Journal of Scientific & Industrial Research Vol. 6 1, April 2002, pp 280-285

Development of Wavelength Dispersive X-ray Spectrometer

Nathai Ram·, R R Dongaonkar, Raghbir Singh, Rama Nand and R P Baj pai

Microelectronics Instrumentation Division ,Central Scientific Instrumen ts Organization,Sector 30, Chandigarh 160030

Received: 22 August 200 I; accepted:22 January 2002

A linear type wavelength dispersive x-ray spectrometer is designed, fabricated assembled, and tested for elemental analysIs. Four analyzll1g crystals (mounted on a turret) cover .the detection range from at. no. 5 (boron) to at. no. 92 (ural1lum). A dIgItal e~ect ronl c control system IS desIgned and tested for selling the crystal position with the help of a te er motor. The motIOn 01 the turret on lead screw can be controlled to a hi gh degree of accuracy with the electron ic ~O l~t~o l system .. It IS possIble to scan the x-ray spectrulll range, as mentioned above. The abi lity to scan gives easv operational flexlbl'.l~Y and opportul1lty to search over an extensive wavelength range. Samples of brass , stai nless steel , a~d mixture of fi ve adJdcent elements (Fe, Co, NI, eu and Zn) are analysed by thi s instrument. X-ray analysi s peaks of different elements pl esent , In the sample are recorded on a strip chart recorder and well resolved. This instrulllent can be utilized for x- ray fluorescence or an IIltcgral part of scanl1lng electron mIcroscope (SEM) for elemental analysis.

Introduction

X-ray spectrometer is an integra l part of a modem scanning e lectron microscope (SEM) for microanalysis purposes. As the hi gh-energy e lectron beam impinges on a specimen, electron spec imen interaction takes place. Some of the typical interactions are secondary e lectrons, characteristics x-rays, back-scattered electrons, spec imen current, e tc. Their interactions can be utilized for specimen imaging as well as e lemental analysis . Secondary e lectron s provide the best poss ible signal for surface topographic studies and the emt ss ton of characteristics x-ray photons is frequently used for material characterization. By measuring the wavelength of the x-rays and comparing the ir intensttles with suitable standards, e lemental composition in microvo lumes of the specimen can be determined. A SEM , which gives details of surface topography with great depth of focus and high resolution when fitted with x-ray spectrometric system also reveals micro-compos ition and highly accurate material characterization is also poss ible.

The wavelength and the intensity of the characteri sti cs x-ray photons emitted from the specimen are measured by x-ray spectrometer. The x-ray spectrometer can be either wavelength

* Author for correspondence

di spersive (WD) or energy di spers ive (ED). The ED x-ray ~pectrometry differs from the WD spectrometry only 111 the methods used to di sperse the severa l spectral lines emitted by the specimen. T his gives ri se to major difference in the types of in strumentation used in the above techniques. In the energy di spe rsive spectrometer all emitted lines of the specimen elements fallon the detector simultaneously. The detector consists of a solid-state semiconductor, lithium drifted silicon detectors Si (Li), and its preamplifier contained in a vacuum cryostat cooled by liquid nitrogen. It converts each absorbed x-ray photon into a current pul se whose amplitude is proportional to the energy of the charac teri stic x-ray photon . Qualitat ive and quantitati ve energy di spersive analysis is carried out on the bas is of pul se heights and their intens ities respec tively . In the WD (crystal) spectrometers, several wavelengths are separated on the basi s of thei r wavelengths before detection. Thus, an x-ray photon of one wavelength is detected by the proportional counter at a time. The reso lution of the WD spectrometer is better th an that of the ED spec trometer.

Materials and Methods

M echanical System for Lif1.ear x-ray Spectrometer

The bas ic principle of the linear x-ray spectrometer is shown in Figure I. The wavelength

.,. -

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RAM e / 01.: WAVELENGTH DISPERSIVE X-RAY SPECTROMETER 281

dispersive spectrometer complies Johann type focusing arrangement. The mechanism developed moves crystal and detector on the Rowland circle so that Bragg's condition (nA = 2d Sine) is fulfilled at all points for different A, where A is the wavelength of x-ray, d is the interplaner space, e is the Bragg angle and n is the order. The focu sing requirement is maintained by moving the ana lyzing crystal along a straight path, on a lead screw, away from the source and rotating the crystal simultaneously . The angle (e) and the linear displacement (L) are related by the equation L = 2R sine, where R is the radiu s of the focusing circle. It comes out that crystal to source di stance is directly proportional to the wave length (A = dlRL) and thi s di stance is calibrated in terms of wavelength . The crystal is automatically positioned accurately by a digital e lectronic control system with the he lp of a stepper motor. Figure 2 shows the working of the various mechanical parts of the spectrometer. The Rowland circle covers Bragg angle from 15 to 75°. The spec trometer is provided with a turret to hold four analyzing crystals of a d ifferent 'd' values, interchangeable in vacuum with provision for fine adjustment of Bragg angle setting accurately for each crystal independent ly. The ingenuity of the system lies in coupling the linear slide motion on the lead screw and the curve linear assembly with the help of belt and pu lley arrangement to achieve the desired performance. The Rowl and c irc le diam is 300 mm and the analyzing crystals are bent at thi s radius with indicated wavelength range and dispers ion are mentioned in Table I

COUNTER

L=2R Sin

ELECTRON BEAM

FOCUSING CIRCLE

ROWLAND CIRCLE CENTER LOCUS

SPECIMEN

Figure 1- Semi -foc using crystal spectromcter geomctry

Table I-Wavelength range and dispersion of various materials

Topaz

LiF

KAP

Pb St

A

0. 3 - 2.5

0.4 - 3.8

3 - 25

9 - 95

mm/A

125

75

10

3

The Rowland c ircle locus moves on a circular track . To maintain 1:2 ratio in the angular motion of the crystal and detector arms, pulleys of 2: I ratio having 300 mm diam have been used and rotation is given with a steel tape, tensioned by a spring. The alignment of crystal to counter is carried out by a sliding link arrangement. From the crystal x-rays of selected wavelength enter into the detector, which is a proportional counter. The signal from the detector is processed by the conventional e lectronic proportional counting system.

Digital Electronic Control System

A digital electronic control system has been designed for automatic operation of the spectrometer with the help of a stepper motor. The block diagram of the system is shown in Figure 3. The present figure (in terms of distance) computed from standard tab les is stored in the storage regi sters through a deci mal to BCD encoder circuit and then di splayed by seven segment LED disp lays. Also, di gita l pul ses of vary ing frequencies (500 to 10,000 pul ses/min) are generated from a crystal oscillator and a di viding network to vary the motor speed . These pul ses of selected frequency are counted and then compared with the present value by four bit magnitude comparators. When the two va lues equalize the resulting output signal closes the contro l gate, which in turn blocks further passage of pul ses to the stepper motor. Thi s stops the stepper motor and hence the spectrometer instantly . Apart from di splay ing the final set point, provi sion has been made to have a second display to ascertain the actual positi on of the crystal during motion. The present system is capab le of presenting three digits before as well as after the decimal point thus giving a high degree of accuracy . Lowest speed of 2.5 rev/mm is used for actual recording the x-ray spectrum on a chart recorder, whereas hi gher speeds are useful while searching the unknown elements by quickly scanning the whole range.

282 J SCI INO RES VOL 61 APRIL 2002

----ROWLAND ~.A""" CIRCLE

TRACK

SPECIMEN

'Y--+----------~ ______ -L-J._-

/ CRYSTAL ASSY

COUNTER

COUNTER ARM

TAPE

".--------~

\ I

. - .... ,. - --- I 1-'·

LEAD SCREW

=1======1

~ CRYSTAL SLIDE

I

--- ---)

Figure 2- Schematic di agram of linear X-ray spectrometer

COMPARATORS

CONTROL SIGNAL

Figu re 3-Block diagram of the control system

The turre t assembly of the x-ray spectrometer is driven to either s ide on a lead screw by a stepper motor. In order to prevent damage to the stepper motor/spectrometer limiting switches are provided . The turret assemb ly on reaching each of the

extremities operates a pair of micro switches _ The micro switches stop the stopper motor when the e~ treme limit is reached in clockwi se direction , the limit switch operates and stops the c lockwise rotation of the motor. Suppl y voltage fo r the anti-clockwise

RAM et at.: WAVELENGTH DISPERSIVE X-RAY SPECTROMETER 283

X·RAYS

PRE·AMPLIFIER LINEAR AMPLIFIER

GAS COUNfER

HIGH VOLTAGE POWER SUPPLY

SINGLE CHANNEL ANALYSER

COUNT RATE· METER

Figure 4-Block diagram of X-ray detec ti on and analys is

direc tion can still be applied . Similarl y the rotation of the motor is controll able when the limit is reached in the antic lockwise directi oll . The ex treme pos iti on of the crystal/turre t towards the x-ray source is taken as the reference mark fo r measuring the linear di stance ' L '. The complete system has been thorough Iy tested fo r its sati sfactory perfo rmance and repeatability e tc. and is curre ntl y in use with the x-ray spectrometer.

Detection and Recording of x-ray Spectra

Different components used in the x-ray detection, elec tronic signa l process ing and readout systems of the x-ray spectrometer are shown in the simplified bl ock diagram in Figure 4. The three mai n groups of the systems are (i) Detector, ( ii) Preamplifi e r and linear ampli fie r ( iii ) S ingle channe l analyzer, rate meter and strip c hart recorder. The detector used for x-ray spectro-chemical analys is is a gas fill ed (argon and methane) proport ional counter where the incident x-rays a re absorbed by the gas atoms liberati ng e lec trons and causi ng an avalanche . S ince a ll the electrons a re collected at the anode the tota l charge is proport ional to the x-ray photon energy. The proport ional counter (sea led) used is hav ing 50 flm thick Be window. For longer wave length region. however, f!ow-co un ter arrange­ment is made use of.

The output signal fro m a propo rti onal counter is a quantity of charge. A charge-sens iti ve preampli f ier has been des igned and tes ted . The first stage is an FET input high gain Operati onal Amp operated in the charge-sensitive mode, since it integrates the charge on the feedback capac itor. T he charge sensiti ve mode is prefe rred over the vo ltage-sens iti ve technique because its gain is not dependent on the detector capac itance or input capac itance. The charge sensitivity in the present case works out to be 0.20 x I 0 12 VIC . The second stage is a buffer amplifier connected in a bootstrapp ing mode to increase the input impedance of the vo ltage fo ll ower. T he low output impedance of the ou tput stage of the preamplifie r is suitab le to d ri ve a long coax ial cable needed fo r the main ampli fier. T he preampli fier is housed very close to detecto r mounted inside the spectromete r fo r be tte r signal to no ise rati o . The linear ampli fier, si ng le channe l analyzer, ratemeter and st ri p chart recorder have been procured from ECIL, Hyderabad for use in the x-ray spectrometer. T he x-ray spectrum as, recorded on a stri p chart reco rder is shown in hgure. 5 fo r mi xture of adjacent e lement Fe (26), Co (27), N i (28) . Copper (29) and Zn (30), F igure 6 fo r sta inless steel and Figure 7 for brass. The x-ray peab of the d iffere nt e le ments present in sample a re very w II reso lved . The

284 1 SCIIND RES VOL 61 APRIL 2002

fluctuations/noise on the base line is, however, due to the unstabi lized supply of the x-ray source.

Main Physical Parameters of Wavelength Dispersive x-ray Spectrometer

X-ray spectrometer

Take off angle of x-rays

Detectable elements

Diameter of Rowl and circle

Anaiyzing crystal

Crystal size

Crystal changing

X-ray detector

Colli mati on

Scanning speeds

Bragg angle (0) range

Mechanical drive system

Control for stepper mOlOr

11 o u 8 ;

A linear type semi focusin g (Johann design) spectrometer with four crystals on a turret Variable up to 60"

Boron (5) to Uranium (92)

300 mm

LiF, Mi ca , Topaz, PbSt (Lead stearate)

10 mm x 25 mm

Crystals are mounted on a turret and can be interchanged under vacuum

Proportiona l counter

Primary delin ing aperture, secondary collimator located in from of x-ray detector window

Switch selected , six stand ard speeds

15 to 75"

Hi gh precision stepper motor drive

Programmable with two digital displays

SAMPLE· Fe. Co. Ni. CU,Zn

ANALYZING CRYSTAL·LiF TARGET· Copper

15

Scale : 1 mm=O 006 Jl. U.

J N ~

'" M .,

WAVE!-ENGTH $..'"

10

Figure 5 - X-ray specllu lTI of the mi xture (Fe. Co. Ni , Cu , Zn ) as recorded on strip chart recorder

SAMPLE· StainJess Steel

ANALYZING CRYSTAl· liF TARGET- Copper

Scale :1mm=O.OO6 A u.

0<

'" N

'" "'" 'f ., ~

'" ., a

U '" ~

00:: '" U

, , a a .,

~ M

• '" . "­a

:ri

a N a

",'

WA~ELE~TH i ..

20

IS

b a a ~

~ a ~ ~

Figure 6 - X-ray spectrum of the stain less steel, as recorded on strip chart recorder

r---I

SAMPLE· BRASS I ANALYZING I

I ~::STAL' liF ~GET. Copper J

Scale :1mm=O.OOS$. U.

r20

I IS

>-f-ill z W

10 f-~

r /,-1

Fi gure 7 - X-ray spectrum of the brass as recorded on strip chart recorder

RAM et al.: WAVELENGTH DISPERSIVE X-RAY SPECTROMETER 285

Results and Discussion

The performance of the instrument has been

thoroughly checked and sat isfactory working is

establi shed . X-ray spectrum of a mixture of five

adjacent elements, (Fe. Co, Ni , Cu, and Zn) has been

successfully recorded. Analysis of other different

combi nations of elements by fluorescence has also

been carried out.

Conclusion

This in strument covers a wide range for

e lemental analysis, viz. starting from at. no. 5 (boron)

to at. no. 92 (uranium). ]t has good reso luti on to

resolve x-ray spectrum of adjacent elements like Fe,

Co, Ni, Cu, and Zn, etc.

References Goldstein Josep h I & Harvey Yakowitz .. Practical scallning electron microscopy (Plenum Press, New York) 1975 .

2 Bertin Eugene P, Illtroductioll to .l·rav spectrometric analysis (Plenum Press, New York) 1978 .

3 Parrish William, x-ray allolvsis papers (BI Publications, Bombay) 1965.

4 Engstrom A. Cosslett V & Pattee H. x-ray microscopy alld x-ray microallalysis (E lsevier Publishing Company. New York) 1960.

5 . Hai ls L, A precision Iill ear x -rav spectrometer, Fifth 1111 Cong x -ray Opt Microallal. edited by G Mollenstedt and K H Gank ler (Springerverleg Berlin . New York) 1969, p 21 9.

6 Lifshin E & Ciccarelli M F, SEMI1973. Proc Sixth Allllu SEM Symp , edited by 0 Joh ri (I ITR I. Cicago, Illi onoi s) 1973.

7 Barbi N C, Sandborg A O. Russ J C & Soderquist C E, SEMI1974 Proc Sevellth Allllu SEM SYIIIP , ed ited by 0 Johri (IITRI. Chicago, Illionois) 1974.