Doping process for producing homojunctions in semiconductor substrates

In doping process for producing homojunctions in a semiconductor substrate, and the semiconductor substrate, dopants penetrate by way of diffusion employing an ultraviolet light source. A mask is introduced between the light source and the semiconductor which has regions of varying thickness. Dopant material is placed between the mask and the substrate, and the mask is then irradiated by the light source.

BACKGROUND AND SUMMARY OF THE INVENTION
 This application claims the priority of German Patent Document 19534574.6,
 which was filed on Sep. 18, 1995, the disclosure of which is expressly
 incorporated by reference herein.
 The present invention relates to a doping process for producing
 homojunctions in semiconductor substrates, into which dopants penetrate by
 diffusion. Furthermore, a light source is provided, the emission spectrum
 of which contains ultra-violet components, and which is directed at the
 surface of the semiconductor substrate. A process of this type is known,
 by way of illustration from IEEE Trans. Electron Devices, vol., ed. 39,
 1992, pp. 105 to 110.
 Diffusion and/or implantation techniques are employed for producing sharply
 defined adjacent doping regions in semiconductor substrates having
 different concentrations of the same dopants, i.e. p- or n-dopant atoms.
 In order to be able to dope solely the selected regions in the
 semiconductor base substrates, masks which are impenetrable for the dopant
 atoms in the selected diffusion and implantation conditions are provided
 on the substrates surface to be doped.
 In the case of silicon semiconductor base substrates, silicon oxide masks,
 which are either thermally grown or precipitated as layers, are provided
 on the silicon surface. In order to produce masks of this type, a
 photosensitive resist is applied onto a homogeneously precipitated silicon
 oxide layer, with the photosensitive resist being exposed to light with
 the aid of suited shadow masks. On the exposed sites, a subsequent etching
 step removes the oxide layer locally down to the base substrate, which can
 then be enriched with a desired concentration of dopant atoms in the
 course of diffusion and/or implantation. Thus, selective doping of
 semiconductor base substrates using known doping techniques requires
 preliminary structuring measures; the mask layer which is active for
 diffusion has to be locally removed prior to diffusion. Another masking
 step, is required to remove the diffusion block locally.
 In particular, the production of solar cells provided with a two-step
 emitter structure requires two complicated process diffusion steps which
 can be conducted using conventional methods of diffusion.
 FIG. 3 shows the production of a two-step emitter solar cell using the
 known etching diffusion techniques, including the sequence of the
 individual steps a to h. In the course of a high temperature step, in FIG.
 3a an oxide layer 32, which can be generated in a conventional
 process-controlled heating furnace, is applied to the base substrate 31.
 Using conventional spin-on techniques, a photosensitive resist 33 is
 applied evenly onto the oxide layer. In a subsequent photolithographic
 step, the photosensitive resist 33 is exposed to light with the use of
 suited masks in a conventional manner (See FIG. 3c.) At the sites exposed
 to light where the oxide layer is to be removed, the light-sensitive
 photoelectric layer is removed. In a subsequent etching step according to
 FIG. 3d, the oxide layer 32 can be removed at the sites exposed to light
 in a selective manner in such a way that local removal of the oxide layer
 down to the base substrate surface becomes possible. Then, according to
 process step FIG. 3e, the photosensitive resist is removed. Following this
 comes the first diffusion step (3f) with n.sup.++ -dopant atoms which can
 only penetrate into the material via the free base substrate surface (for
 this see the white-dotted n.sup.++ -dopant atoms region 34). In a further
 process step (3g), again corresponding to an etching step, the remaining
 oxide regions are removed from the surface of the base substrate in such a
 manner that a homogeneous n.sup.+ -doping 35 can occur in a subsequent
 second diffusion step, which in this case, according to the representation
 in FIG. 3h, is whole surface n.sup.+ -diffusion.
 In the aforedescribed manner, so-called two-step emitter solar cells
 composed of two adjacent n.sup.+ /n.sup.++ junctions can be produced.
 The described production process of a two-step emitter comprises two
 diffusion steps. The first provides for local and deep diffusion, and the
 second provides for whole surface homogeneous diffusion. Both diffusion
 and oxidation are conducted in classical diffusion and oxidation furnaces
 at very high process temperatures of &gt;1000.degree. C.
 In the development of components and the use of novel materials (and in
 order to improve the effectiveness and the reliability of the
 semiconductor components, which due to the hitherto prevailing conditions
 depend on the production of thermal doping processes), it is necessary to
 lower the thermal input during production. Thermal input refers to the
 duration of the thermal diffusion and oxidation steps including the
 absolute temperature level predominating during the diffusion and
 oxidation procedure. In order to meet these requirements, "rapid thermal
 processing" (RTP), in which the to-be-processed semiconductor substrates
 are individually optically heated, has been utilized.
 Contrary to conventional heating methods, which are essentially based on
 the influence of infrared radiation on the to-be-heated substrate, the RTP
 method employs radiation intensive illumination units, which essentially
 emit in the ultraviolet spectral range. A more detailed explanation is
 given in R. Singh's article, Development Trends in Rapid Isothermal
 Processing (RIP) dominated Semiconductor Maufacturing" in, 1st Int. Rapid
 Thermal Processing Conf. RTP 93" Scotsdale, Ariz., Eds. Richard Fair and
 Bohumil Lojek, September 1993, pp. 31 to 42.
 In particular in RTP methods, the diffusion properties of the dopant atoms
 penetrating the to-be-doped semiconductor material can be decisively
 influenced by the characteristic heating rates which can be obtained by
 means of the new "rapid thermal processing" techniques. By way of
 illustration, A. Usami's article, Shallow-Junction Formation on Silicon by
 Rapid Thermal Diffusion of Impurities from a Spin-On Source", IEEE
 Transactions on Electron Devices, vol. ED 39, 1992, pp. 105 ff. explains
 that the diffusion coefficients can be set in a specific range in a
 selective manner by employing new, optical heating methods.
 One object of the present invention is to provide an improved doping
 process for producing homojunctions in semiconductor substrates into which
 dopants penetrate in the course of diffusion, using a light source which
 has an emission spectrum containing ultra-violet components and is
 directed at the surface of the semiconductor substrate.
 Another object of the invention is to shorten the time required for
 production of components of this type and essentially involve considerably
 lower costs.
 Still another object of the invention is to reduce thermal input on the
 semiconductor components during their production.
 With the aid of new RTP methods described in the introduction, the
 production of semiconductor components having homojunctions and, in
 particular, two-step emitter solar cells can be simplified so that only a
 single thermal processing step is required. According to the present
 invention, a mask is placed between a semiconductor and a light source,
 dopant atoms are introduced between the mask and the semiconductor
 substrate to be doped, and rapid thermal processing is used, irradiating
 the mask with the light source.
 The process uses a light source, which has an emission spectrum containing
 ultra-violet components and which is directed at the semiconductor
 substrate. Depending on the to-be-doped regions with the same dopant
 concentration, the mask has regions of varying thickness, and is placed
 between the to-be-doped semiconductor substrate and the light source. The
 light coming from the illumination source impinges at suited sites, where
 the mask has holes through it, directly upon the base substrate and
 immediately interacts with the surface of the substrate. At sites where
 the mask covers the surface of the substrate, only the parts which are of
 the emission spectrum of the illumination unit which are not absorbed by
 the mask material reach the surface of the substrate. In addition, the
 thermal radiation of the mask itself acts on the regions of the
 semiconductor substrate covered by the mask so that, in addition to the
 electromagnetic radiation, the black body radiation coming from the mask
 acts on the surface of the semiconductor substrate and influences the
 temperature profile on the surface of the semiconductor substrate.
 In the invented combination of the method of rapid thermal processing and
 using suited masking, the surface of the to-be processed semiconductor
 substrate is impinged upon with the desired varying spectral distribution
 of the radiation field acting on the semiconductor surface in such a
 manner that different diffusion constants, which determine the diffusion
 process, can be set in various regions of the to-be-doped semiconductor
 substrate surface.
 Because an intermediate layer containing the dopant atoms is placed between
 the mask and the to-be doped semi-conductor substrate surface, depending
 on the diffusion constants setting in due to the different spectral
 distribution of the radiation acting on the substrate surface, the speed
 of the diffusion process responsible for introducing the dopant atoms into
 the semiconductor base substrate varies at different sites.
 For instance, the diffusion processes at the sites of the semiconductor
 base substrate irradiated without hindrance by the illumination unit are
 faster due to the optical excitation by short-wave light. On the other
 hand, at the sites provided with the masking material, the dopant atoms
 penetrate more slowly into the surface of the substrate.
 In this manner, a suited selection of the mask permits generating dopant
 profiles within the semiconductor base substrate requiring only a single
 process step, namely the aforedescribed illumination process with an
 illumination time of approximately 10 seconds and a process temperature of
 about 900.degree. C.
 Other objects, advantages and novel features of the present invention will
 become apparent from the following detailed description of the invention
 when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The sections a to h of FIG. 3 show the production steps for two
 homojunctions in a semiconductor substrate, which are inserted into
 two-step emitter solar cells. An essential drawback of the known process
 is that two separate diffusion steps have to be carried out for producing
 the first deep n.sup.++ -diffusion region and the subsequent laterally
 adjacent n.sup.+ -diffusion regions to be maintained within the scope of a
 homogeneous diffusion. Both diffusion processes require process
 temperatures of above 1000.degree. C. In order to prevent such high
 temperatures and, in particular, to carry out the second diffusion
 process, the to-be-doped semiconductor base substrate is processed using
 RTP techniques.
 According to FIG. 1a, a filmlike coat 2 containing the dopant atoms is
 applied onto semiconductor base substrate 1. Preferably, the coat is
 applied onto the semiconductor substrate by means of a centrifugal method
 such as spin-on techniques. A mask 3 provided with through holes 3 is
 disposed directly or spaced over the filmlike coat 2. An illumination unit
 (not shown), the emission spectrum of which contains ultra-violet
 components, is provided above the mask. The beams emitted from the light
 source impinge upon the masking material at sites on the mask 3 and are
 absorbed by it in a suited manner. The radiation parts penetrating through
 the mask reach through the dopant layer into the semiconductor substrate
 1, where they interact with the semiconductor material. At sites where the
 mask 3 has holes, the beams emitted from the light source reach directly
 through the dopant layer 2, without hindrance into the semiconductor
 material 1. Depending on the radiation parts, suited diffusion constants
 determining the diffusion process set in the semiconductor layers near the
 surface.
 In the regions where the radiation parts can penetrate the semiconductor
 substrate without hindrance, the diffusion process is faster than in the
 regions covered by the mask 3. By virtue of the optically induced
 diffusion of the dopant and the locally varying diffusion process setting
 in on the semiconductor substrate due to the spectral distribution, the
 same results can be achieved using only a single process step that are
 obtained with the sequence of process steps shown in FIGS. 3a to 3h. The
 invented procedure permits considerably shortening the known doping
 processes for producing homojunctions and thereby reducing production
 costs considerably.
 In order to rule out contamination problems in manufacturing semiconductor
 components, the same material is employed for the masking material as is
 used for the semiconductor. Therefore, according to FIG. 2b, the wafer
 sections 3 can lie directly on top of the dopant layer 2 on the
 semiconductor base substrate 1. This direct contact of mask 3 to the
 semiconductor substrate permits intensifying the diffusion yielded to the
 varying distribution of radiation due to the previously mentioned
 temperature effect. Although light absorption is greatest on the surface
 of the mask opposite the light source, it decreases exponentially in
 dependence on the wavelength with increasing penetration depth. The
 related generation of heat in the materials is inhomogeneous corresponding
 to the decrease in light absorption resulting in, as previously described,
 two different diffusion constants. Due to the direct contacting of the
 masking materials with the surface of the substrate, inhomogeneous heating
 of the semiconductor substrate occurs due to the altered thermal mass,
 resulting in the radiation action in the semiconductor material in
 different diffusion constants.
 FIG. 2 is a diagram showing the penetration depth of individual dopant
 atoms in the region of the semiconductor substrate without masking (see
 line with boxes) in relation to the penetration depth of the dopant atoms
 in the semiconductor region covered by the mask (see line with dots).
 Comparison of both graphs clearly shows that in the line with boxes, the
 increase in concentration in the individual depth regions in the
 semiconductor substrate of the dopant atoms is greater than in the line
 with dots. This means that the diffusion at sites not covered by the mask
 runs more effectively and faster than at sites where the masks covers the
 semiconductor substrate. Therefore, doping regions can be produced in
 semiconductor substrates in a single process step which generates,
 following termination of the illuminator time, adjacent doping regions
 each with different dopant concentrations. Essential parameters
 influencing the diffusion processes are the radiation and temperature
 conditions at the individual semiconductor substrate surfaces, which can
 be suitably set by a characteristic design of the mask.
 The aforedescribed process also permits production so-called back surface
 field solar cells, whose rear cell side is characterized by p/p.sup.+
 -homojunctions.
 The foregoing disclosure has been set forth merely to illustrate the
 invention and is not intended to be limiting. Since modifications of the
 disclosed embodiments incorporating the spirit and substance of the
 invention may occur to persons skilled in the art, the invention should be
 construed to include everything within the scope of the appended claims
 and equivalents thereof.