Hybrid substrate

Abstract: A hybrid substrate, for example for use in a head of an ink jet printer, comprises an aluminum-based ceramic base substrate and a silicon layer provided on the base substrate. The substrate can be constructed to provide desired levels of flatness, workability, strength and thermal conductivity.

Description: This invention relates to a hybrid substrate for use in an active device having a fine pattern. In the prior art, for substrates for forming large area active device having fine patterns such as a heat-sensitive head or an ink jet head, an alumina substrate or a silicon substrate has been employed. However, on the alumina substrate, due to poor flatness of the surface, it was impossible to form a device having fine line width of about 10 µm. Also, for improvement of the surface flatness, a substrate having molten glass coated on the surface of alumina was proposed. However, the thermal conductivity of glass is as considerably poor as about 1/20 of that of alumina. Moreover, when a heat-generating device was formed on glass, due to the heat accumulating effect, it was difficult to control on-off switch of heat generation of the device by high frequency. On the other hand, although silicon wafer has a good surface flatness as a substrate, it is high in cost. Further, polysilicon wafer, which is slightly less expensive than silicon wafer, has the drawback that it has remarkably low mechanical strength. On the other hand, substrates with a constitution having a polycrystalline film deposited on a base substrate has been proposed in the prior art. For example, there has been a substrate deposited a polycrystalline silicon film on a quartz base substrate, and thin film transistors formed on the substrate have been practically applied. However, in this example, the grain size of the polycrystal is substantially uniform over the film thickness direction. Accordingly, when a polycrystalline silicon film was deposited to a film thickness of 30 µm or more, the following problems arose. That is, in case where the grain size was so fine as 10⁻⁷ metres or less, thermal conductivity was worsened due to the presence of a large number of grain boundaries, which caused a problem in forming a heat-generating device on the substrate surface. On the other hand, for example, when the grain size was as large as 3 µm or more, a problem that the flatness of the substrate surface was remarkably poor existed. Also, because of the difference in coefficient of thermal expansion between the base substrate and the polycrystalline silicon film to be deposited, crack or peel off was caused to occur in the polycrystalline silicon film in the cooling step after the formation of deposition film with heating. According to the present invention a hybrid substrate comprising a ceramic base substrate and a multi-layers structure of a different material from said ceramic base substrate, is characterised in that said multi-layers structure has a first polycrystalline layer formed on said ceramic base substrate, a second polycrystalline layer formed on said first layer, and a third polycrystalline layer formed on said second layer, that the crystal grain size of said first layer is 0.5 µm or less, that the crystal grain size of said second layer is 2 µm or more, and that the crystal grain size of said third layer is 0.5 µm or less. How the invention may be carried out will now be described with reference to and as shown in the accompanying drawings in which: Fig. 1 shows a sectional structure of a ceramic/polycrystalline silicon hybrid substrate of a preferred embodiment of the present invention; Fig. 2 is a schematic illustration showing an epitaxial crystal growth device for deposition of the polycrystalline silicon film controlled in grain size; Fig. 3 is a graph showing the relationship between the polycrystalline silicon grain size and the HCl content in the starting gas; Fig. 4 is a graph showing the relationship between the polycrystalline silicon grain size and the substrate temperature; and Fig. 5 is a graph showing the relationship between the polycrystalline silicon grain size and the reaction pressure. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 is a sectional structure showing the present invention. The polycrystal of the lowest layer 502 formed on the surface of the ceramic base substrate 501 has a small grain size of 0.5 µm or less, and such small grains fill the voids, etc. on the surface of the ceramic base substrate 501, and further relax the strain between different kinds of materials due to difference in coefficient of linear expansion. The second layer 503 has a grain size of 2 µm or more to be reduced in grain boundaries, and maintains high thermal conductivity. The third layer 504 has a small grain size of 0.5 µm or less to improve flatness of the substrate surface. Thus, the hybrid substrate shown in Fig. 1 has a flat surface and a good thermal conductivity because a polycrystalline layer of multilayer constitution of plural layers having different polycrystalline grain sizes laminated in the film thickness direction are provided on the surface. Fig. 2 is a schematic illustration of a silicon epitaxial growth device for preparation of the hybrid substrate of the present invention shown in Fig. 1. In Fig. 2, the quartz bell jar 601 is evacuated internally by a vacuum pump, and a polycrystalline silicon film is deposited on an alumina base substrate 602 under the state of reduced pressure by introducing reactive gases such as SiH₂Cl₂, HCl, H₂, etc. The alumina base substrate 602 is held on a carbon susceptor 603 heated by a high frequency coil 604. Next, the process of deposition of polycrystalline silicon film is to be described below. First, the alumina base substrate 501 is heated to a high temperature (900 to 1100 °C) and the surface of the substrate is cleaned by flowing HCl, followed by the deposition of polycrystalline silicon layer on the alumina base substrate 501 under an atmosphere of a gas mixture of SiH₂Cl₂ and HCl diluted with H₂. The grain size of each layer (layers 502, 503 and 504) can be controlled by the mixing ratio of the respective gases, the substrate temperature, pressure, etc. Fig. 3 is the result of examination of the relationship between the flow rate of HCl and the grain size, from which there can be seen a tendency that the grain size becomes greater as the flow rate of HCl is increased. Fig. 4 is the result of examination of the relationship between the substrate temperature and the grain size, from which it can be seen that the grain size becomes smaller as the substrate temperature becomes higher. Fig. 5 is the result of examination of the relationship between the pressure and the grain size, which shows that the grain size becomes smaller as the pressure becomes higher. By controlling these parameters, a polycrystalline silicon layer constituted by polycrystalline grain having a desired grain size can be laminated to give a desired hybrid substrate suitable to the object of the present invention. As described above with regard to one example by referring to Figs. 1 to 5 , according to the hybrid substrate of the present invention, a polycrystalline film can be deposited with a flat surface on a ceramic substrate having poor surface flatness, and yet a large area hybrid substrate which is highly heat-resistant with good thermal conductivity can be obtained.

Claims: A hybrid substrate comprising a ceramic base substrate and a multi-layers structure of a different material from said ceramic base substrate, characterised in that said multi-layers structure has a first polycrystalline layer formed on said ceramic base substrate, a second polycrystalline layer formed on said first layer, and a third polycrystalline layer formed on said second layer, that the crystal grain size of said first layer is 0.5 µm or less, that the crystal grain size of said second layer is 2 µm or more, and that the crystal grain size of said third layer is 0.5 µm or less. A heat-sensitive head comprising a hybrid substrate as defined in claim 1. A hybrid substrate according to claim 1 wherein the ceramic base substrate is an aluminum oxide polycrystalline sintered product. A hybrid substrate according to claim 1, wherein the polycrystalline layer is polycrystalline silicon. An ink jet head comprising a hybrid substrate as defined in claim 2.

Inventors: TOKUNAGA HIROYUKI, TOKUNAGA, HIROYUKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: B32B18
IPC codes: C04B35, B32B18, C04B41, H05K1