Phase analysis and thermal stability of thin films synthesized via solid state reaction of Ni with Si_1 − xGe_x substrate

Highlights

• The reaction of ultrathin Ni film with Si_1 − xGe_x layer (x: 25 and 55 at.%) is studied.

• Depending of Ge%, NiSi or NiGe is obtained but with different formation temperatures.

• The morphology, roughness and resistivity were found to be the same for NiSi and NiGe.

• NiGe on Si₄₅Ge₅₅ has limited thermal stability compared to NiSi on Si₇₅Ge₂₅.

Abstract

The products of the solid state reaction involving ultra-thin Ni film (6 nm) and Si_1 − xGe_x layers (Ge 25 and 55 at.%), were analysed using sheet resistance (Rs), glancing angle X-ray diffraction (GIXRD), scanning electron and atomic force microscopy (SEM, AFM) techniques. The reaction was carried out via rapid thermal process (RTP) annealing using two different steps (RTP1 and RTP2) while applying a selective etch (SE) in between them. The intermediate and the end reaction products resulting after RTP1 and RTP2 were found to be dependent on the Ge content, forming Ni-rich silicide (Ni₂Si) and NiSi on Si₇₅Ge₂₅, while Ni-rich germanide (Ni₅Ge₃) and NiGe were obtained by using Si₄₅Ge₅₅. Though the onset of intermediate Ni-rich silicide or germanide phase formation occurs at similar RTP1 temperature (275 °C), the reaction completion to yield low resistive phase NiSi or NiGe phase results at different RTP2 temperatures (400 °C vs 350 °C). Based on the volume expansion, a resistivity value of 25 μΩ cm was obtained for the synthesized NiGe (12 nm) and NiSi (14 nm) layers. Independent of the phases obtained, the films were found to be closed and homogeneous and exhibit rms roughness of 0.5–0.8 nm as evidenced by SEM and AFM analysis. Thermal stability studies carried out on NiSi and NiGe thin films, post RTP1/RTP2, show the latter phase to have limited stability and result in Rs degradation starting already at 475 °C due to phase decomposition.

Introduction

Si_1 − xGe_x heterostructures, due to their high mobility, tunable band gap and large valence band offsets, are becoming increasingly important both for electronic and optoelectronic devices [1], [2], [3], [4], [5]. They are implemented as either channel material or stressor in downscaled complementary-metal-oxide-semiconductor (CMOS) transistor device integration [1], [4], [5], [6], [7], [8]. Traditionally, and in advanced CMOS technology, metal-silicides have been widely used to reduce the contact resistance which thereby plays a major role in enhanced device functionality and performances [2]. In nanoscale CMOS devices, NiSi replaced Co- and Ti-silicides as contact material due to the advantages such as low sheet resistance, low formation temperature, and low Si consumption[9], [10], [11], [12], [13], [14]. Until now, the limited studies that have been reported on self-aligned reaction of Ni with Si_1 − xGe_x substrates, deal either with higher thicknesses of Ni (≥ 10 nm) or lower Ge content (≤ 30%) substrate[2], [4], [5], [15], [16], [17], [18], [19]. With the progressive scaling of the transistors into smaller dimensions coupled with different designs and device architectures, studies on the development of ultrathin metal-silicide formation on higher Ge content substrate becomes more relevant [20], [21], [22], [23], [24] and this, to the knowledge of the authors, is still lacking in the literature [25], [26]

It is well known that the thin film reaction of Ni with Si or Ge by itself is evidenced by a complex phase formation sequence due to the existence of several equilibrium and meta stable phases (Ni₅Si₂, Ni₃Si₂, Ni₂Si, NiSi, NiSi₂ and Ni₅Ge₃, Ni₂Ge, Ni₃Ge₂, Ni₅Ge₂, Ni₁₉Ge₁₂) [27], [28], [29], [30], [31], [32]. The phase sequence and end product formation shows a strong dependency on the adopted annealing conditions (ramp/duration), temperature, deposition technique, Ni film thickness, surface preparation and nature of semiconducting material (amorphous, mono or polycrystalline) [33], [34]. It is self-evident that the reaction of ultra-thin Ni with Si_1 − xGe_x is more complicated than the binary Ni/Si or Ni/Ge system. The difference in entropy of mixing and heats of formation, within ternary Ni(SiGe) system, offers driving force for Ge segregation and instability towards phase and morphology [2], [18]. Hence, a thorough understanding on the phase formation pathways and clarification on the reaction products, due to Ni interaction with Si_1 − xGe_x becomes thus imperative. In this work, we report the self-aligned reaction of ultra-thin Ni film (6 nm) with Si_1 − xGe_x epilayers of different Ge concentration (25 and 55 at.%). By studying the film properties at every consecutive step, involved as in the traditional contact metallization process sequence [20], [10], [35], we show the reaction pathway and terminal phase formation to show a strong dependency on the Ge at.% within the Si_1 − xGe_x. We also show that the thermal stability of the formed low resistive phase is sensitive to the Ge content, remaining more stable for 25% Ge compared to 55% Ge substrate.