METHOD OF FABRICATION OPTOELECTRONIC COMPONENTS

Abstract: A method of fabricating a plurality of optoelectronic components on a semiconductor substrate, each optoelectronic component comprising several layers grown in a reactor. Every layer is being grown under a predetermined individual pressure. The active layers (12, 22) of all the components are lying substantially at the same height. Control of the pressure in the reactor allows the thickness of the layer grown to be constant or to vary over the substrate area.

Description: METHOD OF FABRICATING OPTOELECTRONIC COMPONENTS The invention relates to the fabrication of optoelec¬ tronic components and in particular photonic integrat¬ ed circuits. Photonic integrated circuits (PIC) are semiconductor chips that include several optoelectronic components (e.g. semiconductor lasers, detectors, waveguides, FET devices, etc.). Such chips can be fabricated using various techniques that have been developed for the individual components. However, because each different component (e.g. a laser and a waveguide) requires a different layer structure, that is a stack of several layers having different thicknesses and compositions, the fabrication of PIC's using these known different techniques requires many processing and growth steps. More advanced techniques have been disclosed in the literature that make it possible to grow the layers for the different components over the surface of a sub- strate in a single run. This can be done by controlling the lateral variations in the growth rate over the surface of the substrate, thereby to change the bandgap of the quantum wells in a controllable way laterally over the surface of the substrate. These advanced tech- niques include selective growth, shadow masked growth and non-planar growth using Metal Organic Chemical Vapour Deposition (MOCVD) growth in a reactor. For growing high quality layers, the right growth con- ditions are chosen so as to obtain the best layers. The growth parameters include the temperature, pressure, gas flow. Using the known techniques provides chips including several devices that consist each of a stack of sever¬ al layers having different thicknesses and composi¬ tions. The composition within a few atomic layers can be changed by changing the gas that is switched to the MOCVD reactor. As noted above, the growth rate can be controlled for each device over the surface of the substrate and this results in a complete integrated chip including devices in which the active layers are not lying at the same height. Such a structure has some disadvantages in that the coupling between different devices (e.g. a laser and a semiconductor waveguide) is not perfect and the overall surface is not planar. It is an object of the present invention to provide an improved growth technique that results in a chip struc¬ ture wherein the active layers are all lying at the same height while being grown in a single growth run. This object is achieved according to this invention by a growth method using a MOCVD growth in a reactor in which the pressure is maintained at a different value for growing each individual layer. In particular, a method is provided for fabricating a plurality of optoelectronic components including several layers grown on a semiconductor substrate in a reactor. Every layer is being grown under a predetermined individual pressure whereby the active layers of all the com¬ ponents are lying substantially at the same height. In accordance with this invention, it is possible e.g. to grow a first layer having an equal thickness over the entire surface of a substrate using a low pressure, then a second layer having different thicknesses using an atmospheric pressure, and then again a layer having an equal thickness using a low pressure thereby to realize a structure in which all active layers are lying at the same height. The invention will be more readily understood by refer¬ ence to the drawings and the description of two exem¬ plary embodiments. Fig. 1 shows a semiconductor substrate to be processed together with shadow mask. Fig. 2 is a cross-sectional view along line II-II of Fig. 1, showing a channel. Fig. 3 is a diagram showing the variation in growth rate as a function of the channel width (shadow masked growth) . Figs. 4 and 5 show the layer structures for a laser and a waveguide device grown according to the prior art and the invention respectively. Fig. 6 schematically shows the mask for selective growth. Fig. 7 is a diagram showing the normalized thickness as a function of the oxide width (selective growth) . Figs. 8 and 9 are similar views to Figs. 4 and 5 re-- spectively for a row of lasers. Example 1 Fabrication of an integrated chip including a semi¬ conductor laser coupled to a waveguide by means of shadow masked growth. A substrate 1 (Fig. 1) is partially covered by a mask 2 so as to form channels 3. The purpose in this example is to have the structures of the laser 10 and the wave¬ guide 20 to be deposited in a single growth run. Quantum wells are used in the active layer because a quantum well is sensitive to the thickness variations and allows the thickness variations to be translated into a material bandgap change. In order to cause the waveguide to be transparent to laser light, the quantum wells in the waveguide 20 should be made thinner than in the laser 10. This can be done by growing the laser 10 over a substrate portion not covered by a mask and growing the waveguide 20 using the shadow mask 2. The growth rate in a channel can be varied by changing the structure geometry (depth of well and width of aperture above the well) . For instance, by reducing the aperture it is possible to reduce the growth rate. Fig. 2 shows a cross-sectional view across such a channel. The growth rate is varying with the channel width as can be seen from Fig. 3 which shows the growth rate variation as a function of the channel width at low pressure (76 Torr) and atmospheric pressure (700 Torr) during growth with a fixed 6-micron well depth. Assum¬ ing a 20-micron width for the channels. Fig. 3 shows that the thickness of the grown structure is reduced by ± 50 % when growth is done at atmospheric pressure. This result is quite satisfactory for allowing the quantum wells in the waveguide 20 to be transparent to the laser light. When the complete structure is grown under atmospheric pressure, the resulting structure would look as shown in Fig. 4. The active layers 12 and 22 in the laser structure 10 and the waveguide structure 20 respective¬ ly are not lying at the same height because the thick¬ ness of the active layers as well as of the adjacent cladding layers 11, 13, 21, 23 is reduced by the same factor. Such a result can be avoided in accordance with this invention by first growing the lower cladding layers 11, 21 at low pressure whereby the growth rate for the 20-micron channel is reduced by ± 20 % only. There¬ after, growth is being proceeded further under atmos¬ pheric pressure whereby the active layers 12, 22 are grown at a strongly reduced rate. Finally, growth is done again under low pressure thus resulting in grown upper cladding layers 13, 23 being thicker. The thus obtained structure looks as shown in Fig. 5 : the active layers 12 and 22 of the laser and the waveguide respectively are lying at the same height, thereby resulting in a much better coupling between the two devices. Example 2 Fabrication of a row of lasers emitting light at dif¬ ferent wavelengths by means of selective growth. The surface of substrate 1 is covered with oxide strips 4 whereby growth of the layer is locally prevented (Fig. 6) . The material that comes onto the oxide will diffuse to the uncovered zones thus resulting in an increase in the growth rate there. This method is used for causing the growth rate to vary over the area of the substrate. Fig. 7 shows the variation in growth rate as a function of the width W of the oxide strips with a mask having oxide strips equally spaced apart by an 8-micron distance L. The row of lasers are grown in one single growth run using quantum wells in the active layers of the lasers 10 and using different widths w for the oxide strips over the area of substrate 1. In this example too, the lower cladding layers 11 are grown under low pressure, then the active layers 12 are grown under atmospheric pressure whereby the active layer in the individual lasers are grown at a reduced growth rate. Thereafter growth is done again under low pressure for growing the upper cladding layers 13. The thus obtained structure looks as shown in Fig. 8. It should be recognized that, in this example too, all the active layers 12 are lying at the same height. On the contrary, should growth be done under atmos¬ pheric pressure only for achieving the complete struc¬ ture, the active layers of the individual lasers would be lying at different heights as illustrated in Fig. 9.

Claims: CLAIMS 1. A method of fabricating a plurality of optoelectron¬ ic components including several layers grown on a semi¬ conductor substrate in a reactor, characterized in that every layer is being grown under a predetermined indiv¬ idual pressure such that the active layers (12, 22) of all the components are lying substantially at the same height and such that control of the pressure in the reactor allows the thickness of the layer grown to be constant or to vary over the substrate area. 2. The method as defined in claim 1, characterized in that a first layer (11, 21) is being grown on the sub¬ strate (1) under low pressure (e.g. 76 Torr) , a second active layer (12, 22) is being grown on the first layer (11, 21) under atmospheric pressure (e.g. 700 Torr) and a third layer (13, 23) is being grown on the second layer (12, 22) under low pressure.

Inventors: COUDENYS GEERT FRANS MARIA, DEMEESTER PIET MARIA ALFONS RO, COUDENYS, GEERT, FRANS, MARIA, DEMEESTER, PIET, MARIA, ALFONS, ROSA
Applicants: COUDENYS GEERT FRANS MARIA, DEMEESTER PIET MARIA ALFONS RO, IMEC INTER UNI MICRO ELECTR, INTERUNIVERSITAIR MICRO-ELEKTRONICA
Application year: 1993
Main classification: H01L21
IPC codes: H01L21, H01L27, H01L33, H01S5