Pulsed excimer and Nd:YAG laser crystallization of a-Si:H

Pulsed excimer and Nd:YAG laser crystallization of a-Si:H

378 Applied Surface Science 46 (1990)37X-382 North-Holland Pulsed excimer and Nd:YAG The specific laser crystallization of a-Si:H role of hyd...

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378

Applied

Surface

Science 46

(1990)37X-382 North-Holland

Pulsed excimer and Nd:YAG The

specific

laser crystallization

of a-Si:H

role of hydrogen

M. Elliq, E. Fogarassy,

J.P. Stoquert,

Centre de Recherches Nuckkiaires (INZP3), F-67037 Strasbourg Cedex, France

C. Fuchs, S. de Unamuno,

Laboratowe

PHASE

B. PrCvot *

(UPR du C‘NRS N o 292) and ‘GRPM,

23 rue du f.oes.v.

and H. Pattyn IMEC, Received

Kapeldreef

75, B-3030 Leuoen, Belgium

14 June 1990; accepted

for publication

12 July 1990

We have investigated the possibility of preparing good-quality polycrystalline silicon by laser crystallization of hydrogenated and non-hydrogenated amorphous silicon thin films deposited onto glass substrates. The morphology and surface roughness of the crystallized layer is shown to be a function of the laser fluence and especially of the hydrogen content for both pulsed excimer and scanned Nd:YAG lasers used in this study.

1. Introduction Polycrystalline silicon thin-film transistors (poly-Si TFT’s) have received increasing attention in recent years for different applications including complex liquid-crystal displays (LCD’s) and vertically integrated high-density memories [1,2]. For such types of applications, it appears important to prepare large grain poly-Si thin films giving a high I,,“/I”ff ratio and a high field effect mobility at lower temperature or with a significant reduction in high temperature processing times, which permits the use of inexpensive glass substrates. Among the various techniques developed to prepare poly-Si, pulsed laser crystallization of asdeposited amorphous Si appears very interesting since the high-temperature processing is confined to the near-surface region during very short periods of time [3-51. This communication reports on laser crystallization of both hydrogenated (a-Si:H) and non-hy0169-4332/90/$03.50

Publishers

drogenated (a-Si) amorphous silicon deposited onto different substrates. The amorphous-to-polycrystalline transition, achieved with pulsed ultraviolet or scanned visible lasers, has been determined by optical methods (ultraviolet-visible reflectance, Raman spectroscopy). The influence of the hydrogen content on the surface morphology has been analyzed using a mechanical technique (profilometry). The evolution of hydrogen concentration with laser fluence has been investigated by elastic recoil detection analysis (ERDA).

2. Experimental 2. I. Surnple preparation

(CVD) onto various

B.V. (North-Holland)

(quartz,

deposition SiOz coated

M. Elliq et al. / Pulsed excimer and Nd: YAG laser crystallization

Corning 7059 glass). The first one, 200 nm thick a-Si, is prepared by low-pressure CVD (LPCVD) at 550°C. The second one, 200-1000 nm thick a-Si:H, is deposited by plasma-enhanced CVD (PECVD) at 250 o C. In order to restrict the thermal process to the near-surface region, the laser treatment was performed using either a large area (0.5 x 0.5 cm2) pulsed excimer laser (Lambda Physik, h = 193 nm, T = 20 ns) or a high-repetitive (5 kHz) scanned Q-switched Nd:YAG laser (Quantronix, X = 532 nm, r = 100 ns) which delivers pulses of about 100 pm in diameter. For these two different lasers the thickness of the as-deposited amorphous Si layers is greater than the optical absorption depth, L (X 5 532 nm, L 5 200 nm) [9]. 2.2. Characterization

techniques

2.2.1. Ultraviolet-visible reflectance The ultraviolet-visible reflectance (UVR) spectrum of single crystalline Si (c-Si) is characterized by two prominent peaks E, and E, respectively located at 275 and 365 nm. They are due to optical interband transitions at the X point and along the r-L axis of the Brillouin zone, respectively. The magnitude of these maxima may be reduced by deviation from crystallinity and/or by surface roughness [6]. The UVR spectra were performed at near-normal incidence over the 200-600 nm wavelength range using a Beckman doublebeam spectrophotometer (UV 5270).

of a-SitH

319

films [8]. Raman spectra were measured in a quasi-backscattering (Brewster) geometry using the 458 nm line of an Ar + ion laser. At this wavelength, the optical penetration depth of the light is of the order of 100 nm in a-Si [9]. The light beam was diluted on the sample surface with a cylindrical lens and the total deposited power was limited to 0.15 W. Spectra were recorded with a resolution of 3 cm-‘. 2.2.3. Elastic recoil detection analysis (ERDA) MeV ion-beam analysis with light ions has been demonstrated to be a powerful technique for sensitive measurement of atomic profiles in the nearsurface region (1 pm) of solids. The elastic recoil detection technique has been used to determine the hydrogen profiles in the deposited Si films before and after laser annealing. A monoenergetic 2.9 MeV 4He ion beam, collimated to a spot size of 1 mm2, is incident on a solid target under a grazing angle cx (10 “). The hydrogen nuclei recoil at an angle (Y+ p, where j3 (10 “) is the glancing angle between the surface and the detector direction. In these geometrical conditions, the elastically scattered 4He beam can be stopped by an absorber (Mylar 13 pm thickness) while only the hydrogen atoms can be detected, allowing the spectrum which corresponds to the hydrogen profile in the target to be recorded [lo].

3. Results and discussions 2.2.2. Raman spectroscopy Raman spectroscopy (RS) is known to be a particularly useful technique for identifying whether a film exhibits amorphous or crystalline structure [7]. Indeed, the RS spectrum of c-Si is dominated by a sharp feature at 522 cm-’ while the spectrum of a-Si displays features which resemble the broadened version of the one-phonon density of states (DOS). In the frequency range of the optical vibrations (400-600 cm-‘), the a-Si spectrum shows a very broad and intense structure with a maximum located at an energy of about 10% lower than that of the crystalline Raman peak. Intermediate situations are interpreted as resulting from partially crystallized amorphous

Fig. 1 shows a comparison of the UVR spectra of a-Si:H before and after Nd:YAG laser irradiation. The c-Si UVR spectrum, characterized by the two peaks E, and E,, is also presented. By contrast, the as-deposited a-Si:H reflectance curve does not display any structure in the ultraviolet range and the overall signal is lower than that of c-Si. Following Nd:YAG laser annealing, the appearance of the two peaks characterizes the amorphous-to-polycrystalline transition. The threshold energy density for the crystallization of the near-surface amorphous region is about 0.4 J cmp2. This value is in good agreement with that

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deduced from transmission electron microscopy (TEM) and thermal calculations (not presented here). The lowering of the reflectance signal after laser illumination for fluences above 0.7 J cm-’ is probably due to an overall signal loss induced by a surface degradation process as it will be confirmed by profilometry analysis. A similar qualitative behaviour is found when using the pulsed excimer laser for the crystallization of a-Si:H layers [ll]. In this case a threshold energy of about 0.1 J cm-’ is found. This value compares well to TEM and thermal simulation [ll] too. A more quantitative and detailed analysis of the amorphous-to-polycrystalline transition is achieved by Raman spectroscopy. Fig. 2 presents a series of Raman spectra restricted to the optical frequencies, obtained on a 500 nm thick a-Si:H layer at significant steps of the pulsed Nd:YAG laser processing. The as-deposited film shows the usual amorphous signature which consists in a broad (50 cm-‘) structure with a maximum located at about 480 cm-‘. After irradiation at 0.4 J cme2. the spectrum evolves

significantly and shows a rather narrow line (12 cm- ‘) located on the high energy side of the amorphous band. the intensity of the latter being found to decrease. The narrow component can be assigned to the modified TO phonon of c-Si which has been grown upon laser irradiation in an otherwise a-Si medium. After irradiation performed at 0.5 J cm ‘. the amorphous signal has almost totally disappeared while the crystalline peak shows a high scattering level associated with a very narrow (8 cm ‘) but slightly asymmetric linewidth. Apart from differences in peak intensities and linewidths, it is observed that the c-Si is downshifted by 7 cm ’ with respect to the value measured on single-crystal Si. Within the phonon confinement formalism developed originally by Richter and Ley [12] for Si microcrystals, we thus conclude that the deposited a-Si:H has been crystallized into ordered crystallites with typical dimensions of 557 nm.

Fig. 2. Raman spectra of a 500 nm thick Si layer deposited onto a SiO, (500 nm) coated Corning 7059 glass substrate before and after Nd:YAG laser annealing: (a) as-deposited (a-Si:H), (b) 0.4 J cm-‘. and (c) 0.S J cm l.

M. Elliq et al. / Pulsed exrimer and Nd: YA G laser crystalhation

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Fig. 3. Surface roughness height of a 200 nm thick Si film deposited onto a quartz substrate with (a-Si:H) and without (a-Si) hydrogen after Nd:YAG laser illumination as a function of fluence.

Fig. 4. Surface roughness height of a 500 nm thick a-Si:H film deposited onto a SiO, (500 nm) coated Corning 7059 glass substrate after excimer laser (ArF) treatment versus energy density.

For laser irradiation performed at higher energies (e.g. 0.7 and 0.9 J cmP2) the Raman signature does not evolve but is found to be superimposed on a strong decreasing background. This intense background takes its origin in an increase of the elastically scattered signal due to the surface modification as evidenced by profilometry measurements. Fig. 3 shows a comparison between the surface roughness height (a) of a-Si and a-Si:H films after Nd:YAG laser irradiation performed at increasing fluences. For the a-Si:H, a strong and linear increase of the surface roughness (up to 70 nm for 0.9 J cme2) is observed. By contrast, the non-hydrogenated crystallized layer does not present any significant surface roughness as confirmed by the lower curve of fig. 3 (a < 10 nm at 0.9 J cmP2). It is interesting to notice that surface roughness of the same order of magnitude is also found when using the uniform and large area pulsed excimer laser to crystallize a-Si:H films, as confirmed by the results of fig. 4. This means that the type of the laser (wavelength, size and beam shape) does not influence the formation of roughness. Hydrogen in the amorphous silicon could play an important role in the formation of roughness. Indeed, microscopic bubbles or voids have been previously observed by TEM in the near-surface of the laser-crystallized a-Si:H layers.They have been attributed to the exodiffusion of hydrogen

[13]. In order to confirm this interpretation, hydrogen distribution profiles before and after laser treatment were determined by ERDA experiments. Fig. 5 presents the hydrogen profile in the 1000 nm a-Si:H layer deposited onto SiO, (500 nm) coated Corning 7059 glass substrate before and after laser treatments. Before illumination. the hydrogen is found to be uniformally distributed within the a-Si:H film with a content of about 15%. After Nd:YAG laser irradiation at 2 J cm-* we observe a strong decrease of the hydrogen concentration associated with some accumulation

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382

M. Eihq et ul. / Pulsed

excmrer

and Nd: YAG her

towards the surface. A similar quantitative behaviour is observed after excimer laser annealing at 0.42 J cm-‘. Hydrogen loss can be reasonably attributed to an exodiffusion process. Hydrogen surface accumulation could be at the origin of formation of bubbles or voids (observed by TEM [13]) and be able to disrupt the surface.

4. Conclusion The possibility of preparing good-quality polySi by the laser-crystallization technique of aSi and a-Si:H thin films has been demonstrated. Our experimental observations demonstrate that the amorphous-to-polycrystalline transition is achieved above the surface melting threshold of amorphous Si induced by a pulsed excimer or scanned Nd:YAG laser. In the case of a-Si:H films the formation of surface roughness can be directly related to the surface accumulation and exodiffusion of hydrogen during the resolidification of the melted zone.

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References 111H.W. Lam and M.J. Thompson,

PI

[31

[41

[51 [61 [71 PI PI [lOI

1111 [I21 [I31

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