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Vacuum/volume 33/number 1/2/pages 49 to 52/1983 0042-207X/83/010049-04503.00/0 Printed in Great Britain Pergamon Press Ltd P1 2 Segregation phenomena in thin NiCr layers M Koltai, I Trifonov and M Czermann, Industrial Research Institute for Electronics HIKI, H-1393 Budapest 62, POB 348, Hungary In the case of NiCr resistor layers sputtered or evaporated a certain change occurs in their structure in the course of heat treatment and reactive deposition. The degree of this change depends on the parameters of deposition and heat treatment and it has a great effect on the TCR of the resistor layers. The depth profiles and electron-optical studies show a segregation of alloy components. Comparing the results of AES and electron- optical examinations to electrical parameters we can explain the positive shift of the TCR values by the segregation of the poorly conducting phases from the inside of the layers. Introduction The conditions of deposition have a great effect on the features of thin films because their lattice structure (which frequently differs from the bulk material), chemical composition, defect distri- bution, grain boundary, interfacial and surface properties depend on them. In a previous paper x it was suggested that the TCR and its change for thin NiCr layers are connected with an oxygen-driven Cr segregation. The aim of the present paper is to come closer to an understanding of the relationship between the electrical parameters and the chemical and crystalline structure of NiCr layers having different TCR values and obtained under different sputtering parameters. 150 I00 ._u 50 u o O -Sq After heal treatment ~t/¢ ~ ~'~----~6o/ ~5o zoo zso Sheet resistance (ohm/l~) o___~__~ Oxo~ × ~ o o Before h!at treatment Figure 1. The dependence of TCR values on sheet resistance of sputtered NiCr layers. Experimental The NiCr films were deposited in a rf sputtering plant equipped with a load-lock system and turbomolecular vacuum pump. In order to produce different NiCr films the following sputtering parameters were varied: cathode voltage (U~), argon gas pressure (PAr), partial pressure of oxygen (Po2), and sputtering time (t). Before filling the equipment with sputtering gas the apparatus was pumped down to a vacuum better than 2 x 10 -6 mbar. Apart from the parameter being changed all other parameters were held constant. The purity of argon and oxygen gases was 99.995 ~. The composition of the target was 50 wt% Cr and 50 wt% Ni. The substrates for electrical measurements and AUGER examinations were Corning 7059 glasses. For the electrical-optical exami- nations the NiCr films were deposited on Pt screens coated with amorphous SiOx. After the deposition the resistances of the layers were measured by a four point probe. The TCR values of the films were measured before and after heat treatment between 298 K and 398 K. The heat treatment of the layers took place in air in the following way: the samples were rapidly heated up to 563 K 15 5 o -IO < Figure 2. The dependence of resistance change on sheet resistance. 49

Segregation phenomena in thin NiCr layers

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Page 1: Segregation phenomena in thin NiCr layers

Vacuum/volume 33/number 1/2/pages 49 to 52/1983 0042-207X/83/010049-04503.00/0 Printed in Great Britain Pergamon Press Ltd

P1 2

Segregation phenomena in thin NiCr layers M K o l t a i , I T r i f o n o v a n d M C z e r m a n n , Industrial Research Institute for Electronics HIKI, H-1393 Budapest 62, POB 348, Hungary

In the case of NiCr resistor layers sputtered or evaporated a certain change occurs in their structure in the course of heat treatment and reactive deposition. The degree of this change depends on the parameters of deposition and heat treatment and it has a great effect on the TCR of the resistor layers. The depth profiles and electron-optical studies show a segregation of alloy components. Comparing the results of AES and electron- optical examinations to electrical parameters we can explain the positive shift of the TCR values by the segregation of the poorly conducting phases from the inside of the layers.

Introduction

The conditions of deposition have a great effect on the features of thin films because their lattice structure (which frequently differs from the bulk material), chemical composition, defect distri- bution, grain boundary, interfacial and surface properties depend on them.

In a previous paper x it was suggested that the TCR and its change for thin NiCr layers are connected with an oxygen-driven Cr segregation. The aim of the present paper is to come closer to an understanding of the relationship between the electrical parameters and the chemical and crystalline structure of NiCr layers having different TCR values and obtained under different sputtering parameters.

150

I00

._u 50 u

o O

-Sq

After heal treatment ~ t / ¢

~ ~ ' ~ - - - - ~ 6 o / ~5o zoo zso Sheet resistance (ohm/l~) o___~__~ O x o ~ × ~ o o

Before h!at treatment

Figure 1. The dependence of TCR values on sheet resistance of sputtered NiCr layers.

Experimental

The NiCr films were deposited in a rf sputtering plant equipped with a load-lock system and turbomolecular vacuum pump. In order to produce different NiCr films the following sputtering parameters were varied: cathode voltage (U~), argon gas pressure (PAr), partial pressure of oxygen (Po2), and sputtering time (t).

Before filling the equipment with sputtering gas the apparatus was pumped down to a vacuum better than 2 x 10 - 6 mbar. Apart from the parameter being changed all other parameters were held constant. The purity of argon and oxygen gases was 99.995 ~. The composition of the target was 50 wt% Cr and 50 wt% Ni. The substrates for electrical measurements and AUGER examinations were Corning 7059 glasses. For the electrical-optical exami- nations the NiCr films were deposited on Pt screens coated with amorphous SiOx. After the deposition the resistances of the layers were measured by a four point probe. The TCR values of the films were measured before and after heat treatment between 298 K and 398 K. The heat treatment of the layers took place in air in the following way: the samples were rapidly heated up to 563 K

15

5

o

-IO < Figure 2. The dependence of resistance change on sheet resistance.

49

Page 2: Segregation phenomena in thin NiCr layers

M KoltaL I Trifonov and M Czermann: Segregation phenomena in thin NiCr layers

(about ½ h) then they were kept at this temperature for 6 h. The samples were cooled down to room temperature slowly (about 4 h).

Results

Our main results illustrating the influence of film resistance on TCR and r6sistance change during heat treatment are plotted in Figures I and 2, respectively.

The TCR value for every 'as sputtered' layer is almost the same and varies between - 2 5 and - 3 5 ppm K-~ independently of sputtering conditions in the sheet resistance range of 10- 250 ohm/i--l (Figure 1 ). The grain size of every film 'as sputtered' is _< 5 nm (Figure 3). The diffraction patterns of these films show diffuse lines of NiCr. According. to the microanalyses the composition of the layers is 504-5~ Cr and Ni, respectively.

After annealing the film for 6 h and at 563 K the TCR values are very different (Figure 1). They mainly depend on sheet resistance and there are minor differences among curves characterizing the different sputtering parameters. The resistance change obtained as an effect of heat treatment gives a similar result. The curves have maxima at about 120 ohm/F] and, above a certain value of sheet resistance, the changes become negative.

According to the electron-optical examinations of these layers the sharp lines ofa Ni-rich phase appear. Beside these and beside the lines of the original NiCr matrix other weak diffuse lines can be

seen which can be indexed as Ni and/or Cr oxides (Figure 4). Simultaneously the grain size grew up to about 20 nm.

There was little difference in the quantity of the segregated Ni- rich phase for layers having different sheet resistance (R[]) but, at the highest resistance, the lines of the oxide phase were sharper. The depth compositions of the layer (I00 ohm/[]) before and after annealing are given in Figure 5. (The data are calculated from AES depth profiles.) By comparing these two figures it is seen that, as a result of annealing, a Crand oxygen-rich layer can be found at the surface and there is a Cr depletion in the inside of the layer, but the lowest value of Cr/Ni atomic ratio is found near to the surface.

Figure 6 shows the influence of oxygen (present in the sputtering atmosphere) on the electrical properties of the NiCr films. It is seen that every layer has a negative TCR value and, above a certain oxygen concentration, the TCR becomes more negative for 'as sputtered' films, while that of the annealed films hardly differs from the TCR values of the films deposited with less oxygen content.

The resistance change of oxygen-doped layers decreases for films made at higher oxygen levels. The effect of oxygen on sheet resistance is to increase it. It has to be remarked here that a pre- sputtering in argon gas was made before every charge, it lasted 1 h. Comparing the structures of these films before and after annealing (Figures 7 and 8) some small (5 nm) Ni-rich crystalline regions appear after heat treatment at 563 K for 16 h. The diffraction patterns show weak diffuse lines of NiCr and oxide phases.

Figure 3. The electronmicrograph (a) and diffraction pattern (b) of a NiCr layer before annealing. Figure 4. The elect ronmicrograph (a) and diffraction pattern (b) of a NiCr layer after annealing.

50

Page 3: Segregation phenomena in thin NiCr layers

M Koltai, I Trifonov and M Czermann: Segregation phenomena in thin NiCr layers

(o l 15 70

o 60

~ I.C 50 'd

15 40

3 0 c

zo 8

o I I I 1 5 I0 15 20 Sputtering Lime (rain)

b) t I 9o | I Cr /Ni 80

1,5 o 70

60

Eo ,0 50 ~o 2 40 c

6 30 o O5

0_.~. 20

I0

0 . I 5 I0 15 20 SpuLLerin 9 Lime (rnin)

Figure 5. The depth profile of NiCr layer (a) before and (b) after heat treatment.

[£ <~

&

6 / S - - ohm

5 • _

4 - -

3

° o!5

-i Oxygen content (%1

-I0

- 20

E - 3 o T Q.

-40 -- t~

-5O --

-60

~70

Oxygen content ( % )

05 i.O i l - -

" ~ ' ~ x ~ A f t e r heot treotment.

u

Figure 6. The influence of oxygen on electrical parameters.

7 0 A

[ 3

60 E

5 0 ~

4o g 30 ~

x= (/3

IO

Figure 7. The structure of a 1% oxygen d~ped NiCr layer before heat ,treatment: (a) electronmicrograph, (b) diffraction pattern. Figure 8. The structure of 1% oxygen doped NiCr layer after heattreatment: (a)electronmicrograph, (b) diffraction pattern.

5 1

Page 4: Segregation phenomena in thin NiCr layers

M Ko/tai, / Trifonov and M Czermann: Segregation phenomena in thin NiCr layers

L5

o

t~

~6

,3 0.5

( o )

I I I 5 IO 15 Spu t te r i ng t ime (rain)

7o

6e *~

4o g

3o ~

2 0

I0

.l o

u

G 0•5

( b ) I I I I I

C r / N i . . ~

I I I 5 I0 15

S p u t t e r i n g t ime (rain)

9 O

- 6 o aa

so ~ ~° ~ 3o ~

2 O

- - I 0

Figure 9. The depth profiles of NiCr layers doped with 1% oxygen (a) before and (b) after annealing.

As can be seen from the depth profiles of the layer doped with 1% oxygen (Figure 9) the oxygen content of the films increased and remained unchanged over a certain depth interval after annealing. The Cr/Ni atomic ratio in this interval is low and it is even lower after heat treatment. According to the microanalyses the Cr/Ni ratio for the whole layer does not differ from that of the target (i.e. it was 50/50 Cr/Ni). The width of Cr and oxygen-rich layers is about the same as the ones obtained after annealing when there was no oxygen doping.

D i s c u s s i o n

Comparing the results obtained on layers made under different sputtering conditions it can be concluded that the films having a grain size of _<5 nm have about the same TCR values. As a result of annealing a surface oxidation occurs and, because the activity of Cr to oxygen is higher than that for Ni, Cr moves out to the surface leaving the inside of the layer rich in Ni over a certain region. Because of the fact that the layers with higher Ni content have lower resistivity and more positive TCR values and the fact that the thickness of the surface oxide is about the same at different layers, the change of TCR values and resistance during annealing can be explained by a certain separation of the alloy components. The higher the surface and interfacial fractions of the total films the higher the effect of the segregation on films. That means in our case that with increasing sheet resistance (between 10 and 25 ohm/[]) resulting as an effect of heat treatment metallic properties get more emphasized. This is shown by the more positive TCR values and by the resistance change curves with their maxima.

Apart from oxidation oxygen has another influence on the NiCr layers: it can stabilize a structure having a very small grain size. This phenomenon was observed earlier for evaporated NiCr layers 2-4. This is the reason for the TCR values remaining negative for oxygen doped layers.

A c k n o w l e d g e m e n t s

We are very grateful to O Geszti of the Research Institute for Technical Physics of the Hungarian Academy of Sciences for the electron-optical investigations and to M MenyhArd, T Karfinyi of the Research Institute for Technical Physics of the Hungarian Academy of Sciences for the AES investigations.

References

A Hanusovszky, M Koltai and I Trifonov, International Summer School on Processes of Thin Film Formation. 28 September-4 October, 1980 Fony6d, Lake Balaton, Hungary. Thin Solid Films (to be published). 2 L Lassak and K Hieber, Thin Solid Films, 17, 105 (1973). 3 K Hieber and L Lassak, Thin Solid Films, 20, 63 (1974). 4M I Birjega, C A Constantin, M M Paraschiv and N G Popescu- Pogrion, Rev Poum Phys, 16, 1229 (1971).

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