Interface reactions between CVD and PVD tungsten and aluminium

Applied

Surface Science 73 (190.1) 290-2Y4

applied surface science

North-Holland

Interface

reactions

W. Gruenewald, Tcchnische D-09009

Received

S.E.

Unil~ersitijt

Chrmnirz,

29 March

Selective

CVD

were investigated.

Schulz,

B. Hintze

Chrmnitz-Zwickau,

Fachhereich

1993; accepted

CVD

for publication

on aluminium

tungsten depositions The

interfaces

sive X-ray spectroscopy (EDX)

and aluminium

T. Gessner Zc~rum

fiir

Mlkrote~knolofiitn,

PSF 964 /3/050.

Such an interlayer

was proved in all investigated

25 May 1YY3

offers the opportunity

to realize

reliable

and PVD-W/AI

AI-F interface

compound

was detected

a crystalline

layer combinations.

The diffusion

because of their

by cross section TEM

at the interface

intermetallic

was found already after annealing

contacts (via fill) lor suhmlcron

compound

multllc\cl

tungsten glue layer on top of the aluminium

are of special interest

The interfaces were examined

and AES depth profiling of thin blanket deposited

An amorphous

and AI.%. At the PVD-W/AISi treatment.

and

Elektrotrchnik,

directly on Al (1 % Si) and on a sputtered

of CVD-W/AI

resistance and stability of the interconnect. at both interfaces.

CVD and PVD tungsten

Germany

tungsten

metallizations.

between

influence

and additionally

tungsten layers. Intermediate of CVD-W

deposited

layer\ were tound

by silane reduction

of W and Al can form depending treatment

ot WF,:,,

(111the thermal

twice at 400°C for 30 min. A diffusion of aluminium depth depends on thermal

on contact

by cncrgy-disper-

into tungsten

and on the \tructurc

(11 the

tungsten layer.

1. Introduction

2. Specimen

Using multilevel metallization schemes in 1C’s the contact between different metallization levels especially in submicron dimensions becomes more critical concerning reliability and electromigration. A technique which should be able to meet the requirements is via filling using selective CVD tungsten. Vias between two Al (1% Si) metallization levels have to be filled with CVD tungsten plugs deposited on the lower AlSi interconnect partly coated with special glue layers (e.g. sputtered W). XTEM investigations of CVD tungsten blanket deposited onto AlSi as well as of sputtered tungsten on AlSi were carried out focusing on interfaces and interface reactions. To support and verify the results of the XTEM analysis several unpatterned wafers with reduced tungsten layer thicknesses were studied by AES depth profiling. EDX analyses of the cross section specimens were performed in a TEM with a spatial resolution of about 10 nm.

All investigations were done using 4 inch oxidized wafers. The PVD layers were dcpositcd in a batch type sputter equipment. The sputtered double layers (W on AlSi) were dcpositcd in situ (without leaving the deposition chamber). Heat treatments were done as a furnace anneal in nitrogen atmosphere at 400°C for 30 min. In case of a second anneal hydrogen was used as anncaling atmosphere with the same parameters. Apart from the wafers investigated as deposited all wafers were coated with CVD-W in a single wafer LPCVD equipment. This process was pcrformcd as a blanket deposition on AlSi and as a sclectivc deposition on sputtered tungsten in vias. The parameters arc summarized in table 1. The cross section preparation was performed in the usual way of mechanical thinning and :I following ion milling. The characterization of the tungsten structure and of the interface to the other layers involves problems in cast of the cross section analysis of material combinations as used

0169.4.732/9.1/$06.00

5’ 1993

Elsevier Science Publishers

B.V. All rights reserved

preparation

W Gruenewald Table 1 Deposition parameters tered tungsten layers

of CVD

tungsten

et al. / Interface reactions between CVD and PVD Wand Al

on AlSi and sput-

Wafer temperature

Total pressure

Gas flow (seem)

CT)

(Pa)

WF,

SiH4

H,

20

20

500

20

10

250

Blanket W CVD on AlSi 280 33 SelectiL:e W CVLI on sputtered 230 33

291

W

SPUTTER

here. The two main problems are the lower sputter rate and the significant higher atom mass of tungsten compared with the other materials contained in the multilayer scheme. A careful preparation with optimized parameters of ion milling is mandatory. Such parameter are an incidence angle of the ion beam I 5” (measured from the sample surface) and preventing an incidence of the ion beam parallel to the glue line.

3. Results and discussion 3.1. CVD tungsten / aluminium interface Fig. 1 shows a cross section of a 500 nm tungsten layer deposited on AlSi at about 280°C wafer temperature. At the interface an amor-

Fig. 1. TEM cross section

of the CVD-W/AlSi

interface

TIME

lMIN.1

Fig. 2. AES depth profile of a thin (100 nm) CVD tungsten layer deposited at 280°C onto AlSi (fluorine peak in the interface region indicates an A1-F compound).

phous intermediate layer of 10 nm thickness was formed. EDX analysis of TEM cross section specimen showed this interlayer to be an Al-F compound, which was confirmed by AES depth profiles of a 100 nm thick layer. The AES depth profiles indicated an increased fluorine concentration (about 1%) at the interface to aluminium (fig. 2). The formation of such an Al-F compound was already described earlier [ll and results from the aluminium reduction of WF, which takes place preferentially to the silane reduction (more negative free energy of the aluminium reduction of WF, [2]). At the initial deposition stage of tungsten basically these two reactions contribute to

with an amorphous

intermediate

layer (AI-F

compound),

W. Gruenewald et al. / Interface reactions between CVD und PVD Wand Al

292

Fig. 3. TEM cross section of a thin (50 nm) as-deposited

tungsten growth. The hydrogen reduction can be neglected in the used temperature range (below 300°C) [3]. Hydrogen acts more as a carrier gas. The formation of the nonvolatile aluminium fluoride (AIF,) can deteriorate the contact properties dramatically. A solution could be application of additional top layers (glue layers) on aluminium or higher deposition temperatures (formation of volatile aluminium subfluorides). EDX line scan investigations of a TEM cross section specimen indicated diffusion of aluminium into the tungsten layer up to a depth of

Fig. 4. TEM cross section

of a sputtered

W/AISi

double

PVD-W

film on AlSi without

intermediate

layer.

50 nm. Thus the existing amorphous intermediate layer does not act as an effective barrier. The relatively high diffusion depth compared with several sputtered tungsten layers (see section 3.2) could be due to the columnar structure of the CVD tungsten. 3.2. PVD tungsten / aluminium interface The stability and reactions in multilayer interconnect structures are of great interest for resistivity (including contact resistance) and reliability

layer after annealing intermediate layer.

(400°C

2 x 30 min) with a clearly visible crystalline

W. Gruenewald

Fig. 5. TEM cross section

of a sputtered

et ul. / Interface reactions between CVLI and PVLI Wand Al

W/AISi

double

layer after annealing

aspects. Between aluminium and other metals (e.g. Ti, W) the formation of intermetallic compounds with relatively high resistivity can occur. Interfaces of sputtered tungsten films (50, 70, and 100 nm thickness) and AlSi were investigated as-deposited and after heat treatments. As-deposited layers (process temperatures below 200°C) of 50 nm thickness did not form any intermediate layer (fig. 3). EDX line scan analysis of TEM cross sections showed a very slight diffusion of Al into the sputtered W layer (about 10 nm). AES depth profiles in this case showed a thin interface indicating very slight diffusion, too. The interface of a 100 nm thick tungsten film on aluminium (annealing at 400°C 30 min, twice) is depicted in the X-TEM micrograph in fig. 4. Already at this temperature a crystalline interlayer was formed. With the help of EDX analysis this layer was identified as an intermetallic compound of W and Al. The most probable phase is WAI,,. This could not be established yet because of the large probe size of the TEM compared to the thickness of the intermediate layer. Other workers [4-61 report this formation only at 450°C and above. The thickness of the interlayer is determined by the grain size and fluctuates between 10 and 2.5 nm. Aluminium diffusion into the tungsten layer up to 30 nm was found by EDX line scans.

(400°C

30 min) without

293

visible intermediate

layer.

Fig. 5 shows the interface between PVD-W (70 nm) and AlSi after annealing at 400°C for 30 min without a detectable intermediate layer. But in this case the structure of the sputtered tungsten film differs significantly from the other layers described before. Deep voids could be seen between the tungsten grains. The observed high diffusion depth of 60 nm of Al into W can be explained by these diffusion paths. Similar results were found for WSi,/Al double layers by Mitwalsky et al. [7]. AES depth profiles proved this result showing a broad interface. The reason for the different structure of the sputtered tungsten layer is not fully understood till now, because deposition and other processing conditions were the same compared with the other samples.

4. Conclusion For CVD tungsten layers deposited on AlSi by silane reduction of WF, the formation of an aluminium fluoride interlayer was established and shown by X-TEM. At the interface between PVD-W and AlSi an intermetallic compound can be formed depending on the thermal treatment. A crystalline intermediate layer was found after annealing at 400°C (twice 30 min), whereas an

interface between as-deposited double layers showed no interlayer. For all investigated samples a diffusion of aluminium into the tungsten layer was observed. The diffusion depth depends on heat treatment and tungsten structure. Intermediate layers - if they exist - limit the diffusion but do not act as a diffusion barrier.

Acknowledgements

References [I] S. Kang. R. Chow. R.11. Wilson, B. GorowitL Williams. .I. Electron. Mater. 17 (1988) 3, 213. [2] R.V. Joshi, S.B. Brodsky. T. Bucelot. M.A. Jao Uttecht, Proc. VI. Int. IEEE V-MIC’ Conf., IEEE

113. [3] T. Ohba, T. Suzuki and T. Ha-a. in: Tungsten

[3] [S]

The authors are indebted to T. Scholz (Institut Frcsenius Dresden) for AES depth profiling and to H. Berck (NE-Metal1 GmbH Freiberg) for doing the EDX line scan investigations. This work was supported by the Federal Ministry of Research and Technology of the Federal Republic of Germany (FKZ: NT 27790).

[h]

(71

and

A.(;.

and R. 1989. p.

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