JPS6322457B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a mesa-type semiconductor device, particularly
This invention relates to a method for forming a glass protective film having a uniform thickness on a mesa surface where the end of a PN junction is exposed.

In order to increase the durability of semiconductor elements, methods of forming a glass film with good electrical insulation properties as a protective film for surface stabilization have recently been in the spotlight. Examples of methods for forming this glass film include electrophoresis, printing, centrifugal precipitation,
Known methods include a spin-on method using a spinner and a doctor blade method in which a suspension of glass powder is sprayed and then the sprayed surface is leveled. By the way, among the above-mentioned methods, electrophoresis tends to be particularly frequently used because it allows glass coatings to be formed on necessary parts and has high accuracy.

FIG. 1 is a diagram showing an outline of a method for forming a glass film by electrophoresis. For example, glass particles commercially available from Innotek Co., Ltd. of the United States under the trade name IP760 are mixed with acetone, isopropyl alcohol, methanol, or acetate ether. Suspension 1 is suspended in a non-aqueous solvent, and a small amount of a strong acid such as hydrochloric acid or nitric acid or a strong alkali such as aqueous ammonia or calcium hydroxide is added as an electrolyte. The glass particles in the liquid are charged positively ( ) or negatively ( ). After this, a silicon wafer 3 in which a large number of mesa-type semiconductor devices have been fabricated is placed in the suspension 2.
Then, an electrode plate 4 formed of a platinum plate or the like is immersed, and a DC voltage is applied between the two by a DC power source 5. This voltage application causes electrophoresis between the silicon wafer 3 and the electrode plate 4. Note that if the silicon wafer 3 is used as the positive electrode when the glass particles are negatively charged, and the silicon wafer 3 is used as the negative electrode when the glass particles are positively charged, then the charged glass particles always discharge their charge on the silicon wafer. It loses its charge and aggregates on the silicon wafer.

According to this electrophoresis method, a glass film is formed as described above, but when glass particles are suspended in a nonaqueous solvent, the type of glass, composition, particle size, or combination of solvents Impurities contained in the material itself and pollution from the atmosphere affect the electrolyte, causing the charged state of the glass particles to differ, resulting in variations in the aggregation rate of the glass film, and areas where a glass film should not originally be formed. There was a risk that problems such as the formation of a glass film on the surface of the substrate may occur.

For example, when the aforementioned glass particles (IP760) are suspended in acetone and hydrochloric acid is added as an electrolyte, a good concentration is 100 ± 20 ppm, and a lower concentration of 80 ppm not only slows down the glass deposition rate, but also This results in non-selective adhesion, that is, glass adhesion occurs even in unnecessary areas. On the other hand, at a concentration of 120 ppm, the glass adhesion speed is fast;
Selective glass adhesion progresses, resulting in variations in the thickness of the glass coating. That is, conventional electrolytes are
These are strong acids such as hydrochloric acid and nitric acid, or strong alkalis such as ammonia and slaked lime.These not only react with dust and carbon dioxide gas in the air during the formation of a glass film by electrophoresis, but also cause charged glass particles to become electrically charged. Hydrogen gas (H ₂ ) generated when discharging
and disappears into the air. This causes a change in electrolyte concentration. This caused the above-mentioned inconvenience.

The present invention improves the formation state of the glass film formed by the above-mentioned electrophoresis method, and manufactures a mesa-type semiconductor device in which the mesa surface can be covered with a glass protective film that is highly uniform and has excellent reproducibility. A feature of the present invention is that in place of the strong acid or strong alkaline electrolyte that is added to the suspension during the formation of a glass film by the usual electrophoresis method, strong acid salts or alkali metals are removed. A strong base salt is added to form a suspension, and a glass protective film is formed on a predetermined portion of a mesa-type semiconductor element by electrophoresis in this suspension.

By the way, the present invention has been made based on the following study and observation results. In other words, when we investigated the relationship between the aforementioned glass particles (IP760) and the electrolyte in an acetone solvent system, we found that these glass particles are normally negatively charged ( ), but when strong acids such as hydrochloric acid or nitric acid are added, they become positively charged ( ). It was found that it is charged, and that it becomes strongly negatively charged when aqueous ammonia is added. Furthermore, it has been found that even when the glass particles are positively charged by adding an acid, the amount of charge varies depending on the type of acid, and decreases in the order of nitric acid > hydrofluoric acid > hydrochloric acid. This means that the counter ion of the hydrogen ion H ⁺ adsorbed on the surface of the glass particles
This means that NO ₃ ^- , F ^- , and Cl ^- have an order of ease of specific adsorption (NO ₃ ^- > F ^- > Cl ^- ).

On the other hand, when potassium salt of nitric acid KNO ₃ is added as an electrolyte, compared to when nitric acid is added,
Since the difference between the adsorption forces of K ⁺ and NO ₃ ^- is larger than the difference between the adsorption forces of H ⁺ and NO ₃ ^- , the electrophoretic phenomenon becomes relatively insensitive. In other words, it has been found that the salt acts as a buffer against strong acids and has the effect of controlling the influence of the surrounding atmosphere.
This is necessary and extremely important in continuously forming a glass coating. Incidentally, the same tendency was shown to a lesser extent not only with such strong acid salts but also with strong alkali salts, and even when isopropyl alcohol or methanol was used instead of acentone as the non-aqueous solvent solution.

In other words, it was confirmed that hydrogen ions (H ⁺ ) are dominant in electrophoretic phenomena using non-aqueous suspensions.

In the present invention, based on these study results, the amount of charge on glass particles in the suspension is controlled by adjusting the amount of hydrogen ions (H ⁺ ), and the state of adhesion and formation of a glass film by electrophoresis is determined. is controlled.
By the way, the hydrogen ion (H ⁺ ) concentration can be measured using, for example, a commercially available PH meter, but since it takes time to prepare and calibrate, it is not preferable to use this for control in the actual manufacturing process. do not have.
Therefore, in the present invention, the hydrogen ion concentration is controlled within a predetermined range by continuously measuring the electrical conductivity of the suspension.

Figure 2 shows the change in conductivity of the suspension formed by suspending the glass particles (IP760) mentioned above in acetone and adding boron trichloride as an electrolyte, and the variation in the deposition rate and thickness of the glass coating. This ^is a diagram showing the relationship between
Curves B and C show changes in the maximum and minimum values of the film thickness. As is clear from the diagram,
Although there is a nearly proportional relationship between the conductivity of the suspension and the deposition rate, there is no such relationship between the maximum and minimum values of the film thickness with respect to changes in the conductivity. That is, when the conductivity is less than 200 μcm ⁻¹ , there is almost no change, and the difference Δt between the two is also extremely small. However, 200μ
When it exceeds cm ^-1 , it changes rapidly and Δt becomes extremely large. This means that the variation in the thickness of the glass coating increases. When the above-mentioned conductivity is low, 100 μcm ^-1 or less, a glass film is formed with a low deposition rate but with little variation. A glass coating is also formed on the parts, for example the SiO ₂ film, which has an adverse effect on the subsequent manufacturing process.

On the other hand, when the conductivity exceeds 200 μcm ^-1 , although the deposition rate increases, selectivity occurs between the n-type semiconductor layer side and the p-type semiconductor layer side of the mesa surface to which the glass coating is to be attached, and in some cases. As a result, the desired glass coating cannot be formed. Therefore, in order to reliably eliminate the above-mentioned disadvantages, it is desirable that the conductivity of the suspension is selected to be approximately 150±50μ. Regarding the conductivity of this suspension, almost the same results were obtained even when the electrolyte and solvent were changed.

Next, the present invention will be explained using examples.

Example 1 10 g of glass powder (IP-760), 0.1 g of NH ₄ NO ₃ as an electrolytic control agent and 1 part of isopropyl alcohol
was added to obtain a suspension with a conductivity of 150μ. A silicon wafer with a plurality of mesa-shaped semiconductor elements fabricated in it is immersed in this suspension, and the silicon wafer is used as a negative electrode and the platinum electrode plate is used as a positive electrode, and a voltage is applied at a distance of 40 mm between them. voltage
The glass powder (particles) were deposited on the silicon wafer by electrophoresis at a voltage of 40 V for 60 seconds. The glass adhesion rate at this time is 3 mg/cm ²
It was hot in minutes. After this, the silicon wafer was heat-treated at 900°C for 15 minutes in air.
When the attached glass powder was sintered, a glass coating of approximately 15 μm was formed. The silicon wafer with the glass protective film formed on the mesa surface through the above processing is divided to obtain mesa-type semiconductor devices, which are then TO-
The mesa-type semiconductor device, which was completed by resin-sealing in a 220-inch package, had extremely low leakage current at high temperatures.

Example 2 10g of glass powder (IP760) as electric field control agent
Add 0.03 g of BCl ₃ to 1 methanol, and the conductivity
A 180μ suspension was obtained. A silicon wafer with a plurality of mesa-shaped semiconductor elements fabricated in it is immersed in this suspension, and the silicon wafer is used as a negative electrode and the platinum electrode plate is used as a positive electrode, and a voltage is applied at a distance of 40 mm between them. The voltage was set at 30 V and electrophoresis was performed for 60 seconds to deposit the glass powder (particles) on the silicon wafer. The glass deposition rate at this time was 2.7 mg/cm ² minutes. Thereafter, the silicon wafer was subjected to heat treatment at 700°C in air for 15 minutes to sinter the attached glass powder, forming a glass coating of approximately 17 μm. The silicon wafer with a glass protective film formed on the mesa surface through the above process is divided into mesa semiconductor elements, which are then sealed in resin in a TO-220 package to complete the high temperature mesa semiconductor device. The Cooke current was extremely small.

Example 3 10 g of glass powder (IP760) and 0.15 g of carbonate (PbCO ₃ ) as an electrolytic control agent were added to 1 methanol to obtain a suspension having a conductivity of 130μ. A silicon wafer with a built-in element is immersed, and the silicon wafer is used as a positive electrode and the platinum electrode plate is used as a negative electrode.The opposing distance between the two is 40 mm, and the applied voltage is applied.
The glass powder (particles) were deposited on the silicon wafer by electrophoresis at a voltage of 40 V for 80 seconds. The adhesion rate of glass at this time was 3.1 mg/cm ²
It was hot in minutes. After this, the silicon wafer was subjected to heat treatment at 700°C for 15 minutes in air.
When the attached glass powder was sintered, a glass coating of approximately 15 μm was formed. The silicon wafer with the glass protective film formed on the mesa surface through the above processing is divided to obtain mesa-type semiconductor devices, which are then TO-
The mesa-type semiconductor device, which was completed by resin-sealing in a 220-inch package, had extremely low leakage current at high temperatures.

As explained above using the examples, when a glass film is deposited and formed by the method of the present invention, not only a glass film with a uniform thickness is formed, but also the continuous electrophoresis solves the conventional problem. Variations in the amount of glass coating deposited between aged silicon wafers are also significantly reduced. In addition, glass particles
It is also possible to eliminate the phenomenon of abnormal adhesion that occurs on areas where the formation of a glass film is originally unnecessary, such as on the SiO ₂ film.

FIG. 3 is a diagram showing a cross-sectional structure of a mesa-type semiconductor element, for example, an npn mesa-type transistor, formed by the method of the present invention, where 6 is an n-type silicon substrate serving as a collector region, and 7 is a p-type base layer. , 8 is an n-type emitter region, 9 and 10 are electrodes, and 11 is an n-type emitter region.
The SiO ₂ film, 12 is a collector base junction whose end is exposed on the mesa surface, and 13 is a glass protective film. By the way, if electrochemically active ions adhere to the surface on which the illustrated glass protective film 13 is formed, the characteristics of the formed element will deteriorate. Therefore, when using a strong base salt as an electrolyte, alkali metals such as lithium, sodium, and potassium must be excluded.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an overview of the method for forming a glass film by electrophoresis, Figure 2 is a diagram showing changes in deposition speed and thickness with respect to changes in conductivity of a suspension, and Figure 3 is a diagram showing the present invention. FIG. 3 is a cross-sectional view of a mesa transistor formed by the method of FIG. 1... Suspension, 2... Container, 3... Silicon wafer, 4... Platinum electrode plate, 5... DC power supply, 6
...n-type silicon substrate, 7...p-type base layer, 8
... n-type emitter region, 9, 10 ... electrode, 11
...SiO ₂ film, 12 ... Collector base junction, 1
3...Glass coating.

Claims (1)

[Claims] 1. A mesa-type semiconductor device with at least one PN junction exposed on its surface is suspended in a non-aqueous solution containing glass powder and a strong acid salt or a strong base salt other than an alkali metal. and increase the conductivity to 150±
A semiconductor device characterized in that a glass film is formed on the PN junction end by immersing it in a suspension of 50 μcm ⁻¹ and using the same mesa-shaped semiconductor element as one electrode by electrophoresis. manufacturing method. 2. The method for manufacturing a semiconductor device according to claim 1, wherein the nonaqueous solvent solution is one of acetone, methanol, or isopropyl alcohol.