Semiconductor device

A semiconductor device comprises a glass substrate serving as a substrate having an insulated surface and a silicon layer located on a position overlapping with this glass substrate. The silicon layer includes an amorphous gettering region. Preferably, the silicon layer includes a main region serving as an active element region, and the gettering region is preferably included in the remaining portion of the silicon layer excluding the main region. Preferably, the silicon layer may include a portion serving as an active region of a thin-film transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device. More specifically, it relates to a semiconductor device including a thin-film transistor (TFT) prepared from crystallized silicon.

2. Description of the Background Art

In order to prepare a thin-film transistor from polycrystalline silicon, a step of crystallizing silicon is generally carried out. In this crystallization step, a treatment referred to as laser annealing is performed by applying a laser beam emitted from an excimer laser such as an Xe—Cl laser to an amorphous silicon film for melting the amorphous silicon film with heat resulting from this lasing and crystallizing the silicon in subsequent cooling. A polycrystalline silicon film can be obtained through this treatment. When the polycrystalline silicon film is prepared by this method, a substrate itself is hardly exposed to heat and hence a material having a low heat-resistant temperature can be employed for the substrate. Thus, a thin-film transistor can be formed on a glass substrate having a low heat-resistant temperature.

However, the laser beam emitted from the excimer laser such as an Xe—Cl laser and applied to the amorphous silicon film reaches only a portion of the silicon layer close to the surface thereof, and hence a layer having a large crystal grain size is formed only around the surface of the silicon layer. In relation to laser annealing, therefore, proposed is application of a YAG laser beam in place of the excimer laser beam.

According to a technique disclosed in Japanese Patent Laying-Open No. 2002-367904, a polycrystalline film formed by solid phase growth is extremely thinly left on a lower portion of a semiconductor film while the remaining region is melted for growing crystals from the left polycrystalline film formed by solid phase growth, as described in section 0033 with reference toFIGS. 1 and 4. In an embodiment of this technique, a heat treatment is performed on an amorphous semiconductor film for crystallizing the amorphous film in a solid phase (section 0059) and applying the second harmonic of an Nd:YAG laser beam to the intrinsic polycrystalline silicon film obtained by solid phase growth for melting/crystallizing the same (section 0060). The aforementioned gazette describes that about 80% of the semiconductor film is melted.

According to a technique disclosed in Japanese Patent Laying-Open No. 2000-269133, the second harmonic of an Nd:YAG laser beam is applied to an intrinsic amorphous silicon film for melting/recrystallizing the same (section 0023). This gazette describes that about 92% of the semiconductor film is melted.

A transistor formed on a silicon substrate has such a property that portions of crystal defects easily trap unnecessary impurities causing deterioration of transistor characteristics. Gettering can be performed through this property. In the transistor formed on the silicon substrate, a gettering site is constituted by depositing polycrystalline silicon on the back surface of the silicon substrate or a forming portion having a large number of crystal defects on the back surface of the silicon substrate by sandblasting or the like.

In a thin-film transistor formed on a glass substrate, on the other hand, the thickness of a silicon layer is so small that it is theoretically possible but inefficient to intentionally deposit a polycrystalline silicon film on the back surface of the silicon layer in consideration of a step necessary for this working. Further, it is impossible to perform a treatment such as sandblasting on the back surface of the silicon layer. In the thin-film transistor formed on a glass substrate, therefore, a gettering site must be constituted by another method.

SUMMARY OF THE INVENTION

An object of the present invention is to implement a portion serving as a gettering site in a semiconductor device formed with a substrate such as a glass substrate having an insulated surface.

In order to attain the aforementioned object, a semiconductor device according to the present invention comprises a substrate having an insulated surface and a silicon layer located on a position overlapping with the insulated surface, and the silicon layer includes an amorphous gettering region.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

A semiconductor device according to a first embodiment of the present invention is described with reference toFIGS. 1 and 2. This semiconductor device, forming a top gate thin-film transistor, comprises a glass substrate1having an insulated surface and a silicon layer3overlapping with the glass substrate1, as shown inFIG. 1. A silicon oxide film2serving as the so-called underlayer film is interposed between the glass substrate1and the silicon layer3. The silicon layer3includes a main region serving as an active element region. A gate electrode5is formed on the silicon layer3through a gate insulator film4.FIG. 2partially illustrates the silicon layer3in an enlarged manner.

Grain boundaries6partition the silicon layer3into a large number of crystal grains7. Each crystal grain7has a two-layer structure including a first layer8formed by a substantially perfect crystal on the side closer to the gate electrode5while including a second layer9different from the first layer8on the side opposite to the gate electrode5. Referring toFIG. 2, arrows51and52show the sides closer and opposite to the gate electrode5respectively.

This silicon layer3is formed by applying a YAG-2ω laser beam along arrow10in laser annealing. The transmission property in a polycrystalline silicon portion is improved due to the employment of the YAG-2ω laser beam so that the silicon layer3can be melted and recrystallized up to a deep portion. In this laser annealing, the silicon layer3is not entirely recrystallized but a portion close to the surface opposite to the gate electrode5is intentionally left amorphous with no recrystallization, to form the second layer9. In other words, the second layer9is amorphous. This second layer9serves as a gettering region in the silicon layer3. The gettering region is preferably provided on a portion other than a main region.

The underlayer film, consisting of only the silicon oxide film2according to the first embodiment, may alternatively be constituted of a laminated body of a silicon oxide film2and a silicon nitride film13, as shown inFIG. 3. In a modification of the first embodiment shown inFIG. 3, the silicon nitride film13is arranged between the silicon oxide film2and a glass substrate1.

In the semiconductor device according to the first embodiment, the side of each crystal grain7closer to the gate electrode5forms the first layer8having a large grain size, i.e., excellent crystallinity due to melting and recrystallization resulting from application of the YAG-2ω laser beam. On the side closer to the gate electrode5, therefore, electron mobility can be improved for implementing a state suitable for high-speed operation. On the other hand, the side of each crystal grain7opposite to the gate electrode5forms the amorphous second layer9serving as a gettering site. Thus, the second layer9so captures unnecessary impurities that the operation of the thin-film transistor can be stabilized.

The second layer9, amorphous in the first embodiment, may not be amorphous but may alternatively include a larger number of small crystal defects as compared with the first layer8. Also in this case, the second layer9can serve as the gettering site due to the presence of the small crystal defects. Further alternatively, the second layer9may consist of polycrystalline silicon having a small crystal grain size.

The silicon layer3, irradiated with the YAG-2ω laser beam in the first embodiment, may alternatively be irradiated with another type of laser beam in laser annealing. In particular, the silicon layer3is preferably irradiated with a laser beam having a wavelength λ within the range of at least 370 nm and not more than 710 nm in laser annealing. According to the first embodiment, the silicon layer3is exemplarily irradiated with the YAG-2ω laser beam.

While a silicon oxide film or a silicon nitride film is generally arranged between a glass substrate and a silicon layer as an underlayer film in a conventional semiconductor device, this underlayer film may be separated from the glass substrate or the silicon layer in laser annealing due to the difference in expansion coefficient between these layers. In order to prevent this separation, adhesion between these layers must be reinforced. According to the first embodiment of the present invention, the amorphous second layer9or an alternative second layer including a larger number of small crystal defects than the first layer8is provided in the lower surface of the silicon layer3, thereby reinforcing adhesion between the silicon layer3and the silicon oxide film2serving as the underlayer film.

As shown inFIG. 2, the thickness A of the first layer8is preferably larger than the thickness B of the second layer9. In this case, the bottom surface of a contact hole16partially penetrating the silicon layer3due to excess etching in formation thereof does not pass through the first layer8but remains therein with a high probability, as shown inFIG. 4. When the bottom surface of the contact hole16remains in the first layer8, electric resistance can be suppressed.

Second Embodiment

A semiconductor device according to a second embodiment of the present invention is described with reference toFIG. 5. This semiconductor device, forming an inversely staggered thin-film transistor, comprises a glass substrate1having an insulated surface and a silicon layer3overlapping with the glass substrate1, as shown inFIG. 5. A silicon oxide film2serving as the so-called underlayer film is interposed between the glass substrate1and the silicon layer3, similarly to the top gate thin-film transistor according to the first embodiment. In the inversely staggered thin-film transistor according to the second embodiment, a gate electrode12is held between the glass substrate1and the silicon layer3. The gate electrode12is locally placed on a flat surface of the glass substrate1, and covered with a gate insulator film4from above. The gate insulator film4is covered with the silicon layer3from above. Therefore, the shapes of the gate insulator film4and the silicon layer3reflect the protuberant shape of the gate electrode12with respect to the flat surface of the glass substrate1as such. Also in the semiconductor device according to the second embodiment, grain boundaries6partition the silicon layer3into a large number of crystal grains7, similarly to the first embodiment. Each crystal grain7has a two-layer structure including a first layer8formed by a substantially perfect crystal on the side closer to the gate electrode12while including a second layer9different from the first layer8on the side opposite to the gate electrode12, also similarly to the first embodiment. According to the second embodiment, however, the gate electrode12is provided under the silicon layer3dissimilarly to the first embodiment, and hence the first and second layers8and9are provided on the lower and upper sides respectively in the silicon layer3.

This silicon layer3is formed by applying a YAG-2ω laser beam along arrow111in laser annealing. The applied laser beam is not restricted to the YAG-2ω laser beam but a laser beam having a wavelength λ within the range of at least 370 nm and not more than 710 nm may alternatively be employed. According to the second embodiment, the YAG-2ω laser beam is exemplarily applied. The constitution of the second layer9is similar to that of the second layer9described with reference to the first embodiment.

The underlayer film, consisting of only the silicon oxide film2according to the second embodiment, may alternatively be constituted of a laminated body of a silicon oxide film2and a silicon nitride film13, as shown inFIG. 6. In a modification of the second embodiment shown inFIG. 6, the silicon nitride film13is arranged between the silicon oxide film2and a glass substrate1.

In the semiconductor device according to the second embodiment, each crystal grain7including the first layer8having excellent crystallinity and the second layer9serving as a gettering site can suppress electric resistance, enable high-speed operation and stabilize the operation of the thin-film transistor due to reliable gettering.

Third Embodiment

A semiconductor device according to a third embodiment of the present invention is described with reference toFIG. 7. This semiconductor device, forming a top gate thin-film transistor, comprises a structure identical to that described with reference to the first embodiment in a silicon layer3, while a silicon nitride film13is interposed between a glass substrate1and a silicon oxide film2, as shown inFIG. 7. An oxynitride film14ais interposed between the silicon nitride film13and the glass substrate1. Another oxynitride film14bis interposed between the silicon nitride film13and the silicon oxide film2. The thickness of the silicon nitride film13is 50 nm to 100 nm, while those of the oxynitride films14aand14bare several nm to several 10 nm respectively.

According to the third embodiment of the present invention, the oxynitride films14aand14bhaving intermediate expansion coefficients between those of the silicon oxide film2and the silicon nitride film13are formed on lower and upper interfaces of the silicon nitride film13respectively, thereby reinforcing adhesion between the silicon oxide film2and the silicon nitride film13as well as that between the silicon nitride film13and the glass substrate1. Thus, the silicon layer3and the glass substrate1can be prevented from separating from each other.

The silicon nitride film13may be so omitted that a glass substrate1and a silicon oxide film2are in contact with each other through an oxynitride film15as in a modification of the third embodiment shown inFIG. 8. Also in this case, a silicon layer3and the glass substrate1can be prevented from separating from each other.

The third embodiment of the present invention, applied to a top gate thin-film transistor as shown inFIG. 7or8, is also applicable to an inversely staggered thin-film transistor as in another modification of the third embodiment shown inFIG. 9. Referring toFIG. 9, a glass substrate1and a silicon oxide film2hold a silicon nitride film13therebetween, while oxynitride films14aand14bare formed on lower and upper interfaces of the silicon nitride film13respectively. Also in this case, the glass substrate1can be prevented from separating from a silicon layer3.

The semiconductor device according to each of the aforementioned embodiments forms a thin-film transistor. In this case, the silicon layer3includes a portion serving as an active region of the thin-film transistor. However, the present invention is not restricted to the thin-film transistor. The surface of the silicon layer3according to the present invention exposing the first layer8is also employable for another application as a silicon active region. For example, the present invention can also be employed for forming an element such as a capacitor. In this case, the silicon layer3includes a portion forming one of electrodes constituting the capacitor. Further alternatively, the silicon layer3may include a portion serving as a contact part with another layer, for example.