HVTH: Vo Thi Ngoc ThuyGVHD: PGS.TS L Vn HiuNG DUNG PHAT XA NHIT IN T TRONG SEM

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(1889-1982), Russia

1. GII THIU V KINH HIN VI IN T QUET SEMKinh hin vi in t SEM u tin c phat trin bi Zworykin vao nm 1942.

n nm 1948 C. W. Oatley phat trin kinh hin vi in t quet trn m hinh nay vi chum in t hep co phn giai n 500 A0.

Kinh hin vi in t quet thng phm u tin c san xut vao nm 1965 bi Cambridge Scientific Instrument Mark I. LC S V KINH HIN VI IN T SEM Charles Oatley (1904-1996)

2. Cu tao cua SEM Kinh hin vi in t gm co cac b phn sau:

Sung in t

H thu kinh t. (H thng cac cun quet, c t gia thu kinh th hai va th ba).

B phn gi mu

H thng thu nhn anh. (ng nhn quang in, dung ghi nhn chum in t th cp).

2.1 Sung in tSung phong in t tao ra chum in t vi kich thc im nho, nng lng co th iu chinh c va tan sc nho. Sung in t thng gm ba phn: si t (tungsten) hinh ch V, hinh tru Wehnelt (iu chinh dong phat xa e-) va anode Cng chm in t pht ra s tun theo nh lut Richardson: J = A.T2.exp(-F/kT)

Comparison of Electron Sources at 20kV

UnitsTungstenLaB6FEG (cold)FEG (thermal)FEG (Schottky)Work FunctioneV4.52.44.5--Operating TemperatureK27001700300-1750

Current DensityA/m25*1041061010--Crossover Size m5010

2.2. H thu kinh tH thu kinh t co tac dung tp trung chm in t va c pht ra khi sng phng in t v iu khin kch thc cng nh hi t ca chm tia

Thu kinh tu sang (condenser lens): Tp trung chm in t va pht ra khi sng phng v iu khin kch thc cng nh hi t ca chm tia. Vt kinh (Objective len): Chum in t tip tuc phn ky sau khi i qua thu kinh tu sang Vt kinh thng c dung hi tu chum in t vao im do trn b mt mu giup hi tu tt hn.

Thu knh t thc cht l mt nam chm in, c cu trc l mt cun dy cun trn li lm bng vt liu t mm. V cun dy mang dng in nn n ta rt nhiu nhit v i hi mt h lm lnh (bng nc hoc Nit lng)Hinh 2.4: Cu tao cua thu kinh tHot ng da trn nguyn l lch ng i ca in t trong t trng di tc dng ca lc Lorentz. T trng trong thu knh c thay i bng cch thay i cng dng in trong cun dy =>Tiu c cua thu kinh c thay i bng dong dung iu khin nam chm. Ban kinh quy ao cua in t c xac inh theo cng thc:

. Khe t to ra t trng co phn b thch hp iu khin qu o ca in t.Ta c th iu khin qu o ca in t bng cch iu khin s phn b ca t trng B trong khe t. Hinh 2.5: S truyn qua cua in t qua thu kinh t

2.3. H THNG THU NHN VA TAO ANH

3. NGUYN TC HOAT NG CUA KINH HIN VI IN T SEM

phn gii cua kinh hin vi SEM phn giai cua SEM ph thuc vo kch thc ca chm in t p vo mu. Thng thng, kch thc chm in t ln hn kch thc nguyn t , do o SEM khng phn tch c cp nguyn t. Cac kinh SEM hin ai co phn giai trong khoang 1nm-10nmCac yu t anh hng n phn giai cua kinh SEM: ng kinh chum in t, th gia tc, dong do .v.vgiam ng kinh chum in t n mu , dung khu

Hin tng quang sai (Aberration)Hin tng quang sai la hin tng sai lch anh thu c qua dung cu quang hoc. Cu sai Sc saicc chm tia xa trc chnh s hi t km hn so vi cc chm tia i gn trc chnh v do cng to ra mt a tn rng thay v hi t ti mt im.Do s km n sc trong bc sng ca chm ht mang in.

The focal length of near axis electrons is longer than that of off axis electrons

All lenses have spherical aberration -minimum spot size dmin = 0.5Cs a3

Cs is a lens constant equal to the working distance of the lens

Spherical aberration makes the probe larger, degrades the beam profile, and limits the numerical aperture (a) of the probe lens. This reduces the current IB which varies as a2Cu sai ( Spherical aberration)DOLCa

Sc sai (Chromatic aberration)The focal length of higher energy electrons is longer than that for lower energy electronsChromatic aberration puts a skirt around the beam and reduces image contrastThe minimum spot size at DOLC is dmin= Cca.DE/E0 which increase at low energies and when using sources such as thermionic emitters with a high energy spread DEa

Cach khc phuc quang sai1/ n sc Chon ngun phat xa nhit thich hp Giam Giam quang sai va tng phn giai

2/ Chiu chum xung, giam th ap vao in cc, giup cho vic hi tu cac e- chm va e- nhanh hn cung luc.

3/ Giam tiu c vt kinh (bng cach iu chinh dong (th).

4/ t thm khu .

Mu c t vao trong Tk -> giam f-> giam cu sai

4. U IM CUA SEMKinh hin vi in t chung ta co th bit c cu truc tinh th cung nh b mt cua mu vt.

Phn tch m khng cn ph hy mu vt

Hot ng chn khng thp.

Tc thu d liu nhanh.

phn giai cao

Vic chun bi mu n gian Gi thnh ca SEM thp hn rt nhiu so vi TEM.