A METHOD OF MAKING A COMPONENT FOR A MICROELECTRONIC CIRCUIT AND A SEMICONDUCTOR DEVICE AND AN OPTICAL WAVEGUIDE MADE BY THAT METHOD

Abstract: A portion of a first layer (14; 34) exposed through an open­ ing (20; 40) in a second layer (18; 38) is chemically converted to a compound having properties different to those of the first layer. By arranging that the conversion to the compound extends beyond the boundary of the opening and removing the com­ pound, further material (24) can be depositied onto the sub­ strate (12) through the opening (20). In this manner, a self-­ aligned semiconductor device can be formed with the further material (24) serving as the gate of the device. If the conversion to the compound is limited to the portion of the first layer directly below the opening (40) and the com­ pound has an index of refraction different from that of the first layer, the converted portion (36) can serve as an optical wave­ guide.

Description: A METHOD OF MAKING A COMPONENT FOR A MICROELECTRONIC CIRCUIT AND A SEMICONDUCTOR DEVICE AND AN OPTICAL WAVEGUIDE MADE BY THAT METHOD This invention relates to a method of making component for a microelectronic circuit and a semiconductor device and an optical waveguide made by that method. Self-aligned semiconductor devices have been prepared by methods that are now considered conventional. Self-alignment generally refers to the positioning of source and drain electrodes next to a gate electrode in a FET device. The self-alignment technique was developed in order to position electrodes relatively close to the gate to avoid high resistance between the electrode and the gate. The resistance between the source electrode and the gate is called the source resistance whereas the resistance between the drain electrode and the gate is called the drain resistance. Although other factors influence source resistance and drain resistance, the distance between the source and drain electrodes and the gate is the most critical factor. When the source resistance and/or the drain resistance is too high, the performance of the FET device may be seriously impaired. The prior art self-alignment technique was achieved by depositing an electrode layer on a substrate and positioning a second layer over the electrode layer, providing an opening in the second layer and etching the first layer through the opening. The etching compound is selected to remove the first layer and not the second layer thereby undercutting the second layer. Since the etching step is followed by the deposition of a material through the opening to form a gate on the substrate and the gate material does not spread out beyond the edges of the opening through which it is deposited, the degree of undercutting (i.e., the degree of removal of the first layer under the second layer) determines the distance of the first layer from the gate or stated otherwise the positioning of the source or drain electrodes with respect to the gate. This procedure is what is known in the art as self-alignment and is relatively more precise than mechanical methods for placing a source and/or drain electrode next to a gate. Mechanical methods for aligning the electrodes are known and involve for example masking the semiconductor device and depositing the electrodes and/or the gate through an opening in the mask which is difficult because alignment of the openings in the mask is done mechanically and calls for extremely high precision and close tolerances for the constructions of semiconductor devices and especially microcircuits. Self-aligning methods of the prior art permit separation of elements (e.g., source and drain electrodes relative to a gate electrode in a FET) of fairly close tolerances. Even though this permits construction of microelectronic circuits, it is still desirable to position semiconductor device elements even closer to reduce the resistance between them as for example the source resistance or the drain resistance in a FET. It is therefore desirable to seek a method that would allow this type of positioning especially positioning of elements within several hundreds of nanometers of one another which is not possible with the self-aligning procedures of the prior art. Self-alignment structures that can be built within these tolerances would provide for the construction of even smaller semiconductor devices than can be made with known methods. One of the difficulties with the methods presently employed for the construction of self-aligned structures is that an acid etching step is generally used to remove the first layer to provide the opening to the substrate and the acid may remove other elements of the circuit that are to be retained unless etching is properly controlled. Acid etching is difficult to control, however. Additionally, the acids that are commonly employed to remove electrodes may also attack the substrate. The removal of the electrode materials by chemical means such as acid etching according to the prior art process has to be carefully controlled so that the degree of removal of the first layer and consequent undercutting of the second layer is not done to any great extreme not only because of the problems of removing other elements in the microcircuit but also because excess removal of the first layer or electrode material will increase the distance between the electrode and the gate subsequently formed on the device and the source resistance or the drain resistance of the device would be too high due to the excessive distance between the electrodes and the gate. The utilization of light as a signal transfer means for the transmission and processing of information has been significantly developed over the past years. Fiber optic and light emitting units (e.g. lasers) have been developed, that, in combination withmicrocircuits, transfer and process information. These microcircuits require components (wave guides) for receiving and/or transmitting light signals transmitted through fibers or by lasers which are not easily provided because of the size of the microcircuit. Stated otherwise the delivery of such light signals to various parts of the microcircuit is difficult because of the relatively small size of such circuits compared with the systems used for the transmission of these light signals. According to the invention, there is provided a method of making a component for a microelectronic circuit including forming a first layer (14;34) of material on a substrate (12;32), forming on the first layer a second layer (18;38) of material which is different from that of the first layer, forming an opening (20;40) in the second layer to expose a portion of the first layer, and contacting the portion of the first layer exposed by the opening with a substance with which it chemically reacts so that said portion of the first layer is converted to a compound having properties different from those of the first and second materials. A self-aligned semiconductor device can be made by a method embodying the invention by depositing a conductor on a semiconductor substrate and a layer over the conductor. The layer is opened over a region of the substrate on which an additional element is to be placed such as a gate for a field effect transistor (FET). A portion of the conductor is then chemically converted through the opening in the layer to a compound that is removed by a reagent that does not remove the conductor. An element is then applied to the surface of the substrate through the opening. Self alignment is obtained by converting the conductor into a compound to a sufficient distance from the edge of the opening that a controlled amount of undercutting of the layer is provided when the compound is subsequently removed. When the element is applied to the surface of the substrate through the opening, such as the application of a gate for a FET, the edge of the conductor is separated from the element that is deposited through the opening but yet close enough to minimize any electrical resistance between the conductor and the element deposited through the opening. The layer may be of any conducting material which is not chemically converted when the conversion of the underlying conductor into the compound is carried out. In a specific example a germanium conductor is placed on top of a semiconductor substrate and a gold, molybdenum or palladium layer placed over the germanium. The gold, molybdenum or palladium layer is opened and the germanium underlying the opening is converted to germanium oxide to a predetermined distance beyond the edge of the opening. The germanium oxide is then removed by water or hydrogen chloride or a plasma after which a gate e.g. an electrode, for example of aluminum is deposited through the opening onto the surface of the semiconductor substrate. By chemically converting a portion of the conductor into a compound that is readily removed by the reagent a high degree of control is possible in placing the conductor in a position next to an element deposited on the semiconductor substrate such as a gate. Control of the chemical conversion of the conductor is generally more precise and less difficult than the prior art etching methods which were employed to remove the conductor. Additionally, the etching reagents of the prior art tended to remove elements of the device or circuit that are to be retained. A further application of the invention is in the production of wave guides. In this application of the invention a first layer of for example germanium, silicon, aluminum, tantalum or titanium is deposited onto a substrate which may be a semiconductor device. A second layer is placed over the first layer and an opening provided in the second layer in the shape of a wave guide. The first layer is then converted by a chemical reaction in the area below the opening to a compound conforming in shape to the opening and having an index of refraction different from that of the first layer. Where germanium, silicon, aluminum, tantalum and titanium may be used as the first layer, they may be chemically converted to their respective oxides. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which ' Figure 1 represents a sectioned side view of a semiconductor substrate having a first and second layer deposited there over; Figure 2 is a sectioned side view of the structure illustrated in Figure 1 after an opening has been formed in the second layer, and a portion of the first layer chemically converted through the opening; Figure 3 is a sectioned side view of a self-aligned semiconductor device formed from the structure illustrated in Figure 2; and Figure 4 represents a sectioned side view of a wave guide device comprising a substrate having a first and a second layer thereon, the second layer having an opening through which the region of the first layer underneath the opening is chemically converted to a compound having a index of refraction different from that of the first layer. Referring to Figures 1, 2 and 3, a semiconductor device 10 (Figure 3) is formed by a method embodying the present invention. A substrate 12, for example of gallium-arsenide or silicon, is coated with a thin layer of a conductive material 14, e.g., of germanium. A second layer 18 is formed on the layer 14 in a region over and next adjacent to desired location for a circuit component on the substrate 12. The substrate 12 is normally high in electrical resistance and may contain a region which is lower in electrical resistance than the substrate 12. Doped region 8 is produced by ion implantation, diffusion or epitaxial growth techniques. Region 8 is formed before layer 14 is placed on substrate 12. Doped region 8 is typically formed with either silicon, selenium, sulfur, tellurium or tin. The location of the circuit means is defined by producing an opening 20 in layer 18 extending over the region and formed by standard lithographic procedures for semi conductive device purposes. The layer 18 may be a photoresist layer which may be subsequently removed or it may be of a material such as gold, tin, molybdenum, tungsten, or palladium. Once the opening 20 is formed, the structure is placed in an ambient with which the exposed part of the layer 14 reacts and is thereby chemically converted below the opening 20 and in directions extending outwardly from the opening 20 to a compound 16 that is removable from the device by a reagent that will remove the compound 16 at a faster rate then the layer 14 or the layer 18 by which it is meant that the reagent will have substantially minimal to no effect on the layers 14 and 18 as well as the device 10 and substrate 12. In one embodiment, the layer 14 comprises germanium and the ambient in which the device is placed comprises an oxidizing medium such as air or oxygen for the conversion of the layer 14 beneath opening 20 into a portion 16 of germanium oxide. The conductor 14 is then divided into two regions 14A and 14B. Thus when germanium is used as the layer 14 it will be divided into two germanium regions 14A and 14B after conversion of the portion 16 into germanium oxide as described herein above. The conversion of a portion of the layer 14 into a compound 16 is controlled for a distance extending under the layer 18 adjacent the opening 20 in directions substantially parallel to the plane of the device 10 and within a predetermined distance from the edge of opening 20 by controlling the conditions under which the layer 14 is exposed to an ambient through the opening 20. In one example exposure of the layer 14 through the opening 20 to air and/or other oxygen sources can be effected by the pressure or the partial pressure of the oxidizing ambient, by placement of the device in an autoclave or other pressure device or by mixing the ambient with other nonreactive compounds, such as in the case of an oxidizing gas, e.g., oxygen mixed with nitrogen or a rare gas to control the partial pressure of the oxygen. Compounds consisting of oxides result from oxygen-bearing ambients, while compounds consisting of nitrides result from, say, ammonia-containing ambients with active nitrogen content. The temperature of the ambient can also be adjusted to change the reactivity thereof. Generally, by increasing the temperature and/or pressure of the ambient, the rate of reaction will increase and by reducing the temperature and/or pressure of the ambient the rate of reaction will decrease. The partial pressures may vary from anywhere from about 0.1 atmospheres to slightly less than 1 atmosphere pressure, the pressure range for the ambient, including the partial pressure ranges being anywhere from about 0.1 atmospheres to about 1,000 atomospheres. The ambient may also be ionized by methods well known in the art which increases the reaction rate and allows the reaction to proceed at lower temperatures. The time period over which the layer 14 is exposed to the ambient through the opening 20 is also a factor that will control the extent of the development of the layer 14 into a compound 16 underneath the layer 18. As the time of exposure of layer 14 through opening 20 to the ambient increases, the degree of formation of compound 16 also increases. All of these are parameters that can be adjusted by a person having ordinary skill in the art. In any event, it is critical that the compound 16 is formed to some degree underneath the layer 18 so that subsequent removal of the compound 16 by the reagent will result in some undercutting of the layer 18. Once the compound 16 is formed, to the degree and the extent determined for the production of a self-aligned semi-conductor or other device, the compound 16 is removed. In the case where layer 14 comprises germanium, the compound 16 may consist of germanium oxide which may be removed from the substrate 12 through opening 20 by means of a reagent such as water, hydrogen peroxide, hydrochloric acid or by plasma etching techniques. The plasma employed for removal of the compound 16 may comprise a plasma of a haloflouro lower alkane where the halogen compound in addition to flourine preferably comprises chlorine such as, for example carbon tetraflouride, monochlorotriflouromethane, dichlorodiflouromethane, and trichloromonofluoromethane. After the compound 16 is removed, the substrate 12 may be converted to a semi-conductor device by deposition of a conducting material 24 through opening 20 by a conventional process. Layers 24 and 24' are of the same material and are deposited simultaneously and may comprise, for example tungsten or aluminum, or suitable mixtures such as tungsten - silicon or tungsten-aluminum which is placed on the semi-conductor device 10 by a known method, for example by vapor deposition. Upon heating the device for a time and temperature determinable by a person with ordinary skill in the art, for example for 1 to 5 minutes at 400 to 5000 C, ohmic contact between the layer 14 and the region 8 can be formed such as in the case where layer 14A and 14B comprise germanium and the substrate 12 and region 8 comprise gallium arsenide. In this case, a FET is formed in which, after a lead is attached to the element 24' it can serve as a gate whereas the conductor 14A will serve as a source electrode and the conductor 14B will serve as a drain electrode. The distances 26 and 28 between elements 14A and 24 and elements 14B and 24 can be controlled within several hundred nanometers thereby reducing the source resistance between elements 14A and 24' and the drain resistance between elements 14B and 24 to a degree not heretofore readily achieved by known methods. Thus, there has been described herein a self-aligned structure, in which semi-conductor devices can be fabricated and the components of the device can be positioned closer to one another by a method whereby the distances between components are more readily and easily controlled than previously possible. In another embodiment of the invention, the element 24' may comprise any circuit component for controlling power through the device 10. When element 24' does not comprise a gate, it may comprise another circuit component such as a conductor, a capacitor, a resistor or a ###c####ctsr Jift Fr E. As another example of the present invention the layer 14 may comprise a titanium or a tantalum layer and the layer 18 may comprise tin, gold, palladium or silver. A portion 16 of the titanium or tantalum will be converted to a compound such as titanium dioxide or tantalum oxide by anodization or oxidation through the opening 20 in the tin, palladium, gold or silver layer after which the oxide may be removed by hydrogen flouride. Alternatively, the tantalum oxide or the titanium dioxide may be removed by a plasma. The layer 14 may also comprise an aluminum layer which may be converted into a compound comprising aluminum oxide which may be removed by a plasma. Alternatively layer 14 may comprise a silicon layer which may be converted into a compound comprising silicon dioxide which is removed by hydrogen fluoride. Lastly, the layer 14 may comprise either silicon, tantalum, titanium, or hafnium, all of which may be converted to compounds comprising their respective oxides and nitrides and the oxides and nitrides removed by a plasma. The method of the present invention is also applicable to the formation of wave guides on semiconductor or other devices as illustrated by device 30 in Figure 4 in which a layer 34 is placed on a substrate 32, for example of silicon, of a device 30 over which a layer 38 is also placed. An opening 40 is formed in the layer 38 in a similar way to the opening 20 in the device illustrated in Figure 2. The opening 40 in layer 38 extends downwardly to the first layer 34 and has the width and length of a wave guide. The layer 34 is then reacted through the opening 40 to convert the portion of the layer 34 directly below the opening 40 to a compound 36 conforming in size to the length and width of opening 40, compound 36 having an index of refraction different from that of layer 34. Layer 34 may comprise a thin layer of german am, al¯mir,un, titanium or tantalum each of which may be converted through opening 40 to its respective oxide. The oxides of these metals have an index of refraction different from that of the metals from which they are formed. Layer 38 may comprise for example either tin, palladium, tungsten, or gold but may also comprise a photoresist layer which is subsequently removed so that an approximately planar surface is formed over which subsequent semiconductor device members may be placed, for example conductors and insulators. The self-aligned semiconductor devices as well as the wave guide devices formed by a method embodying the present invention have application in any of the known electronic circuits employing semiconductors or wave guide means. In one embodiment, the substrate 32 may comprise a semiconductor device for the transmission and processing of information utilizing light as a signal transfer means. The substrate 32 may contain, for example, a semiconductor laser device as well. where the device 30 is to receive light signals by means of fiber optic members operatively associated therewith, the opening 40 can be arranged at one end of the device to extend for a width sufficient to receive such a signal from a fiber optic device and then narrow down or neck in to a width more consistent with the width of a wave guide on a microcircuit semiconductor device (other means are well known to those skilled in the art for coupling light from optical fibers into waveguide elements on semiconductor substrates). The increased width of opening 40 may also provide an area having a different index of refraction suitable for receiving laser signals for subsequent transmission through the device where the opening 40 subsequently is constricted so as to produce a wave guide of small dimensions suitable for a wave guide on a microcircuit semiconductor device. The wave guides can also be employed for the interconnection of microcircuit chips where each of the chips have such wave guides in them. The wave guides can therefore be used in lieu of electrical conductors.

Claims: CLAIMS 1. A method of making a component for a microelectronic circuit including forming a first layer (14;34) of material on a substrate (12;32), forming on the first layer a second layer (18;38) of material which is different from that of the first layer, forming an opening (20;40) in the second layer to expose a portion of the first layer, and contacting the portion of the first layer exposed by the opening with a substance with which it chemically reacts so that said portion of the first layer is converted to a compound having properties different from those of the first and second materials. 2. A method as claimed in claim 1, in which the chemical reation in the first layer spreads laterally beyond the boundary of the portion of that layer exposed by the opening (20) in the second layer, the method further including removing the compound and depositing fresh material (24) on to the substrate through the opening in the second layer. 3. A method as claimed in claim 2, in which the compound is removed by contacting it with a solvent. 4. A method as claimed in claim 2, in which the compound is removed by contacting it with a plasma. 5. A method as claimed in any of claims 2 to 4, in which the first layer consists of germanium, the second layer consists of gold, molybdenum, palladium or tungsten, and the compound produced by the chemical reaction in the germanium layer is germanium oxide. 6. A method as claimed in claim 5, in which the germanium oxide is removed by contacting it with water or hydrochloric acid. 7. A method as claimed in claim 5, in which the first layer consists of titanium or tantalum, the second layer consists of tin, palladium, gold or silver and the compound produced by the chemical reaction in the titanium or tantulum layer is titanium or tantalum oxide, respectively. 8. A method as claimed in claim 7, in which the titanium or tantulum oxide is removed by contacting it with hydrogen fluoride. 9. A self-aligned semiconductor device formed by a method as claimed in any of claims 2 to 8, in which the substrate consists of semiconductor material, the first layer consists of a conductive material, and the fresh material deposited onto the substrate is conductive material to form the gate of the semiconductor device. 10. A method as claimed in claim 1, in which the compound has a different index of refraction to that of the material of the first layer and in which the chemical reaction is limited to the portion of the first layer directly below the opening in the second layer. 11. A method as claimed in claim 10, in which the compound is an oxide. 12. A method as claimed in claim 11, in which the first layer consists of silicon, germanium, aluminium, titanium or tantalum. 13. A method as claimed in any of claims 10 to 12, in which the second layer consists of gold, tin, palladium or tungsten. 14. A method as claimed in any of claims 10 to 12, in which the second layer consists of a photoresist. 15. A method as claimed in any of claims 10 to 14, in which the substrate is a semiconductor device. 16. A method as claimed in any of claim 10 to 15, when used to form an optical waveguide on the substrate, the opening in the second layer being of a size suitable to receive a light signal input from an optical fibre. 17. An optical waveguide formed by a method as claimed in any of claims 10 to 16, the waveguide comprising the portion of the first layer which is converted to the compound.

Inventors: HOVEL, HAROLD JOHN, KUECH, THOMAS FRANCIS
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L29, H01L21, H01S5, G02B6