DE3225484A1 - Process for diffusing aluminium into silicon - Google Patents

Abstract

To obtain a doping structure with a precise lateral boundary in the diffusion of aluminium into silicon wafers, an aluminium layer having a thickness of less than 5 mu m is deposited on at least one principal face of the silicon wafers and is patterned by means of a photomasking process. The patterned silicon wafers are then heat-treated in an inert gas atmosphere containing an oxidising gas additive at a temperature slightly below the eutectic temperature of the Al-Si system and subsequently heated to the diffusion temperature at a low heating rate. For 50 mm diameter silicon wafers, the chosen heating rate should be less than 5 K/min, in particular less than/equal to 1 K/min.

Description

Method of Diffusing Aluminum in Silicon The Invention relates to a method for diffusing aluminum. with a laterally limited Doping structure in silicon wafers, with the Silicon wafers deposit an aluminum layer with a thickness of less than 5 m and is structured by means of a photomask process, and then the silicon wafers in an inert gas atmosphere containing an oxidizing gas additive at diffusion temperature heated and kept at this temperature for a sufficiently long period of time will.

Such a method is known from DE-AS 28 12 658. In the known method is applied to a main surface polished to optical quality a single crystal silicon semiconductor substrate by vacuum evaporation an aluminum layer educated. The aluminum layer is patterned in a conventional photo mask process Then the semiconductor substrate is kept at a temperature of 12500C, where the aluminum is in Gas silicon substrate diffuses. To prevent, that the edges of the aluminum layer should fray irregularly during diffusion the thickness of the aluminum layer is limited to a maximum of 2 # m, as a diffusion atmosphere an inert gas with an addition of 0.05 to 10 vol .-% oxygen is used and the Alignment of the aluminum pattern in line with the Kristallor.4 lentation will.

From DE-OS 28 01 862 a method for the production of metal samples is known on silicon wafers as preparation for thermomigration. Here is a 5 to 100 ts m thick aluminum layer on the surface of the silicon wafer applied and a pattern corresponding to the desired pattern on the aluminum layer Photoresist layer generated. The areas of the not covered by the photoresist layer Aluminum layers are etched away. The photoresist layer is then removed and the remaining aluminum layer to an annealing process in an inert gas atmosphere at a temperature of 560 to 5700C for about 30 minutes. As preferred The surface plane of the silicon wafer is indicated as the (111) plane.

From US-PS 39 01 736 and 40 07 987 and from DE-OS 26 52 667 various processes for thermomigration are known. All procedures What is common is that there is an aluminum layer on one surface of a silicon wafer is formed, the layer in a recess either in the silicon surface introduced itself or in a silicon oxide layer covering the silicon surface will. The silicon wafer is controlled by the eutectic temperature of the Al-Si system heated so that a melt of aluminum-rich silicon material is formed. Then the main surface opposite the aluminum coating becomes the Silicon wafer heated up so much that a temperature gradient runs right through it the silicon wafer forms, whereupon the aluminum runs right through the silicon body migrates, essentially maintaining its lateral structure. As preferred Strongly dispersed infrared radiation is suggested as a heating source.

It has been found that when aluminum diffuses into Silicon compliance with the process parameters described in the prior art is not sufficient to generate doping patterns with sharply defined edge contours. So occurs undesirably during the heating before the diffusion process lateral thermal migration of the aluminum; the aluminum migrates from areas which are covered with aluminum after the photo mask process, actually aluminum-free Areas in. The originally sharp contours of the aluminum pattern are fraying the end. This fraying cannot be prevented by those mentioned in the introduction DE-AS 28 12 658 proposed admixture of 0.05 to 10 vol. T of oxygen for Diffusion atmosphere or by limiting the aluminum thickness to a maximum of 2 m prevent.

It is known that the metal used as a diffusion source Aluminum coating with silicon at a temperature above 5770C, which represents the eutectic temperature for the Al-Si system, a liquid eutectic forms. At slightly higher temperatures, the eutectic changes to a liquid Al-Si mixed phase over, which for example contains 77% silicon at 12800C.

This liquid layer tends to form droplets, which creates the structure dissolves.

Another difficulty is that at the diffusion temperature of 1250 whether the vapor pressure of Aluminum or of Al-Si mixed phases sufficient to disc as a gaseous doping source on the entire silicon act, whereby an exact lateral structure is prevented.

If oxygen is present in the doping atmosphere, both Both silicon and aluminum are oxidized. The silicon oxide layer that builds up is masked to some extent against aluminum vapor; at the same time aluminum gets through Oxidation to A1203 disabled. Because the oxidation of aluminum is very rapid is going on, there is a risk that too much aluminum will be converted into Al203 is lost. Aluminum layer thickness and oxygen concentration must be chosen so that a sufficient masking of the aluminum-free surfaces against aluminum vapor is guaranteed and that during the diffusion process enough metallic aluminum is available as a diffusion source. The aluminum layer thickness however, it must be kept as small as possible because of the risk of droplet formation, etc.

The present invention is based on the object of a method of the type mentioned at the beginning, with which laterally precisely delimited doping structures made of aluminum in silicon.

This object is achieved in that the structured silicon wafers initially at a temperature a little below the eutectic temperature of the system Al-Si are tempered and that the heating from the tempering temperature to the diffusion temperature takes place with a small heating rate.

This results in the advantages that both the preparation of the silicon wafers, the application and structural ture the aluminum pattern as well as the actual diffusion process according to tried and tested conventional methods in tried and tested conventional devices can be carried out and that the fraying of the Structures through lateral thermal migration and redoping through gaseous Aluminum can be safely avoided.

The invention makes use of the knowledge that a certain critical temperature gradient must be exceeded in order to prevent lateral thermomigration to get going. The value of this critical temperature gradient depends on Among other things, from the surface properties of the silicon wafer and the crystal orientation away.

A theoretical determination is because of the multitude of influences currently not possible According to an advantageous development of the invention the heating rate Rg for silicon wafers with a diameter of 50 mm is less than 5 t / min, in particular set less than / equal to 1 K / min. In the case of silicon wafers with a diameter D of Deviates 50 mm, the heating rate R becomes according to the relationship R = RO # 50mm / D set. If the operating parameters remain the same, the heating rate must of the furnace, the smaller the selected, the larger the silicon wafers are.

The silicon wafers are preferably also cooled after the Diffusion process with a small cooling rate.

Based on the drawings s @ ll the invention in the form of an exemplary embodiment explained.

It shows: Fig. 1 the result of a conventional diffusion procedure when diffusing small-area aluminum structures in silicon, Fig. 2 in stronger Enlargement of the result of the method according to the invention when diffusing the the same small-area aluminum structures in silicon, FIG. 3 shows a cross section through a silicon disk diffused on one side, FIG. a shows a cross section through a silicon wafer diffused on both sides.

Fig. 1 shows a photograph of a silicon wafer after a conventional diffusion method was treated. On the silicon wafer surface was a regular pattern of square surfaces using photo mask technology with an edge length of 250 tm. You can see that the originally Square point structure with increasing approach to the edge of the pane, strongly frayed is due, in particular, as a result of undesired lateral thermal migration too high a heating rate of the diffusion furnace.

Pig. 2 shows an enlarged view of the edge section of a silicon wafer diffused according to the method according to the invention. One recognises here, that the square aluminum patches retain their original contours exactly to have. (In contrast to FIG. 1, the Al spots appear in this representation as white spots.) FIG. 3 shows a cross section through a silicon wafer 1 with a thickness H, on the main surface of which aluminum spots 2 are used as doping sources are appropriate. It can be seen that the aluminum penetrates the silicon wafer to a depth h 1 is diffused. Due to the inevitable transverse diffusion, the p-doped area 3 extended over the width 1 of the aluminum layer 2 to a width L. This transverse diffusion is about 80; the penetration depth h and is due to the isotropic diffusion of aluminum in silicon. The jumping back of the diffusion front in the immediate vicinity The proximity of the surface comes from the diffusion of aluminum out of the silicon conditions. Also of importance is the fact that the diffusion front in the Depth h is straight.

Fig. 4 shows a cross section through a silicon wafer 1 with the Thickness D, on the opposite main surfaces of which aluminum patches 2 are used as Doping sources are applied. You can see that the aluminum is in front of both sides has diffused so deep into the silicon wafer 1 that a continuously p-doped Region 3 is formed, the diffused p-zones in the Ge @ iet 4 3 overlap.

The invention is intended to be more detailed on the basis of the following exemplary embodiment explained.

Lapped monocrystalline silicon wafers with a diameter of 50.8 mm and an orientation (111) were treated as follows: - Etching the surface by 7.56 m / side; - Full surface vapor deposition with aluminum of 2 / sm Starch, - application of a photo mask for the aluminum structure; - Etching with one Aluminum etching solution at 450C for 8 to 10 minutes; - Rinse and dry the etched discs, - stripping of the photoresist; - Put on the silicon wafers Quartz carrier, with the axes of rotation of the disks parallel to the long side of the Carrier lying; Distance between the panes 3mm, - Slide in of the equipped quartz carrier into the diffusion tube heated to 500oC; the diffusion tube is flushed with about 300 liters of nitrogen per hour with an addition of oxidizing gas, - heating of the diffusion tube at 1 K / min to 12500C; - Deep diffusion at 12500C for 72 hours, optionally dispensing with the addition of oxidizing gas will; Slow cooling to room temperature.

After this process, the pn junction was at a depth of h = 200 m; the appearance on the surface as well as in the depth of the silicon wafer corresponded that of FIGS. 2 and 11.

Claims (4)

Appropriate method for diffusing aluminum with a lateral limited doping structure in silicon wafers, with at least one flauptflache An aluminum sheet with a thickness of less than 5 µm is deposited on the silicon wafers and is structured by means of a photo mask, and then the silicon wafers in an inert gas atmosphere containing an oxidizing gas additive at diffusion temperature heated and held at this temperature for a sufficiently long period of time be, characterized in that the structured silicon wafers initially at a temperature a little below the eutectic temperature of the Al-Si system are tempered and that the heating from the tempering temperature to the diffusion temperature takes place with a small heating rate. 2. The method according to claim 1, characterized in that the heating rate (Ro) for silicon wafers with a diameter of 50 mm less than 5 K / min, in particular less than / equal to 1 fireplace is set. 3. The method according to claim 1 or 2, characterized in that the heating rate (R) for silicon wafers. with other diameters (D) according to the Relationship R = RO # 50mm / D is set. 4. The method according to claim 1, 2 or 3, characterized in that the silicon wafers after the diffusion process with a heating rate CR) corresponding speed be cooled down.