Buried Si3N4 layers in silicon produced by high-intensity implantation and rapid thermal annealing

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Buried Si3N4 layers in silicon produced by highintensity implantation and rapid thermal annealing R V Gribkovskii, Kurchatova

F F Komarov

7, SU-220064

and A P Novikov,

Minsk,

institute

ofApplied

Physics

Problems,

USSR

It is established that stoichiometric fl-SI,N, layers are formed by in-situ implantation 7~ 1017 cmM2 and implantation temperatures equal to or higher than 800°C. At lower formed after RTA at all implantation temperatures.

1. introduction The formation of SiOZ or Si,N, buried layers in silicon by highcurrent ion implantation of oxygen or nitrogen is one of the promising techniques to fabricate SOI structures. However, there are some processes in silicon crystals under high-intensity implantation conditions which are not well understood”. The present work aims to study the dependences of the implanted nitrogen distribution and buried layer structure on the ion dose and beam current density. 2. Experimental Silicon single crystals of 1 x 1 cm’ were implanted with a stationary beam of Nt ions at energy of 140 keV to doses ranging from 10” to 10” at cm-‘. The temperature due to ion beam heating was estimated to be about 600,800 and IOOO’C for the ion current density of 50, 100, 250 ,uA crn~.‘, respectively. Rapid thermal annealing was performed by a halogen lamp system’ at 1300°C for 20 s with a ramp-rate of 130°C s- ‘. Before and after annealing the samples were examined by Rutherford backscattering, ir

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Figure 1. Depth distribution of nitrogen implanted in silicon with doses : (a) and (c)-l.5 x IO”; (b) and (d)-9.5 x 10” and corresponding irspectra(Ti:...,600;--~,SOO;and~.~, 1OOO’C).

at doses higher than doses, thin cx-Si,N, layers are

transmission and transmission electron microscopy/transmission high energy electron diffraction. 3. Results and discussion The distribution of nitrogen atoms in silicon implanted with the lowest and the highest doses of N: ions is shown in Figure 1. At the initial stage of irradiation an increase in ion current density is accompanied by disappearance of the asymmetry and diffusive blurring of the profiles. These effects are due to the temperature rise with ion current density. High implantation doses cause the profiles shape to transform from a Guassian-like to a trapezoidal one after the implantation temperature reached 800°C. The constant concentration region is formed from the area of the maximum exclusively towards the surface. Simultaneously, a small general profile shift is observed which is also directed towards the surface. With ion implantation at a temperature of IOOOC, nitrogen saturation takes place at a somewhat smaller concentration and the profile shift is slightly greater. The shift of the distribution towards the surface may be due to a more effective sputtering of silicon as the beam power increases. The transformation of the distribution is caused by the simultaneous action of the high temperatures and the effective generation of nonequilibrium point defects. The direction of under-stoichiometric nitrogen redistribution is determined by an increased concentration of vacancies on the silicon overlayer. The same figure shows the ir-transmission spectra for these samples. The spectra obtained for the samples irradiated with the lowest nitrogen doses have a wide absorption band with a to the oscillation maximum at - 800 cm- ’ which corresponds energy of Si-N bonds. As the ion current density increases, the absorption on the N-SiLN bonds also increases (k 500 cm- ‘). Therefore, one can say that there exists a more effective generation of stoichiometric inclusions of Si,N4. The spectra obtained for samples irradiated with the highest doses at implantation temperatures of 800 and 1000°C have a linear structure, characteristic for b-Si,N, (ref 5). Figure 2 shows the evolution of the nitride layer structure formation. Near the rear interface (Figure 2(a)), the buried layer has a clearly defined block structure, disappearing as the layer grows. Near the front boundary, the fi-Si?N, layer is crystallized epitaxialy (Figure 2(b)). The annealing does not cause any structural changes in the buried layers. Figure 3 shows the typical distribution of the nitrogen atoms in silicon after implantation at medium ( - 7 x IO”) doses of 111

R V Gribkovskii

et al: Burled S&N,

layers in silicon 1

(a)

I

I-HEED

(b)

1

e Figure 2. Evolution

1

(c)

THEED

v (cm-’

of the but-icd nitride structure

Figure 3. Formation

THEED

I

of’r-Si ?N, layers by RTA

(.

._as imp1:

aIIII

T = x00 C). NT ions before and after RTA. A narrow layer of stoichiomctric Si,N., is formed and one can also observe an increased nitrogen concentration near the surface associated with nitrogen tending to migrate along extended defects. It follows from Figures 3(b) and (c) that the layers being formed under such conditions (medium dose, implantation temperature from 600 to 1000’ C and RTA) are crystallized into sc-modification and have the structure shown in Figure 3(c). 4. Conclusions The formation of /GSi,N, buried layers by in-situ implantation is determined by the ion current density and implantation dose.

112

RTA enables thin x-Si,N, tation dose is about equal

layers to bc formed when the implanthe stoichiomctric threshold.

to