Contamination of Samples in Sem

8/13/2019 Contamination of Samples in Sem

1/19

Electron Beam-Induced ContaminationElectron Beam-Induced Contamination

in the Scanning Electron Microscope:in the Scanning Electron Microscope:

a Thing of the Pasta Thing of the Past

Andrs E. ladr! "a#uri P. Purushotham

and Michael T. Poste$

%anometer Scale Metrolog& 'roup

National Institute of Standards and Technology (NIST)

andras(nist.go#

Certain commercial e)uipment is identified in this *or$ to ade)uatel& descri+e the e,perimental procedure. Such identification does not impl& recommendation orendorsement +& the %ational Institute of Standards and Technolog&! nor does it impl& that the e)uipment identified is necessaril& the +est a#aila+le for the purpose.


2/19

SEM imensional Metrolog& Challenge

Integrated circuit and nano-technolog& samples:

real three-dimensional structures *ith much smaller than // nm in si0e

1anted: Accurac& and precision in si0e and shape determination at atomic le#els

2o* close can *e get to this goal *ith an SEM3


3/19

Electron-Beam-Induced Contamination

4 Electron +eam-induced contamination is one of the most +othersome pro+lemsof the scanning electron microscopes 5SEMs6. E#en in clean-#acuum

instruments a pol&meri0ed h&drocar+on la&er *ith lo* secondar& electron &ield

can get deposited under the electron +eam.

4 This means that in the SEMs! repeated measurements cannot +e done *ithout

e,tra! sometimes unaccepta+l& high measurement errors. uring the time

necessar& for e#en one measurement! the sample dimension can change.

4 The e,tent of this change remains un$no*n unless a suita+le contamination

deposition measurement techni)ue is found and regular monitoring is

implemented.4 There are se#eral methods to diminish the rate and amount of contamination

deposited under the electron +eam of the SEM.

The +est is to get rid of the pro+lem altogether! *hich is no* possi+le.


4/19

Contamination Caused +& Electron BeamContamination Caused +& Electron Beam

4 A d&namic process of adsorption and desorption of a mi,ture oft&picall& organic molecules

4 Mostl& secondar& 57 8/ e6 electrons cause the +uild-up

4 Mostl& primar& 5high energ&6 electron $noc$ out molecules and cleanthe surface

4 The molecules ma& ha#e #er& high surface mo+ilit&4 Causes 9dimensional gro*th: as high /. nm;sec < gro*th rate has

+een o+ser#ed

4 Sources

= sample: surface 52>?! C?>! process materials6 and +ul$ 5resist! sol#ents!

etc.6= en#ironment: oil& residues on the sample stage and cham+er and the

9good and 9+ad 5and ugl&6 #acuum

@egular monitoring and effecti#e cleaning are indispensa+le


5/19

Electron Beam-induced Contamination

C-SEMs usea rectangular

+eam retrace

par$ing

pattern.

Some

la+orator&

SEMs par$

the +eam

mostl& at the

right for

retrace.


6/19

Electron Beam-induced Contamination Can

Be Trul& Bad

Contamination pattern formed on a silicon chip sample during > hours of

continuous +om+ardment. The right image is *ith 8 sample tilt.


7/19

nrelia+le Critical imension

Measurements ue to Contamination

The results of 8/ repeated line *idth measurements *ith t*o C-SEMs

using the same 5D nm6 photoresist *afer.


8/19

Contamination Pattern at 2igh Magnifications

At high magnifications! i.e. small field-of-#ie*s the deposited contamination pattern

does not follo* the irradiation pattern. A 8/ $, magnification image of the Si 9grass

sample after / minutes of continuous 8 $ / pA +om+ardment. The insert and the

*hite frame sho* the 8// $, image.


9/19

Fighting Contamination - Gi)uid %>Trap;Cold Finger

Silicon 9grass sample irradiated for / minutes *ithout 5left6 H *ith

5right6 using li)uid %>cooled anti-contamination de#ice! 8/ $,


10/19

Fighting Contamination - Go*-Energ& ?,&gen

Plasma

Silicon 9grass sample irradiated for / minutes +efore 5left6 and after the

use of E#actron anti-contamination de#ice! 8/ $,


11/19

@emo#ing Contamination - Go*-Energ& ?

Plasma

Silicon 9grass sample *as irradiated for / minutes. The left image *as ta$en

after contamination deposition and is sho*n untreated. The right image *as ta$en

after a /-minute in-situ treatment of the sample *ith E#actron anti-contamination

de#ice. 8/ $, magnifications


12/19

JEI Scientific E#actron Anti-contaminators

4 Automatic cleaning de#ices use lo*-

energ& 58 to / 16 o,&gen plasma

4 se the o,&gen in the air! +ut other

gases can +e used as *ell

4 Plasma and nitrogen cleaning c&cles can+e com+ined

4 ?nce the cham+er has +een cleaned up!

usuall& a fe* minutes are enough to get

+ac$ to contamination-free operation

Schematic diagram of the E#actron

cleaning head

?ther manufacturers ma$e similar de#ices


13/19

%IST Contamination Specification

4 %IST is ad#ocating a contamination specification to +e included among otherspecifications of all SEMs.

4 %IST SEM cleaning procedures and a #ia+le contamination specification offer

an effecti#e solution for this pro+lem.

4 Specification:= on a sample of amorphous Si patterns on a Si su+strate 5%IST scale cali+ration chips

or *afers6! using the +est resolution imaging parameters ta$e one image at // ///

times magnification.

= sa#e the image! and *ithout mo#ing to a ne* location! go up to >// /// times

magnification and continuousl& image the sample for / minutes! and finall&!

= go +ac$ to // /// times magnification and ta$e another image.

= If there is an& #isi+le dar$ening! frame! an& contamination structure +e&ond the

sample itself on the last image! the instrument fails to meet this specification.


14/19


4 If the specification *as not met! first clean the sample in the mi,ture of

: concentrated sulphuric acid to / K h&drogen pero,ide solution 5acid

piranha solution6. This ferocious o,idi0er *ill clean all h&drocar+on

residues from the sample in less then / minutes. If the instrument -*ith

the clean sample- fails the test again! it needs to get cleaned *ith a lo*-

energ& plasma cleaning process.

4 At the +eginning an o#ernight cleaning *as needed to meet the

contamination specification. Gater! as the instrument graduall& cleanedup! hour or e#entuall& onl& / minute long cleaning procedures *ere

sufficient.


15/19


4 It is important to point out that the nascent 5ioni0ed atomic6o,&gen generated +& the plasma cleaner o,idi0es man& materials!

+ut the process ad#antageousl& is #er& effecti#e on h&drocar+on

residues.

4 It is recommended to use the minimum! +ut sufficient time and

plasma current.

4 This calls for a relia+le and regular monitoring of the

contamination performance of all SEMs.

4 The %IST scale cali+ration samples 5@M and S@M chips and

*afers6 are suita+le for contamination performance measurementsContamination is unaccepta+le! and no* it is possi+le to

achie#e contamination-free SEM operation.


16/19

%IST Contamination Specification @esults

At high landing energies: essentiall& no contamination

After / minutesAt the +eginning of the test


17/19


At lo* landing energies: actual cleaning is o+ser#a+le



18/19


2elium Ion Microscope 52IM6

Essentiall& no contamination



19/19

Contamination-Free Scanning Electron Microscop&Contamination-Free Scanning Electron Microscop&

Electron +eam-induced contamination is one of the *orst pro+lemsof scanning electron microscop&! especiall& for nano-scalemeasurements.

4 It can +e no* essentiall& eliminated

4 "e& re)uirements to the solution are clean #acuum s&stems! theuse lo*-energ& plasma to clean the sample cham+er of the SEMand a procedure that &ields clean samples.

4 %IST no* has se#eral scanning microscopes that are essentiall&contamination-free.

1e hope that our successful *or$ on the elimination of SEMcontamination *ill ser#e the electron microscopist communit& all

o#er the *orld.

andras(nist go#

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Contamination of Samples in Sem