ^memory

encoding -:- video bank

D

video s i g n a l input

i n t e r f a c e h a r d

d i s k 0 < _computer ^{h o s t}

l o c a l a r e a printer

n e t w o r k p l o t t e r

Fig. 1 Block diagram of image processing system

produces an electron cloud on the active PSD area. This pulse is di- vided into four signals. An analog computer calculates the coordi- nates of the incident particle from those charges. The output vol- tages proportional to the X- and Y-coordinates are digitized by an Wilkinson ADC and fed into the image memory, where the corresponding matrix element is incremented in a fast arithmetic logic unit (ALU).

The double bufferd image memory allows the simultaneous access of a

host computer (e.g. a PDP-11/23), while the other memory is used to

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accumulate the new image. Because of the interlaced structure, any serial combination of both memories can be computed by an output-ALU in the output branch of the system. The resulting image is displayed on a tv-monitor. Therefore the observation of differences between two stored images can be done in real-time without the need of any software interaction from the host computer.

I11 - EXPERIMENTAL APPLICATION

Figure 2 shows an example of the on-line image processing facilities of the system. This is done by programming the output-ALU to compute the absolute difference between the two stored images. Identical ma- trix elements will result in dark pixels, while differences are dis- played as bright spots.

A

short field evaporation between the record- ing of image 2a and 2b produces slightly different micrographs. The obvious change at the upper left plane of the tungsten (011)-oriented tip may be directly observed in picture 2a and 2b. It is also visible in the difference-picture 2c. Even less significant alterations as in the right part of images 2a/b become distinct spots in the difference-picture, This function does the same operation as conven- tional color comparator techniques /ref. 4/ but is much faster.

Another example is the evaluation

of

random walk functions from a ser- ies of stored images which can be done in a very short time.

The combination of PSD data acquisition techniques and digital image storage opens the possibility for gated image recording. First attempts to obtain gated FI-images were made by Kellogg and Tsong /ref.

5 / .

They used a pulsed channelplate stage to synchronize to the species of interest in a pulsed laser time of flight (TOF) experiment.

Due to the high capacity of a channelplate (about 100 pF) gating in- tervalls shorter than several 10 ns are extremly problematic.

The present design circumvents this difficulty by gating the input of the memory bank. Because of the constant computation time of the analog computer (about 2 vs), the correlation of the arrival time of the species of interest to the coordinate output is no serious prob- lem.

We attempted to use this design to record the FIM's produced by

photon desorbed He using synchrotron radiation /ref. 6/. Parallel to

the sampling of the image, the TOF-spectrum was registered making use

of the signal derived from the channelplate. A properly delayed pulse

then enabled the image memory only at the time corresponding to the

arrival of the He-ion. Unfortunately, the photon flux on the tip was

extremly small at the monochromator used. Therefore the count rates

of valid events were only 2 cps leading to long recording times (gre-

ater than 10 h). To test the applicability, a simple experiment was

performed: at a high pressure of He (2*l0-~ Pa) , the TOF-spectrum

shows a flat background due to normal field ionization, where the time

correlated light- and He-gas-phase peaks are 'superimposed (image

fig. 3al. Gating the memory to the longer flight times (only normal

field ionization) results in a "conventional" FIM-image (fig. 3b). If

one masks out only the ~ e + peak, the gas-phase photoionized helium ap-

pears as a blur on a background which still allows to recognize the

tungsten tip (fig. 3c). Using the possibilities of the image proces-

sor, this background may be removed to show the "image" of the

gas-phase ions with an intensity maximum at the irradiated side of the

tip (fig. 3d).

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122-482 J O U R N A L DE PHYSIQUE

i n memory bank A

5 seconds f i e l d e v a p o r a t i o n

Fig. 2a W

in memory bank B

Fig. 2b

p r o c e s s i n g

Fig.

2c

Fig. 2 Example for hardware-controlled image processing:

Subtraction of two FIM images to show the influence

of field evaporation

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Fig. F i g .

Fig. 3 Gated time of flight image 5

n

L

d

U

C

. -

V) C aJ

+

C

-

0 250 50 0 750

^I

1000

T O F [nsl

r

time window in gated mode

J '.

H 1 1

H e / W 1,5x IQ-4Pa 5.68 kV 7 8 K

I 1 I

\

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JOURNAL

DE

PHYSIQUE

IV

-

CONCLUSIONS

PSD techniques are able to record FIM-images at very low ion intensi- ties. The application of the described image memory with hardware processing facilities allows real time image processing with simul- taneous monitoring. Gating experiments are supported by the memory system. This new method allows TOF-imaging with very high time reso- lution (1-2 ns).

REFERENCES:

1. Schiller, Thomas

Thesis; TU-Berlin; 1985

2 .

Series

3392

open face sensor

Surface Science Laboratories Mountain View; CA 94043; USA

3 .

COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY

HAL Id: jpa-00225707

https://hal.archives-ouvertes.fr/jpa-00225707

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY

Th. Schiller, U. Weigmann, S. Jaenicke, J. Block

To cite this version:

Th. Schiller, U. Weigmann, S. Jaenicke, J. Block. COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-479-C2-484.

�10.1051/jphyscol:1986273�. �jpa-00225707�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n°3. Tome 47, mars 1986 page C2-479

COMPUTERIZED IMAGING SYSTEM FOR FIELD ION MICROSCOPY

Th. SCHILLER, U. WEIGMANN, S. JAENICKE and J.H. BLOCK Fritz-Haber Institut der Wax-Planck.-Gesellscha.ft, Faradaywegr 4-6, D-1000 Berlin 33, F.R.G.

I - INTRODUCTION

II - INSTRUMENTATION

The image processing system shown in figure I is divided into the com- ponents: resistive anode PSD /ref.2/, analog computer /ref.3/, digital image memory and host computer. The ion distribution of the FIM-image is converted and intensified by a chevron plate. Each incomming ion

PHYSIQUE

FIM r e a l t i m e

image display

a n a l o g d i g i t a l dual

computer p o s i t i o n - , PSD memory

encoding -:- video bank

D

video s i g n a l input

i n t e r f a c e h a r d

d i s k 0 < computer h o s t

l o c a l a r e a printer

n e t w o r k p l o t t e r

Fig. 1 Block diagram of image processing system

The double bufferd image memory allows the simultaneous access of a

host computer (e.g. a PDP-11/23), while the other memory is used to

I11 - EXPERIMENTAL APPLICATION

Figure 2 shows an example of the on-line image processing facilities of the system. This is done by programming the output-ALU to compute the absolute difference between the two stored images. Identical ma- trix elements will result in dark pixels, while differences are dis- played as bright spots.

Another example is the evaluation

random walk functions from a ser- ies of stored images which can be done in a very short time.

The combination of PSD data acquisition techniques and digital image storage opens the possibility for gated image recording. First attempts to obtain gated FI-images were made by Kellogg and Tsong /ref.

They used a pulsed channelplate stage to synchronize to the species of interest in a pulsed laser time of flight (TOF) experiment.

Due to the high capacity of a channelplate (about 100 pF) gating in- tervalls shorter than several 10 ns are extremly problematic.

The present design circumvents this difficulty by gating the input of the memory bank. Because of the constant computation time of the analog computer (about 2 vs), the correlation of the arrival time of the species of interest to the coordinate output is no serious prob- lem.

We attempted to use this design to record the FIM's produced by

photon desorbed He using synchrotron radiation /ref. 6/. Parallel to

the sampling of the image, the TOF-spectrum was registered making use

of the signal derived from the channelplate. A properly delayed pulse

then enabled the image memory only at the time corresponding to the

arrival of the He-ion. Unfortunately, the photon flux on the tip was

extremly small at the monochromator used. Therefore the count rates

of valid events were only 2 cps leading to long recording times (gre-

ater than 10 h). To test the applicability, a simple experiment was

performed: at a high pressure of He (2*l0-~ Pa) , the TOF-spectrum

shows a flat background due to normal field ionization, where the time

correlated light- and He-gas-phase peaks are 'superimposed (image

fig. 3al. Gating the memory to the longer flight times (only normal

field ionization) results in a "conventional" FIM-image (fig. 3b). If

one masks out only the ~ e + peak, the gas-phase photoionized helium ap-

pears as a blur on a background which still allows to recognize the

tungsten tip (fig. 3c). Using the possibilities of the image proces-

sor, this background may be removed to show the "image" of the

gas-phase ions with an intensity maximum at the irradiated side of the

tip (fig. 3d).

Fig. 2a W

Fig.

Fig. 2 Example for hardware-controlled image processing:

Subtraction of two FIM images to show the influence

of field evaporation

Fig. F i g .

Fig. F i g .

Fig. 3 Gated time of flight image 5

. -

+

-

0 250 50 0 750

1000

T O F [nsl

time window in gated mode

J '.

H 1 1

H e / W 1,5x IQ-4Pa 5.68 kV 7 8 K

\

computer p o s i t i o n - ^, ^PSD ^memory

d i s k 0 < _computer ^{h o s t}