Dielectric substrate with reduced and stabilized surface electrical conductivity, process for its manufacture, and use of the substrate.

(42) Date of commencement of the patent: 15. 6.1990 (45) Date of issue: 10.12.1990

<td>(56) Documents:</td><td>(73) Patent owner:</td>

<td>GB-PS 1236481 US-PS 4263350 US-PS 4274856 EP-Al-0082079</td><td>AVL COMPANY FOR INTERNAL COMBUSTION ENGINES AND MEASUREMENT TECHNOLOGY MBH. PROF.DR.DR.HC HANS LIST A-8020 GRA2, STEIERMARK (AT).</td>

<td></td><td>(72) Inventor:</td>

<td></td><td>SARACOGLU IBRAHIM ADNAN DR. GRAZ, STYRIA (AT).</td>

(54) PROCEDURE FOR REDUCING AND STABILIZING ELECTRICAL CONDUCTIVITY (57) For the sustainable, Solvent-resistant and temperature-resistant reduction and stabilization of the electrical conductivity of the surface of silicate-containing substrates crystalline, polycrystalline or amorphous structure is proposed that the surface of the substrate is chemically cleaned, and with in at least one solvent from the group of the alcohols and halogenated hydrocarbons, preferably low chlorinated hydrocarbons (eg Chloroform, dichloromethane), dissolved chlorotrimethylsilane (CHjCISi) is brought into contact, and the solvent and the resulting in the chemical reaction liquid and gaseous substances are then allowed to evaporate.

QQ

AT 391 859 niBwme

No. 391,859

The invention relates to a method for lowering and stabilizing the electrical conductivity of silicate-containing dielectric substrates of crystalline, polycrystalline or amorphous structure intended for sensor technology, wherein the surface of the substrate is chemically cleaned.

Such substrates require the highest possible high insulation resistance, independent of the temperature and of chemical or mechanical influences. They are z. B. used as a substrate for the common construction of sensor and measuring electronics, or for electrodes or bushings in high-impedance sensors and measuring amplifiers, such. B. in electrochemical measuring systems. They also serve as a dielectric for various sensors, eg. B. of the resistive type in which the measurand to be determined influences the resistance of a conductor or semiconductor deposited on the dielectric substrate, or of the capacitive type in which the measurand to be determined has the capacitance, voltage to be measured on electrodes applied to the dielectric or amount of charge.

A special case of the capacitive sensor type are the piezoelectric sensors, where extremely high insulation resistance is required. For example, a good quartz pressure transducer must have an insulation resistance greater than 10¹⁴ Have ohms, so that sufficient quasi-stationary measurements are possible. To achieve this high value, each of the parallel-connected quartz elements must have a high insulation resistance.

Of course, the substrates mentioned often have a very low electrical conductivity, but the practically effective insulation resistance is significantly reduced in most cases by the surface conductivity. This is due in general to a contamination of the surface, ie substances that are connected to the

Surface of the substrate are attached and which are held by physisorption, chemisorption or by a chemical bond to the surface.

Various methods for lowering the electrical conductivity of silicate-containing substrates are known. Above all, various chemical solvents are used to clean the surface. In particular, attached water must be removed, but also by z. B. Store in dry

Atmosphere at elevated temperatures can be achieved.

Such a cleaned silicate surface has an extremely low stability. Purified silicate surfaces are chemically very active due to their strongly polar character and seek to saturate their activity by the addition of foreign substances. In the first place, water is re-deposited from the ambient air. As a result, the surface electrical conductivity and the

Insulation resistance decreases.

There are several methods to protect silicate surfaces from re-attachment of water molecules after their purification. These are used z. B. to prevent misting with water vapor from optical devices, such as mirrors. In general, while hydrophobic layers are applied to the substrate to be protected, in which case z. As fats or waxes containing substances, and especially substituted silanes and silicones would be mentioned.

For example, in Silicon and Silicones [ISBN 3-540-17565-2] by EG Rochow, Springer Verlag, 1987, in the chapter entitled Some Interesting Applications, how to render silicate surfaces and paper hydrophobic by the action of vapors of various methylchlorosilanes can.

A disadvantage of the said methods is that the applied n layers have insufficient stability at elevated temperatures and mechanical or chemical stress and also disturb other important properties of the substrate, such as the optical or tribological nature, disturbing.

These methods are therefore hardly used in silicate insulators for sensor technology.

Object of the present invention is to develop a method of the type mentioned so that the treated substrate surfaces have a low electrical conductivity, are durable resistant to chemical and mechanical influences and thereby have a high temperature resistance compared to conventional methods.

This object is achieved in that the surface of the substrate with in at least one solvent from the group of alcohols and halogenated hydrocarbons, preferably lower chlorinated hydrocarbons, dissolved chlorotrimethylsilane ((CHß ^ ClSi) is brought into contact and the

Solvent and the resulting in the chemical reaction liquid and gaseous substances are then allowed to evaporate. For example, chloroform or dichloromethane may be used as the solvent.

This results in a stable, hydrophobic and highly insulating substrate surface. The advantageous properties of the substrates treated by the process according to the invention are summarized as follows:

- non-wetting for water, formation of water globules upon condensation of water vapor;

- high insulation resistance (> 10 ^ Ω);

- The degree of isolation decreases with precipitation of water vapor, for example after breather, but reaches within minutes after the evaporation of the condensate back to baseline;

- Resistance of the passivation during storage of the substrate in normal room air;

- Resistance even at higher temperatures up to 300 ° C;

-2Nr. 391 859

- Chemically resistant to common solvents such as water, water vapor, surfactants, detergents, acetone, methanol, gasoline, weak acids and alkalis (pH = 2-11), etc .;

no noticeable change in the mechanical surface properties such as hardness, brittleness, roughness, etc .;

the passivation can also take place after the application of any electrodes, for example vapor-deposited gold electrodes, to the substrate without entailing destruction of the electrodes or weakening of their adhesion to the substrate;

- Cleaning and passivation of the substrate surfaces take place simultaneously.

By the method according to the invention, the high chemical activity of the purified silicate surface is rendered ineffective by the saturation of its endeavor to enter into chemical bonds. In this case, the silicate-containing substrate, or attached to the surface of water is chemically removed and replaced at the same time by an organic hydrophobic radical.

As a solution very suitable for the process, chlorotrimethylsilane dissolved in chloroform (CHCl3) was found. The suspected mode of action of this solution on z. B. a quartz surface with attached water will be described below.

HH 0

Area ΠΙ: H deposited water,

Area Π: physisorption x .0 .0, Q. H '_H \ θ · b H^Z \

Area I: Chemisorption

0H

0H

Students' Union

Sl Sl / I

0 0

Silicate substrate - S i ^- 0 - Si - 0 - Si -

It can be assumed that directly adjoins the silicate surface area I, which is characterized by the chemisorption, ie by the formation of chemical bonds between the substrate and deposited substance. The free valences of the oxygen atoms located on the surface are saturated by hydrogen atoms. Nevertheless, these oxygen atoms retain their strong electrical polarity, so that in the subsequent region II water molecules are distorted by the action of the polarity and adhere by the formation of hydrogen bonds and by physisorption. With growing

-3Nr. 391 859

Distance from the quartz surface becomes more and more ineffective its polarity, until in area ΠΙ attached water is held only by simple hydrogen bonds.

If you treat the surface sketched above with z. B. in chloroform dissolved chlorotrimethylsilane, the following reactions take place:

1. In area III, all of the accumulated water is removed to form trimethylsilyl alcohol and hydrochloric acid:

(CH₃)₃SiCl + H₂O -> (CH₃)₃SiOH + HCl t

The hydrochloric acid escapes and the alcohol remains in the solvent.

Second The physisorbed water in area Π can also be removed in a similar way:

(CH₃)₃SiCl + <H₂O> ------> (CH₃)₃SiOH + HCl t physisorbed

In doing so, some of the (CH₃)₃Groups of silanol alcohol the binding energy of the

Hydrogen bond binding (HBB) and electron cloud distortion (ECD) bound water molecules down so that the H₂O molecules from area Π are pushed into the area KI.

This reaction continues until the layered structure of the water at the interface of regions II and I is disturbed. The release of water molecules corresponds to an increase in their mobility and thus an increase in entropy. Because of AG = ΔΗ - TAS thus the free enthalpy AG is lowered, whereby the reaction is stabilized.

Third By displacing the water molecules from region II, there is the possibility that chlorotrimethylsilane reacts with the surface in region I:

(CH₃)₃SiCl + HO-Si = -> (CH₃)₃Si-O-Si = + HCl t

Quartz surface with chemisorbed hydrogen

Quaiz surface with chemisorbed (CH₃)₃Si groups

At this step of the reaction, gaseous HCl escapes again while the (CH₃)₃Si groups are bound to the free valences of the oxygen atoms of the quartz lattice. The desired effect of permanently increasing the insulation resistance of the surface was thus achieved: all the water responsible for the electrical conductivity was removed and the re-addition of water is due to the hydrophobic property of the electrically non-conductive to the surface chemically bound (CH₃) Groups prevented.

According to the invention, it is provided that a 5 to 20% strength chlorotrimethylsilane solution in chloroform is used.

Advantageously, the method according to the invention can be carried out very rapidly. It is envisaged, for example, that the substrate is immersed for 15 seconds to 2 minutes in the solution of solvent and chlorotrimethylsilane, that the substrate at temperatures between 15 ° and 50 ° C for 15 seconds to 2 minutes in air or dried in a dry atmosphere and then at least stored for 1/2 hour. Measurements taken immediately after the wet-chemical treatment show a still somewhat unstable insulation value. About one hour later you will reach stable values> 10 ^ Ω. An optimally stabilized surface is obtained by storing the freshly passivated substrates for a few hours in room air in the fume cupboard, or when exposed to a dry atmosphere. The reason why it takes some time until the H bound directly to the substrate₂O-layer is replaced by CHyGruppen, or until the covalent bonds have formed on the quartz surface is the following: The closer the chemical reaction reaches the quartz surface, the slower the reaction rate, because on the one hand the outer layers, in which the reaction already done, a hydrophobic barrier for the underlying H₂O layers, and on the other hand, the binding forces of H₂O molecules to the substrate become stronger the closer one gets to the substrate surface.

A particularly advantageous application of the method according to the invention is given when a silicate-containing, piezoelectric element is used as the substrate, or when the piezoelectric element consists of quartz. Especially in the application of piezoelectric quartz crystals in pressure sensors,

-4Nr. 391 859 high insulation values ​​are desired, which must be maintained over long periods of time at higher humidity, or higher temperatures and mechanical stress. With the method according to the invention, the insulation problem in Druckaufnehmem can be solved in extremely cheap, simple, hardly time consuming way.

There are of course many other applications conceivable, it can be used as a substrate optically transparent glass for the production of z. Windows, mirrors, lenses and optical prisms.

As an example, the passivation of a quartz plate, for example a piezoelectric measuring element, is given here:

1) The passivation of the quartz plate is best done in the fume hood, with the quartz plate is dipped into the solution with a glass holder. For a short time metals, eg. B. tweezers are immersed.

2) Due to the high volatility of the chemicals involved, the container should only be opened briefly for immersing and removing the quartz disks.

3) After about one minute in the solution, the quartz discs are carefully lifted out and exposed in the fume cupboard for about one minute to allow the chemical reaction to complete.

4) After about one hour of storage in the air, the quartz disks are completely cleaned and passivated. They can then be fed to their use.

5) The proton affinity of CH 3 groups is quite large, so that a proton is abstracted from the solvent to form silol. Therefore, a pH measurement of the solution should often be carried out. If the pH is greater than 2.3, a fresh solution should be prepared.

Finally, it is also possible according to the invention that the surface of the substrate is brought into contact with chlorotrimethylsilane ((CH 3) .beta.ClSi) and also chemical intermediates attached to the surface with at least one solvent from the group of alcohols and halogenated hydrocarbons, preferably lower chlorinated hydrocarbons (z. B. Chloroform, dichloromethane). The process according to the invention can thus be carried out, if necessary, in such a way that the surface of the substrate is first treated with chlorotrimethylsilane and then purified with chloroform.

Since all the chemicals used or the solution made from them are corrosive and easily flammable, care must be taken that they are not inhaled and always kept closed.