THE INFLUENCE OF X-RAY DAMAGE ON ELECTRON-INDUCED INTERFACE DEGRADATION IN MOS CAPACITORS

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THE INFLUENCE OF X-RAY DAMAGE ON ELECTRON-INDUCED INTERFACE

DEGRADATION IN MOS CAPACITORS

U. Schwalke, E. Jacobs, B. Breithaupt

To cite this version:

U. Schwalke, E. Jacobs, B. Breithaupt. THE INFLUENCE OF X-RAY DAMAGE ON ELECTRON-

INDUCED INTERFACE DEGRADATION IN MOS CAPACITORS. Journal de Physique Colloques,

1988, 49 (C4), pp.C4-299-C4-302. �10.1051/jphyscol:1988462�. �jpa-00227960�

JOURNAL DE PHYSIQUE

Colloque C4, supplhment au n09, Tome 49, septembre 1988

THE INFLUENCE OF X-RAY DAMAGE ON ELECTRON-INDUCED INTERFACE DEGRADATION IN MOS CAPACITORS

U. SCHWALKE, E.P. JACOBS and B. BREITHAUPT*

Siemens AG, Corporate Research and Development, Microelectronics, 0-8000 Miinchen 83, F.R.G.

"~ietec, Arbeitsgruppe Mikrostrukturtechnik, 0-1000 Berlin 13, F.R.G.

Abstract

-

The radiation response of MOS capacitors and their degradation resistance after annealing has been investigated. Compared to unexposed samples, irradiated and subsequently annealed MOS capacitors were found to be more prone to electron-induced interface degradation. The enhanced degradation correlates with the initial radiation damage and is lowest in TaSia silicide gates. To which extent hydrogen contamination of the oxide or mechanical strain may account for the observed results will be discussed.

The realization of fine-line CMOS structures with dimensions < 0.5 pm will require advanced lithography and etching techniques. X-ray lithography has proven to be a promising technique for future submicron semiconductor fabrication /1/ and reactive ion etching is already in use for micron and submicron feature patterning. Since photon energies used in x-ray lithography and the energies of the intense radiation generated in plasma etching systems are well above the SiOz band gap, electron-hole pairs are generated in the oxide during irradiation. As a result of hole-trapping and interface-state generation, structural and chemical changes occur near the SiOz/Si interface /2/ which will damage the oxide. The radiation response is known to be affected by hydrogen contamination /3/ introduced in the oxide during processing as well as mechanical strain /4/ induced by gate electrodes.

Usually, radiation damage is removed by low temperature (300°C I T < 500°C) forming gas metallization anneals. However, little is known regarding the degradation behavior of annealed MOS devices. There is concern that the relaxation of the oxide damage is unstable, leading to enhanced device degradation during operation /5/. Especially in submicron devices with increased internal electrical fields and more severe hot-carrier problems this may become a serious limitation for device reliability.

In this work, the radiation response of MOS capacitors with different gate materials and gate oxide thicknesses as well as their degradation resistance after annealing have been examined.

- ,

MOS capacitors used in this study were prepared by means of a standard CMOS process on 4-inch <loo> Si wafers. Gate oxides of 11 to 45 nm were grown at 900°C in dry O2+HCL ambient and as gate electrodes n+-poly-Si, n+-(TaSi=)- polycides and TaSiz-silicides were used respectively.

All x-ray exposure experiments (EpH=1.6keV) were performed at the lithography beam line of the BESSY Synchrotron Facility in Berlin. Only specified areas of the wafers were irradiated so that several differently exposed as well as unexposed reference chips could be obtained on the same wafer. To be close to practical processing conditions, no bias was applied to the devices during

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988462

JOURNAL DE PHYSIQUE

irradiation and the exposures were in the sensitivity range of the resists envisaged for x-ray lithography. The x-ray doses absorbed in the gate oxide with various gate electrodes were calculated by means of the XMAS program /6/.

Compared to n*-poly-Si gates, the total dose absorbed in SiOa was found to be reduced by = 30% for TaSia silicide and polycide gates. Annealing of the exposed wafers was either performed at 450°C in forming gas for 15 minutes or at 700°C in pure El2

.

Electron-induced oxide degradation was accomplished by Fowler-Nordheim constant-current injection from the Si-substrate (positive gate bias = PGB) as well as from the gate electrode (negative gate bias = NGB)

.

The interface state density (Dl.-) spectrum was calculated from the HF and quasi-static C(V) curves /7/ whereas flatband voltage shifts (

Avaa)

were determined from the HF-C(V) curves.

3

- - - m

In Fig.lA radiation-induced negative flatband-voltage shifts as a function of the incident dose are shown for MOS capacitors with different gate electrodes but the same oxicle thickness of to, = 25 nm. The V,, shifts increase with dose and saturate at ~5 0.3 J/cm2 due to space charge buildup in the Si02 layer. For different gate materials large variations in the saturation

Av,,

values were observed which are paralleled by the DL- spectra shown in Fig.lB. The radiation damage was lowest in T a S L silicide gates which is mainly attributed to the corrective mechanical stress created by the silicide _{/ 8 / .} Consistent with the strainecl-bond model /4/ a corrective compressive strain in the SiOZ induced by the TaSi2 film would compensate for the intrinsic strain induced by oxidation. As a result of the increased Si-0 bond strength at the interface, an increased radiation hardness (Fig.1) and an improved hot-carrier stability /9/ is observed. On the other hand, in n+-polycide gates the mechanical stress of the silicide is expected to be efficiently buffered by the 300 nm poly-Si layer so that the radiation response should be comparable to n+-poly-Si gates.

However, due to the increased amount of hydrogen incorporated into the oxide during processing (900°C silicide formation anneal in forming gas) a largely increased radiation sensitivity is obtained, consistent with previous results / 3 / .

Since oxide thicknesses will be reduced in scaled CMOS devices, we have also studied the radiation response as a function of to,. For tox values of 2 20 nm,

Av,,

is proportional to the oxide thickness, indicating a constant radiation-induced trapped oxide charge of

-

5.5 x 10-8 C/cm2. However, as to,

-

- 1

n+ polycide gate

5

-0.5

1 1

n+ poly-Si gate

I

TaSiz gate

0

;: b:

; I

0 0.5 1

Incident Dose ( ~ / c m ' )

Fig.1: VaB shifts (A) and DL- distributions (B) of MOS capacitors with different gate electrodes after x-ray exposure.

-

_10'~

?E

- -

2 ^.-

..

n 10"

10"

t' X - r a y dose : 0: 95 J /cmZ

I n+ polycide :'

2

1

y'

-

TaSiz

- \ *

*..*...+

t*. 1

. ..

I

I I

Fig.2: Dependence of AvFB (A) and D L ~ (B) on oxide thickness.

- 0 . 9

-0.8

is reduced below 20 nm, a departure from linear dependence is observed (Fig.2A), suggesting a reduced hole trapping in thin oxides. The beneficial effects of thin oxides with respect to radiation hardness are even more obvious in the Dat spectra shown in Fig.2B. Again, reduced interfacial stress present in thin oxides or different growth properties during initial oxidation may account for the increased radiation stability.

-

n+ poly -Si g a t e

-

Absorbed dose : 1700 J/crn3

-

After a brief 450°C forming gas anneal, a complete relaxation of the radiationlinduced damage has occurred, as shown in Fig.3. However, annealing up to 700°C in Hz-free ambients did not result in a complete reduction of the

-0.7 -

- 0 . 6

-

-0.5 -

0 10 20 30 4 0 50 60 tox ( n m )

n+

-

poly -Si gate

exposure

1 1 1 t c 1

- 4 -3 - 2 -1 0 1 2 3 4 Gate Bias ( V )

Fig.3: C(V) curves illustrating recovery from radiation-induced oxide damage after 450°C anneal in Nz-Hz.

-0.6

1

n+ poly - S i g a t e

to, = 15 nm

Absorbed dose : 1700 J/crn3

Irradiated /& annealed

0 1 2 3 4 5 6x16'

Injected Charge ( ~ l c m ' j

Fig.4: Vsa shifts as a function of injected charge.

C4-302 JOURNAL DE PHYSIQUE

radiation damage. This result suggests that H-atoms bonded at defect-sites in Si02 are responsible for the relaxation of the radiation-induced oxide damage.

During subsequent constant-current injection experiments the irradiated MOS capacitors were found to be more prone to interface degradation compared to the unexposed ones, as evident from the VFB shifts (Fig.4) and the Die spectra (Fig.5A). The enhanced degradation is explained by the reduced bond energy of the Si-H bonds ( 5 3 eV) which are therefore more easily broken then Si-0 bonds

( = 5 eV). Consistent with this interpretation, the susceptibility to electron-

induced interface degradation was found to correlate with the initial radiation damage level and was lowest in TaSia silicide gates (Fig.5B).

n+

-

Poly-SI Gate X - r a y dose : 0.95 J/C~'

After X - ray exposure

-

^{10'2 -} and N2-Hz anneal

?E

.-

-

2

-

^.-

a

NO exposure

0 1 2 3 x 1c

lnjected Charge ( c/cm2 ) lnjected Charge ( clcm2 )

-

^10"

?E

-

Fig.5: Electron--induced interface degradation in n+-poly-Si (A) and TaSin gates (B) with tox = 25 nm.

TaSi2 Gate

X - r a y dose : 0.95 ~/cm' -

After X - ray exposure and N2-HZ anneal

4

-

-c

We have shown that low temperature metallization anneals in forming gas will only conceal radiation-induced oxide damage. Compared to unexposed samples, an increased suscep1:ibility to electron-induced interface degradation was observed which was found to correlate with the initial x-ray damage level.

Nevertheless, radiation hardness and hence degradation resistance can be improved by the proper use of advanced gate materials and thinner oxides as well as avoiding H2 contamination of the oxide during processing.

-

2 .- * cl

0 1 2 3 x l o 2

REFERENCES

/1/ A. Heuberger, in Microcircuit Engineering 1986, Ed. H.W. Lehmann North- Holland, Amsterdam.

/2/ E. F. da Silva,.Y. Nishioka and T.P. Ma, Appl. Phys. Lett.

51,

270 (1987).

/3/ Y. Nishioka, E.F. da Silva and T.P. Ma, IEEE Trans. Nucl. Sci., NS-34 (1987) 1166.

/4/ V. Zekeriya and T.P. Ma, IEEE Trans. Nucl. Sci., NS-31 (1984) 1261.

/5/ F.C. Hsu, J. Hui and K.Y. Chiu, IEDM Techn. Dig. (1985) p.48.

/6/ H. Oertel, H. Eietz and A. Heuberger, Microelectr. Eng. 3 (1985) 387 /7/ R. Castagng and A. Vapaille, Surface Sci. 28 (1971) 157y

/8/ F. Neppl and U. Schwabe, IEEE Trans. Electr. Devices, ED-29 (1982) 508.

/9/ A. Lill and M. Orlowski, ESSDERC '85 Europhys. Conf. Abstr., (1985) 106.