DE1521975C - Process for thin etching of silicon wafers - Google Patents

Description

The requirements in the high-frequency behavior of many semiconductor components, especially for memory switching diodes, require defined thin crystal layers of different doping. In addition is still a long service life of the minority charge carriers is required, so that the epitaxial technology is only limited is applicable.

The switching behavior of memory switching diodes is largely determined by the impurity gradient in the area of the pn junction. In the area of optimal diffusion conditions around μ fluctuating .Scheibendicke causes the silicon single crystal, a change j3es_Störstellengradienten by a factor of 10. "

The aim of the present invention is to produce plane-parallel, approximately 45 μ thin, high-resistance silicon single crystal wafers with a thickness tolerance within the wafer of ± 1 μ.

It is very difficult, if not even impossible, to produce thin, high-resistance silicon wafers without a greater destruction depth and waviness or crowning of the crystal surface than 3 μ by mechanical polishing or etching with acidic etching solutions. The most common etching solutions consist of a mixture of concentrated hydrofluoric acid, nitric acid and acetic acid. When etching silicon monocrystals, depending on the acetic acid concentration, spherical surfaces are created and, at higher concentrations, concave surfaces. In addition, the etching is very uneven and is very dependent on the different crystal orientations and dislocations. ■

By means of alkaline etching solutions, e.g. B, with concentrated solutions of KOH or NaOH, at different temperatures one obtains a more favorable temperature constancy than with acidic etching solutions, because the released heat of reaction can easily flow away at low temperatures due to the slow course of the reaction, on the other hand a solution close to the boiling point can no longer be significantly heat. Nevertheless, even with alkaline etching solutions, crystal disks do not result in plane-parallel surfaces, because of the wavy surface of more than 3 μ that already exists after the polishing process. In addition, the etching in pure KOH or NaOH solution results in a stronger attack on the material from the edge of the disk, which is oriented in the (III) direction as usual, than on the crystal surface. (

The removal rate can be reduced by adding oxidizing or reducing agents to etching solutions can be influenced in the case of thickness etching. By oxy-. ding or reducing additives to alkaline Etching solutions can change the thickness etching speed on silicon wafers oriented according to (III) planes the factor 2 and more can be reduced or increased, while the etching from the edge only relatively is little influenced. The strongest oxidizing agents even bring the reaction to a standstill.

If this type of surface etching is used to produce thin silicon single crystal wafers made of high-resistance material for crystals whose lapped surfaces are oriented in the (III) plane, etching pits are created on the surfaces that have a surface roughness, i.e. a depth of the unevenness found , which is 8 to 12 μ .

The invention now relates to a method for producing thin, plane-parallel, in particular high-resistance wafers of silicon single crystals with a surface roughness of less than 2 μ and a thickness of 5 to 100 μ, preferably 45 #, for producing ν of semiconductor components by etching the single crystal wafers after lapping and fine polishing by means of an oxidizing agent containing alkaline

see etching solution, in which the advantage of the alkaline Etching solutions with oxidizing additives at higher temperatures on lapped silicon single crystal wafers is associated with a particularly favorable crystal orientation for the etching removal.

According to the teaching of the invention, single crystal disks oriented in the (HO) or (511) plane are etched with a solution which contains bromine and / or higher-value oxygen compounds of bromine, lead or manganese as the oxidizing agent. The most suitable is potassium dichromate K ₂ Cr ₂ O ₇ , which converts into its less strongly oxidizing decomposition products in the potassium hydroxide solution, which reach a certain high concentration and slow down the decomposition reaction. As long as there is still something that has not disintegrated

K ₂ Cr ₂ O _{7 is present} in the etching solution, even boiling, highly concentrated KOH or NaOH hardly affects the surface of the silicon.

According to a further development of the invention is

seen that the production of the silicon single crystals oriented in the (511) plane by cutting from a (III) -oriented crystal, which along a (111) surface inclined at about 20 ° to the rod axis in such a way What is complicated is that most of the panes have a (HI) and a 16 ° misoriented (100) surface portion (actually (511) surface) show, takes place.

The concentration of alkaline constituents in the etching solution is set between 10 and 50 percent, preferably 40 percent, in the method on which the invention is based. The addition of oxidizing agents takes place in excess, so that a sediment of, for example, K ₂ Cr ₂ O ₇ or K ₂ CrO _{4 is} present at the beginning of the reaction. The temperature during the etching process is kept constant in the vicinity of the boiling point of the etching solution, for example, at 12O ⁰ C.

The aim of the present invention, plane-parallel, about 45 μ thin, high-resistance silicon wafers with a specific resistance greater than 20 ohms. To produce cm with a thickness tolerance of ± 1 μ within the panes, was etched to the desired thickness with alkaline solutions with reaction-inhibiting additives of oxidizing agents.

The method on which the invention is based is to be explained in more detail using an exemplary embodiment will.

The silicon single crystal wafers, lapped to 180 ± 1 μ and with as little dislocation as possible, with a specific resistance greater than 20 OHm.cm and a (HO) -oriented surface, are placed upright in a Teflon holder in an aluminum oxide vessel with 40 percent NaOH solution containing an excess of K ₂ CrO ₄ or Na ₂ Cr ₂ O ₇ is added, treated and etched down to the desired slice thickness, for example up to 45 μ. An excess of K ₂ CrO ₄ or Na ₂ Cr ₂ O ₇ means that enough chromium salt is added that a sediment is always present during the etching reaction. The etching vessel made of alumina is immersed in a 125 ⁰ C hot glycerine bath, which is held with heating coil and contact thermometer at a constant temperature. The etching vessel is provided with a cover to avoid excessive evaporation and the formation of carbonates. Instead of NaOH, KOH can also be used. After dialcal etching, a brief overetching in a hydrofluoric acid-nitric acid mixture is carried out to remove heavy metal deposits on the silicon single crystal wafers before further processing. The duration of the entire etching process depends on the temperature of the etching solution and is 40 to 45 minutes with an initial thickness of the silicon wafers of 180 ± 1 μ up to the desired wafer thickness of, for example, 45 ± 1 μ under the conditions given above.

The thin silicon wafers obtained by the method according to the invention have a surface roughness of less than 2 μ without additional crowning and waviness of the wafer.

If one again assumes finely lapped, but spherical (511) -oriented disks as a result of the fine lapping, disks of almost any thickness with a surface roughness of less than 1 μ are obtained with the etching solution on which the method according to the invention is based and under the same working conditions.

It did not succeed, both on (100) surfaces and on (III) surfaces without misorientation a better leveling effect for thickness etching than a surface roughness of 8 to 12 μ also corresponds to oxidizing or reducing To achieve additives in the etching solution.

The method according to the invention can be applied in a very advantageous manner to the production of semiconductor components where very thin crystal layers are required, such as Memory switching diodes.

It is also possible to use the method according to the invention for the production of piezoresistive To use arrangements, for example for strain gauges.

Claims (6)

1 521 S75 claims: 1. A method for producing thin, plane-parallel, in particular high-resistance wafers of silicon single crystals with a surface roughness of less than 2 μ and a thickness of 5 to 100 μ, preferably 45 μ, for the production of semiconductor components, by etching the single crystal wafers after lapping and fine polishing by means of a Alkaline etching solution containing oxidizing agent, characterized in that single crystal disks oriented in the (HO) or (511) plane are etched with a solution which contain bromine and / or higher-valent oxygen compounds as oxidizing agent. 2. The method according to claim 1, characterized in that the production in the (511) plane oriented silicon single crystals by cutting from one with a twinning provided, (11 l) -oriented crystal takes place. 3. The method according to claim 1, characterized in that the single crystal wafers are etched with a solution which contains K ₂ Cr ₂ O ₇ or K ₂ CrO ₄ as a higher-value oxygen compound. 4. The method according to at least one of claims 1 to 3, characterized in that the Single crystal wafers are etched with a solution which, in terms of their alkaline components has a concentration of 10 to 50%, preferably 40%. 5. The method according to at least one of claims 1 to 4, characterized in that the Etching close to the boiling point of the etching solution while keeping the temperature constant, preferably at 120 ° C. 6. The method according to at least one of claims 1 to 5, characterized in that the higher-value oxygen compounds are added in supersaturated solution.