JP2569871B2 - Reference electrode - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a reference electrode used for controlling an electrochemical reaction.

(Prior Art) Conventionally, a semiconductor acceleration sensor has been manufactured by making full use of micromachining technology called micromachining of single crystal silicon. The acceleration sensor has a beam structure having a scoop-like weight at the tip. When acceleration is applied to the structure, a force proportional to the acceleration is generated at the tip weight, and a stress proportional to the force is also generated at the supporting portion of the beam supporting the tip weight. Therefore, it is possible to convert a change in the stress generated in the support portion into a change in the electric resistance value.
A piezoresistor, which is a so-called pressure-sensitive element, is formed on a support portion of the beam, and is finally converted into an electric signal to detect acceleration. The earliest accelerometer structures include IEEE TRANSACTIONS ON ELECTRON DE
VICES), Vol.ED-26, P1911, 1979. Acceleration sensors include a process for forming a thin film of silicon. In this thin film forming process, a technique is used in which anisotropic etching is performed on silicon and the etching is stopped when a desired thin film is formed. Conventionally, a process using an anodic oxidation reaction of silicon has been used for this stopping technique. This etching stop technique is achieved by etching a silicon substrate in an etching solution while applying a voltage necessary to cause oxidation. Since the etching solution does not dissolve silicon oxide, it is a technique for stopping the etching by forcibly forming an oxide film. Usually, a chemically inert platinum electrode is used for the counter electrode, and a predetermined voltage is applied between the silicon and the platinum electrode. The normal oxidation reaction is realized by assembling such a simple system, but in general, the voltage at which the anodization reaction starts varies due to the high electric resistance of the etching solution and the distance between silicon and the counter electrode. Is commonly observed. Therefore, recently I Triple E Transactions on Electron Devices, Vol.
As described in 36 (4), P663 (1989), attempts have been made to reproducibly carry out an anodic oxidation reaction of silicon, which is an electrochemical reaction, using a reference electrode, which is a standard for electrochemistry. In this case, a silver-silver oxide electrode or a saturated calomel electrode is used as a reference electrode. This reference electrode uses a silver elution reaction. Silver chloride is a poorly soluble salt, and a saturated solution can be easily obtained. Therefore, when used with silver, a voltage with good reproducibility is generated. The structure is shown in FIG. At the center of the electrode is a silver electrode 10. A silver chloride crystal 11 is stuck around it, and a saturated potassium chloride solution 12 surrounds it. The electrode is housed in a glass tube 13 and a porous pipe 14 made of ceramic is provided below to maintain electrical contact with an external solution. A solution of potassium chloride is constantly flowing out of the hole. The solution of potassium chloride continually escapes and must be replenished before it runs out. Therefore, a hole 15 for replenishing potassium chloride is provided above. Lid for storage
16 is tightened, but the lid is opened during the experiment to smooth out the potassium chloride.

(Problems to be Solved by the Invention) In a method of controlling an etching stop reaction using a reference electrode, which has appeared instead of the conventional etching stop technology, a silver-silver chloride electrode or a saturated calomel electrode is used as a reference electrode. Because of its use, there is a problem in that the solution is contaminated by potassium chloride oozing out of the electrode, thereby contaminating the semiconductor manufacturing process line. Further, since the etching is usually performed at a high temperature of about 90 degrees, the life of the reference electrode has been significantly shortened. Furthermore, during the experiment, it is necessary to open the lid to smooth out the flow of potassium chloride.Therefore, there is a problem that the etching solution penetrates into the reference electrode and the measured value fluctuates, which is meaningless. Was.

An object of the present invention is to overcome the above problems, prevent contamination of a semiconductor manufacturing process line, and provide an easy-to-use reference electrode.

(Means for Solving the Problems) In the present invention, a semiconductor having a property of dissolving in the above solution is used as a material of the reference electrode. Note that only the crystal plane of the semiconductor, for example, only the <111> crystal plane may be exposed.

(Action) Generally, when a metal or the like is put into an acid or alkali solution, a phenomenon of dissolution is observed. When different metals are placed in the same solution, a voltage is created between their electrodes, a well-known Volta battery. The voltage obtained by such electricity has the property of being substantially determined by the metal used. Therefore, in the electrochemistry, a voltage obtained when hydrogen having a certain concentration is used as one electrode is defined as a standard starting force and used as a difference in the electrochemical reaction. As described above, once a reference is determined, a specific oxidation-reduction reaction occurs at a specific potential. Therefore, in an electrochemical experiment, data is described based on this potential. Originally, the reference electrode is based on hydrogen as described above. However, since the usability of the hydrogen electrode is unusually poor, an electrode that is easier to use is actually used as the reference electrode.

Since the reference electrode is required to have a constant potential regardless of the surrounding environment as a fixed point of the electrochemical, the reference electrode is filled with a saturated solution containing an alkali metal, in which a reference electrode reaction is performed. It has a structure in which electrodes for raising are installed. In order to easily obtain a saturated solution, a saturated solution of a hardly soluble substance is used, and silver is frequently used for an electrode. As described above, since the conventional reference electrode contains a large amount of alkali metal and becomes a source of contamination in the semiconductor process, the present invention has considered a disposable reference electrode from a practical viewpoint. It is known that silicon itself is dissolved in an alkaline solution to perform an electrochemical reaction for emitting electrons. The potential of this reaction is (-1.4 V) vs (saturated calomel electrode). Therefore, a method is conceivable in which a piece of silicon is prepared separately from the silicon substrate to be used for etching and is used as an electrode. If the same material as the substrate material is used for the electrode, the electrode does not become a source of contamination in the semiconductor manufacturing process. In addition, since the etching state of the reference electrode and the etching state of the semiconductor wafer used for the etching are the same or substantially similar, the change is automatically added to the fluctuation of the solution or the fluctuation of the temperature. Is done. Therefore, good etching can be performed.

(Example) FIG. 1 shows a first example of the present invention. It has a structure in which a part of the elongated silicon 1 is exposed and the other part is covered. Since the etching occurs from all places without the coating 2, the wiring is detached with the progress of the etching and good results cannot be obtained. Therefore, it is necessary to cover the wiring with a chemically inert material. Teflon (registered trademark) is good. A lead wire 3 for extracting a voltage to the outside is connected from the end of the silicon electrode. Since this wire needs to be chemically inert, a platinum wire is preferably used. The lead wire is connected to a region of the substrate which has been reduced in resistance by diffusion in advance.

FIG. 2 shows a second embodiment. This electrode is characterized in that an inverted triangular hole prepared in advance by anisotropic etching is used as an electrode. Each plane of the inverted triangle formed by etching is composed of a (111) crystal plane. Since this crystal plane has a very low dissolution rate in an anisotropic etching solution, if this plane is used as a reference electrode, it can be used for a long time.

FIG. 3 shows a third embodiment. In this example, by using a silicon substrate cut out from a predetermined wafer so that the specific crystal plane 5 of silicon becomes the main surface, an electric potential corresponding to the specific crystal plane of silicon is extracted. The feature is that it consists of. This makes it possible to set the potential of the wafer to be etched in accordance with each crystal plane orientation. Wafer orientation is (100) and anisotropic etching is (111)
When performing etching while exposing the surface If the silicon surface of the reference electrode is also set to (100), it is etched in the same etching state while exposing the (111) surface like a wafer. The variation is automatically added to the variation and the like. Therefore, good etching can be performed. Although the surface of the reference electrode is different from that of the wafer, for example, (111), although not as strict as when the surface is the same, the above-mentioned change is added to some extent.

FIG. 5 shows an etching process using these electrodes. Reference numeral 20 denotes a jig for applying a voltage to a silicon wafer substrate, which contains a silicon substrate to be subjected to etching. Wiring is provided inside and outside of the jig so that different voltages can be applied to the epitaxial layer and the substrate. The entire jig is made of a chemically inert material such as Teflon. After setting the silicon wafer on this jig, any one of the reference electrodes 22 shown in FIGS. 1 to 3 is set, and the platinum plate 21 as the counter electrode is further set, and the anisotropic etching solution 30 such as hydrazine or the like is set. The filling,
Etching is performed at a predetermined temperature, for example, 90 ° C. Each electrode is connected to a potentiostat 23, and a predetermined voltage is applied. In this way, an etching stop experiment with good reproducibility can be performed, and the alkali compatibility does not enter the silicon process, so that the process compatibility can be improved. The wiring material is preferably chemically inert, and may be made of platinum or the like, or may be used by coating the surface of another conductor. In the above description, an example in which silicon is used as the material of the reference electrode has been described. However, when a GaAs wafer is etched, GaAs is used as a reference electrode, and when a Ge wafer is used, Ge is used as a reference electrode.

(Effect of the Invention) Since an electrode made of a pure semiconductor is used instead of the silver-silver chloride electrode or the saturated calomel electrode conventionally used as a reference electrode, the electrode does not essentially contain an alkali metal. No contamination of the semiconductor process occurs. Furthermore,
Since a voltage is generated by the actual melting of the semiconductor, a change in the etching solution and an influence on the etching due to a temperature change are added to the generated potential, and an appropriate potential is automatically generated. Since it is only necessary to use a substrate similar to the semiconductor substrate actually used, it is easy and a new electrode can always be used. Since the electrode is a semiconductor, when a conventional reference electrode is used at a high temperature, deterioration is observed, but there is no such concern. In the conventional electrode, it is necessary to open a lid for replenishing potassium chloride at the time of etching, so that it was observed that the etching solution entered the reference electrode during the experiment. No worries. An electrode using the <111> plane obtained by anisotropic etching is difficult to be etched when used as a reference electrode, so that the reduction rate of the reference electrode material is small and the electrode life can be extended.

[Brief description of the drawings]

FIG. 1 is a sensor structure diagram showing a first embodiment of the present invention. FIG. 2 is a structural view of a sensor showing a second embodiment of the present invention. FIG. 3 is a sensor structure diagram showing a third embodiment of the present invention. FIG. 4 is a view showing the structure of a conventional reference electrode. FIG. 5 is a view showing an experimental apparatus using a reference electrode. 1 ... silicon, 2 ... coating, 3 ... lead wire, 4 ...
Etching hole, 5 ... single crystal plane, 10 ... silver electrode, 11 ...
... Silver chloride crystal, 12 ... Saturated potassium chloride solution, 13 ... Glass tube, 14 ... Pipe, 15 ... Hole, 16 ... Lid, 20 ... Jig, 21 ... Counter electrode, 22 ... Reference electrode , 23 ... Potentiometer

Claims (3)

(57) [Claims] In a reference electrode used to control an electrochemical reaction in a solution for etching a semiconductor,
A reference electrode, wherein a semiconductor having a property of dissolving in the solution is used as a material of the reference electrode. 2. The reference electrode according to claim 1, wherein only a specific crystal plane is exposed. 3. The method according to claim 1, wherein said step is performed by anisotropic etching.
> The reference electrode according to claim 2, wherein a crystal plane is used.