GROWTH AND CHARACTERIZATION OF POLYCRYSTALLINE SILICON FILMS DEPOSITED BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION AT LOW TEMPERATURES

(1)

GROWTH AND CHARACTERIZATION OF POLYCRYSTALLINE SILICON FILMS DEPOSITED BY PLASMA ENHANCED

CHEMICAL VAPOR DEPOSITION AT LOW TEMPERATURES

著者 Ahalapitiya Hawage Jayatissa journal or

publication title

静岡大学大学院電子科学研究科研究報告

volume 18

page range 191‑193

year 1997‑03‑29

出版者静岡大学大学院電子科学研究科

URL http://hdl.handle.net/10297/1252

(2)

氏名。(本籍

) AHALAPITIYA HEWAGE JAYATISSA(ス

リランカ)

学位の種類

博

^士

(工

^学⁾

学位記番号

工博甲第

121

号学位授与の日付

平成

7年

12月 22日学位授与の要件

学位規則第4条第 1項_{該当} 研究科・専攻の名称

電子科学研究科

電子応用工学

学位論文題目

GROWTH AND CHARACTERIZAT10N OF POLYCRYS‐

TALLINE SILICON FLMS DEPOSITED BY PLASMA ENE

̲ HANCED CHEMICAL VAPOR DEPOSITION AT LOW TEMPERATURES

(プラズマ化学気相堆積による低温堆積多結晶シリコン薄膜の成長と評価)

論文審査委員

(委

員長)

教授神藤正士

^{教授} 福家俊郎教授

山

^口

十六夫

^{教授}

畑

中

義

式助教授

石

^川

賢

司

^助教授

中

西

^洋一郎

文内容の要旨

The growth of polycrystalline silicon (poly-Si) using plasma enhanced chemical vapor deposition (PECVD) of

silane at temperature below 500"C has been investigated. The system is a capacitively coupled parallel plate rf

glow discharge reactor where the substrates are mounted on the cathode (powered electrode). It was found that the crystallization process in cathode-type rf GD severely depends on power density where low power makes amorphous silicon because of lack of enough H radicals to promote crystallization process and high power makes poor crystallinity due to bombardment of high energy ionic particles. To reduce the effects from ion bombardment, a mesh was attached to the substrate holding elecuode with expecting to reduce the electric field in the film ^growth

zone. When the metal mesh was attached, the crystalline growth was markedly enhanced. These results strongly support the fact that the atomic H radicals play an important role in the crystalline growth in PECVD of silane. In the mesh attached case, the growth of crystalline silicon films was possible at low temperature, low power and high concentration of Sitla. These results are completely different from the results reported by previous workers.

The present system could deposit crystalline silicon films with a deposition rate higher than 30 nm/min using

‑191‑

(3)

lNVo SiH+. It was also found that the highly oriented and epitaxial like films could be grown on single crystal substrates of silicon and sapphire.

The films deposited were studied using various kinds of conventional characterization processes. The crystallinity of films were studied in X-ray, RHEED, UV and Ellipsometry measurements. It was found that the optoelec- tronic and the structural properties of silicon films could be controlled by varying the deposition conditions. X-ray diffraction and spectroscopic ellipsometry measurements were comparatively performed to study the microstruc- ture of silicon films, ^andthese properties were favorably compared with properties of thermally annealed films and poly-Si deposited in conventional CVD.

The optical properties of films were studied by optical transmission and reflectivity measurements. The absorp- tion coefficient of crystalline silicon films deposited was higher than that of a-Si: H in the higher energy range but

it was reduced in the low photon energy range. The optical gap seems to be decreased, compared to a-Si: H according to the Tauc's plot. The UV spectra shows the band structure of silicon films can obviously be described in terms of crystalline silicon assuming the size effect.

The electrical properties of films were studied by means of resistivity, activation energy and Hall effect measurements. The activation energy of crystalline silicon films deposited without-mesh were usual properties which conuolled by the crystalline phase in the films. However, the activation energy of films deposited with-mesh case

is very much different from that of without-mesh case: the activation energy is smaller than 0.4 eV. It can be considered that the films have low activation energy and high conductivity due to the in situ passivation of grain boundaries in the films. An attempt was also paid to explain the electrical properties using the grain boundary trapping model. Phosphorus doped films were fabricated using ^aphosphinesilane gas mixture. The in situ doping effects on crystallinity and, electrical properties of films were investigated. It was found that the density of trapping states at the grain boundaries of poly-Si films deposited in PECVD is smaller than that of poly-Si fabricated in conventional methods. Forgoing effect is attributed to the hydrogenation during the growth of films. The mobility of films deposited at 4O0'C lies in 5 - ^l0^cm2v-ltl^range.^According^to^theproperties of films deposited, high quality silicon films are deposited over 4O0'C and rf power above 0.75Wcm-2. The density of ^grain^boundary trapping states estimated from Hall measurements are in 5xl0l t-2xl0l2 _range_{while it}_is_smaller_{than the}_density of trapping states reported for poly-Si fabricated by LPCVD or normal pressure CVD.

Finally, the applications of these poly-Si films were investigated. It is proposed that the use of poly-Si forpulsed laser annealing can provide several advantages such as the elimination of preannealing step, the possibility to obtain a higher uniform recrystallized layer and lowering the fabrication temperature up to deposition temperature. An attempt was also paid to fabricate thin film transistors using silicon films ^producedin this technique and annealed films. Higher field effect mobility of films compared to the amorphous silicon films ^producedby the same method and the same temperature range was observed.

‑192‑

(4)

論文審査結果の要旨

薄膜トランジスター(・1「1・)をスイッチング素子として用いた、アクティブマトリックス液晶デイスプレイは、現在主として水素化アモルファスシリコン(a―Si:H)・lH・が用いられている。しかし、^a―si:H

はキャリヤの移動度が遅いため、スイッチング速度の点で大画面化、高精細化の限界に達している。

従って、将来の大画面化、高精細化のためにキャリヤ移動度の大きい薄膜物質の探索が求められている。その中で、半導体プロセス技術の点から、多結晶シリコン(poly―Si)薄膜の利用が最も注目されている。本論文はグロー放電法によって、^500℃以下の低温でガラス基板上に^poly―Siを形成する方法とその得られた薄膜の評価について述べている。

本論文は高周波による平板2極のグロー放電化学気相堆積法を用いた実験を主とした研究である。本研究では高周波電力を供給する電極(カソード電極)上へ基板を置くカソード堆積法の有効性について調べ、さらに、イオン衝撃を防ぐためのメッシュ電極の挿入により、低温においてかつ高濃度のシラン原料(100%)から、高速度(30nttmin。)に^poly‐Siを作製できることを実証した。これは新しい記録的な値である。さらに、^poly―Si薄膜の形成機構についても検討され、これは、薄膜前駆体が表面に付着しシリコンとの結合が形成されるとき、弱い結合の水素化シリコンが、プラズマ生成された水素ラジカルにより切り離され、強い結合のSi共有結合のみが残る結果、結品化が進むものと考えた。即ち、表面における水素ラジカルのエッチング作用が結品化の主要な機構であり、イオン衝撃は結晶化を妨げるこ

とも明らかとなった。

基板として単結晶のシリコンを用いれば、エピタキシャル成長も佃 ℃の低温で可能なこと、また、

サファイヤ基板上への高配向な^poly―Siが成長可能なことが確認された。

さらに、薄膜成長時に不純物として、リン(P)、ボロン(B)の添加による結晶性への影響も調べられた。本堆積方法によるとき、不純物Pの添加により結晶性の著しい向上が、Bの添加においても結晶性の向上が認められた。

得られた多結晶薄膜を用いて、・^lH・が製作され、移動度の評価がされた。移動度は5‑10cmηvsecで低温堆積薄膜として良い値がえられた。実用的な値としてはさらに向上が必要であり、エキシマレーザーによるアニーリングの有効性も確かめられた。

以上要するに、多結晶シリコン薄膜を低温で形成する方法と薄膜成長の機構について研究し、多くの新しい知見を得ており、さらに、・^1「^1・への応用の有効性についても確認している。これらは、博士の学位を授与するに十分な内容であることを認定する。

‑193‑

GROWTH AND CHARACTERIZATION OF POLYCRYSTALLINE SILICON FILMS DEPOSITED BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION AT LOW TEMPERATURES