Effect of annealing treatment and nanomechanical properties for multilayer Si(0.8)Ge(0.2)-Si films

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Effect of annealing treatment and nanomechanical properties for multilayer

Si

0.8

Ge

0.2

–Si ﬁlms

Bo-Ching He

a

_{, Hua-Chiang Wen}

_b,*

_{, Meng-Hung Lin}

a

_{, Yi-Shao Lai}

c

_{, Wen-Fa Wu}

d

_{, Chang-Pin Chou}

a

Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, ROC

b

Department of Mechanical Engineering, Chin-Yi University of Technology, Taichung 411, Taiwan, ROC

c

Central Labs, Advanced Semiconductor Engineering, Inc., 26 Chin 3rd Rd., Nantze Export Processing Zone, 811 Nantze, Kaohsiung, Taiwan, ROC

d

National Nano Device Laboratories, Hsinchu 300, Taiwan, ROC

a r t i c l e

i n f o

Article history:

Received 9 September 2009

Received in revised form 22 December 2009 Available online 5 March 2010

a b s t r a c t

Multilayered silicon–germanium (SiGe) films consisting of alternating sublayers with different mechan-ical properties have been epitaxially deposited by an ultra-high vacuum chemmechan-ical vapor deposition (UHV/ CVD) system. We report engineering of the mechanical properties of SiGe multilayer films by a commer-cial nanoindenter. From annealing treatment, it consists of an ex situ thermal treatments in furnace (600 °C) and rapid thermal annealing (800 °C) system. Subsequent roughness and microstructure of SiGe multilayer films were characterized by means of atomic force microscope (AFM) and transmission elec-tron microscopy (TEM).

The annealing treatment not only produced misfit dislocations as a significant role in the critical pile-up event but also promoted hardness. The hardness of the films increased slightly and then gradually achieved a maximum value (from 12.6 ± 0.4 GPa to 14.2 ± 0.7 GPa) with increasing annealing tempera-ture. This may be due to the relaxation effect from thermal annealing and is potential to provide the reli-ability behaviours to design periodical SiGe multilayer structure in further.

Ó 2010 Published by Elsevier Ltd.

1. Introduction

Silicon–germanium (SiGe) is a very attractive semiconductor material which serves adjustable band gaps, enhanced carrier mobility, and higher dopant solubility than that of pure Si. Due to these outstanding properties, SiGe alloys is competitive with III–V alloys and its compatibility with Si processing makes it low cost to integrate with highspeed Si-based technologies applica-tions[1–3]. However, there are several limiting factors for its appli-cations such as the long growth time, high material consumption, rough surface, and partial strain relaxation [4,5]. Above all, the implication of the degree of Si/Ge interdiffusion in microstructure is difficult to decide because it was caused by many factors includ-ing Ge content, temperature, and so on[6,7]. Notice that, the lattice mismatch between SiGe and Si substrate can introduce crystal de-fects such as misfit dislocations and threading dislocations in the microstructure, which results in the degradation of device quali-ties, i.e., these misfit dislocations may exist at several interfaces and thread through the layers. In general, the abruptness of the interface misfit dislocations has the edge component in the active areas of device[8,9]. The high quality of the SiGe is required for

de-vices application, that is to say, crystal defects and non-uniform composition are undesirable[10,11].

In recent, multilayer films with a large number of repeated lay-ers have been intensively studied in the last decade. The enhance-ment of the films performance has been achieved by combining layers with different mechanical properties. The multilayers SiGe/ Si is useful as model systems to improve the films hardness. It was found that the hardness of these multilayer films was signifi-cantly improved compared with that of one monolayer. In contrast, multilayer films in the SiGe/Si system not only has attracted widely interest but also plays a critical role in the monolithic integration of Si-based micro- and opto-electronic devices [12]. Therefore, the relation between hardness enhancement mechanism and the restriction of dislocation movement in multilayer SiGe/Si films is focused. In this case, indentation techniques can be used to delam-inate films from the substrates and measure mechanical properties such as elastic modulus E and hardness H[13]. As ours knowledge, the annealing treated SiGe films is still not yet understood in the periodical structure to undergo a significant indentation process. The annealing treatment is potential to provide the reliability behaviours of periodical SiGe multilayered structure by using a quantitative method.

In this paper, the multilayer structure was deposited by an UHV/CVD system. This system has many beneﬁts compared with

0026-2714/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.microrel.2010.02.013

* Corresponding author.

E-mail address:[email protected](H.-C. Wen).

Contents lists available atScienceDirect

Microelectronics Reliability

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other deposition technologies. For example, the low growth rates allow atomic layer precision in the growth of SiGe films. Steep dop-ing as well as stable Ge profiles is required for high speed devices. In addition, UHVCVD avoids the segregation of Ge due to the low thermal budget. Therefore, the UHUCVD deposited SiGe shows high crystalline quality, smooth surfaces, and a low density of mis-fit dislocations [#2] and is different from sputtered one, which is in an amorphous form [#3]. We present experimental investigations during annealing under the influence of internal mechanical prop-erties of period SiGe multilayer on Si substrate. It is demonstrated that the quantitative stress was inside the layer by means of nan-oindentation technique. As a consequence, the annealing effect from the production of dislocations in the microstructure was evidenced.

2. Experimental procedure

The deposition process of SiGe multilayer ﬁlms on the Si sub-strate was described as the following: (i) the p-type Si(1 0 0) wa-fers were prepared by a standard Radio Corporation of American (RCA) clean and a HF:H2O (1:50) bath for 15 s. The samples were simultaneously introduced into load-lock chamber of ultra-high

vacuum chemical vapor deposition (UHV/CVD) system. After-wards, a 120-nm thick Si0.8Ge0.2layer was deposited at 500 °C for 43 min from pure SiH4(in 85 sccm) and GeH4(in 15 sccm) mix-ing (Ge concentration x = 20), the rate of deposition is 2.8 nm/ min and the vacuum is achieved at 10 7_{mbar. (ii) A 10-nm-thick} Si buffer layer was deposited on the Si substrate at 500 °C for 100 min from pure SiH4(in 85 sccm) gas and the rate of deposition is 0.1 nm/min and the vacuum is achieved at 10 7_mbar. Mean-while, the SiGe and Si buffer layers were deposited following four period cycles, and the structures were terminated by a 10-nm-thick Si cap layer (ca. 530 nm in the total 10-nm-thickness). (iii) In the

Fig. 1. AFM images of surface topography of samples: (a) single-layer at RT, (b) multilayer at RT, (c) multilayer at 600 °C for 0.5 h and (d) multilayer at 800 °C for 20 s. Table 1

Variation of average surface roughness (Ra) and root-mean-square surface roughness

(Rms) at room temperature and various annealing temperatures.

Sample Sample condition Average roughness, Ra(nm) Root mean square roughness, Rms(nm) (a) Single-layer-SiGe RT 0.2 0.4 (b) Multilayer-SiGe RT 3.1 3.6 (c) Multilayer-SiGe (600 °C) 3.2 3.9 (d) Multilayer-SiGe (800 °C) 3.5 4.1

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annealing treatment, the SiGe multilayer ﬁlms were ex situ en-dured thermal treatments in furnace on N2 gas for 10 min (600 °C) and rapid thermal annealing for 20 s (800 °C).

Morphological character, roughness, and microstructure of the SiGe thin ﬁlms were observed by means of scanning electron microscopy (SEM, Hitachi S-4000), atomic force microscope (AFM, Veeco D5000), and transmission electron microscopy (TEM, JEOL, JEM-2100F). From 3D patterns of AFM analysis, we mainly investigated two parameters: the height roughness param-eters (Ra) and the root mean square (Rms). In addition, TEM samples were prepared within mechanical polishing down to 20–30

l

m, followed by Ar ion milling to electron transparency [14]. The observations were made at 200 kV.

Subsequent hardness and elastic modulus of the SiGe thin ﬁlms were studied by using a Nano Indenter XP instrument (MTS Coop-eration, Nano Instruments Innovation Center, TN, USA). The nano-indentation measurements used a diamond Berkovich indenter tip (tip radius 50 nm), suggesting that plastic deformation can be generated at very small load. In addition, hardness data obtained with Berkovich indenter can be transformed to Vickers hardness because of the same shape of a three-sided pyramid, which means a similar area-to-depth function[13,15]. The continuous contact stiffness measurement (CSM) mode, which is executed by

super-imposing small oscillations on the force signal to measure dis-placement responses, offers a direct measure of dynamic contact stiffness during the loading process in the indentation test and is insensitive to thermal drift. Hence, the CSM mode can obtain hard-ness and Young’s modulus relative to the indentation depth and is used in this experiment[16,17].

3. Results and discussion

The AFM system was employed to investigate the annealing process of the SiGe thin films in their surface and microstructure condition.Fig. 1shows the sample of SiGe thin films before and after annealing treatments. Compared atFig. 1a–b, we observe a smooth manner of the single-layer SiGe films and a comparatively rougher SiGe multilayer films. InFig. 1c–d, it can be seen that the surface roughening of SiGe multilayer films is gradually increased at annealing treatment from 600 to 800 °C, as lists inTable 1. As well known, the dislocation nucleation may occur from surface roughening [18–22]. It is suggested that Si1 xGex layers, grown on Si(0 0 1) substrates by UHV/CVD, relaxed either by dislocation multiplication or by dislocation nucleation based on surface rough-ening[23,24]. The surface roughness increases, which may be also

Fig. 2. The cross-sectional TEM image of the SiGe thin ﬁlms samples: (a) single-layer at RT, (b) multilayer at RT, (c) multilayer at 600 °C for 0.5 h and (d) multilayer at 800 °C for 20 s (Si cap: Si capping, Si BL: Si buffer layer, MDs: misﬁt dislocations).

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attributed to the extra thermal budget and the relaxation of the strain[25–28]. Besides, the waved surfaces observed in multilayer ﬁlms may be due to the 2D germanium segregation on surface dur-ing growdur-ing or annealdur-ing process at temperature below 600 °C

[29]. We suggest that the SiGe multilayer films may be more com-plex than that of single films in the segregation mechanism. There-fore, more investigation in the microstructure of SiGe multilayer films is needed.

In order to examine the changes of SiGe films in the structural properties before and after annealing treatments, XTEM examina-tion was employed in this work.Fig. 2a illustrates the profile of a single-layer SiGe films, and virtually no misfit dislocations as well as native dislocations have occurred.Fig. 2b shows the relatively stable in the interface of SiGe multilayer films. The amorphous Si buffer layer is embedded between SiGe layers which are totally crystalline. However, while the sample endured annealing treat-ment at 600 °C, we observe a low density of short misfit segtreat-ments and some interdiffusion from the misfit dislocations (Fig. 2c). Meanwhile, the SiGe multilayer films allow a direct comparison of the dislocation formation while the annealing temperature rises to 800 °C (Fig. 2d). Both the conditions indicate a rapid increase in both misfit segments and the activated nuclei with increase in annealing temperature. This clearly demonstrates that the nucle-ation events and misfit segments are independent processes from thermally activated process in the SiGe multilayer films. Above all, TEM profile tends to display a serial nucleation seed at the each interface of SiGe multilayer films, which induces higher density of misfit dislocations. The similar phenomenon in the SiGe structures grown by means of UHV/CVD method occurred by a modified Frank–Reed mechanism. The dislocations are formed by the repro-duction of corner dislocations [4]. In our experimental results, however, the high density of misfit dislocations can be aggregated between SiGe multilayer films. Thereby, the relative defects may induce the change in mechanical properties of the SiGe multilayer films.

The epilayer–substrate combination which corresponds to a softer epilayer (SiGe) on a harder substrate (Si) is investigated. The harder substrate, relatively difficult to deform, can impose a severe resistance to plastic deformation of the films, which could modify their response. Accordingly, the nanoindentation technique is powerful in probing the behaviours of the films. The samples be-long to two categories: the materials (SiGe single-layer and multi-layer) are investigated by CSM measurement; the penetration depth dependence of the E and H can be obtained. At the same

time, the single-layer SiGe ﬁlms is included for reference data. In

Fig. 3, the same displacements of loading–unloading curves of the (a) single and (b–d) multilayer SiGe films are illustrated. Dis-crepancy in penetration depths of samples (a–d) are exhibited obviously (inset,Fig. 3). From this, we can observe that SiGe sin-gle-layer films is softer than that of all multilayer films due to the maximum penetration depth. Besides, we can also observe that the H of annealed SiGe multilayer films is higher than that of non-annealing one because of the less penetration depth. It is believed that the discrepancies among the multilayer SiGe films are mainly due to the increased activation energy, which is consistent with the kink model[30]for dislocation glide[31,32].

To further investigate this phenomenon, CSM nanoindentation technique is proposed.Fig. 4 shows typical CSM results of SiGe films for single-layer and multilayer structures on Si substrate. The effect of substrate is evident from the contact depth depen-dence of the H, as shown inFig. 4. For indentation depths up to about 20 nm, the H increased as the indentation depth became deeper, which is normally attributed to the transition between purely elastic to elastoplastic contact whereby the H is actually the contact pressure. For indentation depths greater than about 20 nm, the H of SiGe films became constant. The E followed a trend similar to that of the H, which is converged at an indentation depth smaller than that for H (Fig. 5). Herein, H and E were therefore determined by averaging measurements at indentation depths from 100 to 200 nm, considering an adequate depth to achieve a fully developed plastic zone and meanwhile to avoid the more sub-strate effect[33]. However, in the present case the substrate prob-ably affects the values of the elasto-plasticity coefficients since, for the experiments under a 10 mN peak load presented here, the pen-etration depth is ca. 200 nm, which does not exceed half of the film thickness; the influence of the substrate is expected[34]. For the measured results of each sample, the data at a contact depth of 100–200 nm are summarized inTable 2. It is interesting to observe that the H of annealing SiGe multilayer films is higher than non-annealing. In addition, the E decreases slightly with annealing treatment.

It is observed that the SiGe multilayer films can be more stable than that of SiGe single-layer. It is known that the films can relax by either the formation of misfit dislocations or interdiffusion. Misfit dislocations can enhance the H while significant interdiffu-sion exhibits the opposite tendency towards the H [#5]. Obviously, the enhanced H through annealing treatment in our study

Fig. 3. The displacements of loading–unloading curves of the SiGe ﬁlms: (a) single-layer at RT, (b) multisingle-layer at RT, (c) multisingle-layer at 600 °C for 0.5 h and (d) multisingle-layer at 800 °C for 20 s.

Fig. 4. The hardness of the SiGe thin ﬁlms samples: (a) single-layer at RT, (b) multilayer at RT, (c) multilayer at 600 °C for 0.5 h and (d) multilayer at 800 °C for 20 s.

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indicates that misfit dislocations dominate the relaxation mecha-nism. The annealing treatment not only leads nucleation seed but also slightly increased the H of the SiGe multilayer films. It is thus reasonable to conclude that slip and the orientation of the ba-sal planes plays a key role in defining the mechanical properties of crystalline materials. The dislocations are formed by the reproduc-tion of corner dislocareproduc-tions, which is based on the restricreproduc-tion of dis-location movement within and between SiGe multilayer films, as refer in the result ofFig. 4. However, the E of SiGe multilayer films with annealing treatments decreases slightly. The phenomenon may be due to the result of dislocation-induced additional strain, which would cause a decrease in stiffness [#1]. In other words, the E decreases because it is in proportion to stiffness. Neverthe-less, the annealing treatment does not cause a significant decrease in the E in our experiment. When the contact depth exceeds 10– 20% of the film thickness, substrate effect on the contact stiffness and H can be observed. The thickness of SiGe multilayers is approx-imately 530 nm, so the H and E data are obtained at a contact depth of 100 nm to measure these properties with certainty.

This result demonstrates that the SiGe multilayer ﬁlms is more susceptible to plastic deformation and has the higher H in compar-ison to non-annealing samples. While annealing treatments are carried out, multilayer ﬁlms will relax in order to reduce the strain energy; the restriction of dislocation movement therefore serves active role at the interfaces.

4. Conclusions

Our approach on an enhancement of mechanical ability of SiGe layer is based on a multi-heterostructure. High quality relaxed SiGe multilayer structures have been grown epitaxially by UHV/CVD methods. The agglomerated grains of the multilayer SiGe ﬁlms

were investigated by AFM analysis. It is obviously observed that the surface roughening of SiGe multilayer films is gradually in-creased, the dislocation multiplication or by dislocation nucleation may occur from the annealing treatment. The enhancement of the hardness at annealing temperature was achieved in the films by means of nanoindentation techniques. The H of the SiGe multilayer films within annealing treatment is higher due to the relaxed effect and, more susceptible to plastic deformation in comparison to the single-layer sample. Besides, the E decreases slightly through annealing treatment due to dislocation-induced additional strain. Dislocation glide is well understood and several mechanisms for dislocation nucleation have been studied.

Acknowledgements

This research was supported in part by the National Science Council in Taiwan under contract NSC 98-2216-E-009-069. Techni-cal support from the National Nano Device Laboratories contract NDL-97-C05G-087 and NDL-97-C04G-088 is also acknowledged.

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Table 2

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