A scattering chamber for thin film analysis

R. Tonini, M. Benassi, M. Bosi and A. Magnani

Università degli studi di Modena e Reggio Emilia -Dipartimento di Fisica via Campi 213/A – 41100 Modena

INTRODUCTION

Most of the research activities performed by the MOSS (MOdena Stato Solido) group are dealing with thin films: composition, formation, stability, electrical and mechanical properties are some of the aspects investigated. Therefore the research needs the use of various techniques and the ones based on nuclear interactions are important. In fact they are the first to be used because they give information on elemental composition and uniformity. Prompt nuclear reactions (NRA) and Rutherford Back Scattering (RBS) are used for elemental composition, NRA for light elements and RBS for heavy elements; RBS is also used to control the in-depth distribution of elements. Forward scattering, called Elastic Detection Analysis (ERD), is used to profile hydrogen and helium, MeV 4He+ ions are used for hydrogen and 15N++ for helium and hydrogen. Few experiments are also performed to establish the limit of a particular technique or to implement it. Generally we need to analyze many samples, for instance a study of kinetic of a compound formation needs to analyze between 50 and 100 samples. Therefore, the necessity to use the allocated beam time at the best , it is absolutely necessary to have an automatic and/or semi-automatic system. Moreover the exact definition of the geometry is mandatory mainly when complex films with many elements are investigated. To satisfy our needs we decide to build a scattering chamber which allows to automatically analyze many samples, having a reproducible and fixed mechanical geometry.

EXPERIMENTAL SETUP

The requirements for the scattering chamber are: 1. the sample holder should be light and independent

from the whole chamber 2. the possibility to mount more than thirty samples 3. the sample should be changed automatically 4. the surface of the sample should be always at the

centre of the scattering chamber 5. the data should be taken from two different

detectors 6. the possibility to change the tilt angle (direction

of the beam and normal to the sample surface) from -60° up to 180°

Fig. 1 shows a picture of the whole scattering chamber, Fig.2 is a detail of sample holder with the two detectors and Fig. 3 shows the octagonal sample holder alone. In the same picture is also shown one of the eight slides used to mount 4 samples. The samples are placed against a reference plane by a suitable spring; they are changed by two stepping motors operating in vacuum. The detectors

are horizontally moved, from 20° up to 270°, by two stepping motors placed outside the chamber. By software and by mechanical switches the contact between the detectors are prevented. To suppress electrical noise the data acquirements are performed with the motor power switched off.

FIG. 1 General view of the scattering chamber

FIG. 2 Close view of the sample holder and the two detectors. The sample holder can rotate and can move up and down

FIG. 3 The sample holder has eight faces and each one have a slide where four samples can be mounted. The figures shows seven slides mounted and one detached. The samples are pushed toward a reference plane by a spring

Several measurements have been performed to establish the limits and the precision of the scattering chamber. The tests are not finished yet, but several preliminary results show that the reproducibility in sample position is better than 0.2 mm, while the detector angle is known better than 2%. The sample can be tilted with precision less than 0.2 degrees. A red LED, see Fig.2, define the centre of the scattering chamber and is used to position the beam. One of our research projects is the study of stability of chalcogenides (Ge2Sb2Te5-GST) in presence of a titanium film, as a function of the annealing temperature. The samples have been deposited in argon atmosphere using a sputtering apparatus. In Fig. 4 and 5 are reported two spectra taken in the same sample with two different detectors placed at 150 and 110 degrees respectively. The spectra are obtained in a as-deposited and 450 C 15 min annealed samples. The results allow establishing that

450 500 550 600 650Channel

0

2

4

6

8

10

Yie

ld(#

/uC

/keV

/msr

)1/2

1.8 2.0 2.2Energy (MeV)

PV1512 RBS SGS RBS #2 70GST/5Ti/20TiN as prepPV1513 RBS SGS RBS #2 70GST/5Ti/20TiN 450C 15min

FIG.4 RBS spectra taken with detector at 110°

titanium diffuses in the GST layer and a small interaction occurs at the front surface of GST. Moreover comparing the data from the two detectors it is possible to exclude a role of argon.

450 500 550 600 650Channel

0

2

4

6

8

10

Yie

ld(#

/uC

/keV

/msr

)1/2

1.8 2.0 2.2Energy (MeV)

PV1512 RBS SGS RBS #2 70GST/5Ti/20TiN as prepPV1513 RBS SGS RBS #2 70GST/5Ti/20TiN 450C 15min

FIG. 5 RBS spectra taken with detector at 150°