JPH0366609B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor pressure sensor, and particularly to a semiconductor pressure sensor suitable for use in a high temperature and high humidity environment.

As an example of a semiconductor pressure sensor, JP-A-1988-
As disclosed in Japanese Patent No. 62331, a device having the cross-sectional structure shown in FIG. 1 has been devised.

In FIG. 1, a gauge chip 1 as a diaphragm is made of a silicon semiconductor,
In the figure, the center portion of the lower surface is thin and the peripheral portion is thick. This gauge chip 1 is fixed on a glass die 2 made of pirate glass. The thick part of the gauge chip 1 and the glass die 2 are fixed by applying a high DC voltage with the gauge chip 1 as the positive electrode and the glass die 2 as the negative electrode in a high-temperature atmosphere, as shown in FIG. This was accomplished by a method of forming a bonding layer in the boundary layer, the so-called anodic bonding method.

However, when a semiconductor pressure sensor formed by the above-mentioned anodic bonding method is used in a high temperature and high humidity environment, it has the disadvantage that the gauge zero output shifts in the positive direction (so-called upshift). There was a huge problem with the reliability of the system.

An object of the present invention is to provide a semiconductor pressure sensor that can reduce upshift and improve reliability in a high-temperature, high-humidity environment.

The present invention conducted various experiments to investigate the cause of upshift under high temperature and high humidity conditions, and the results showed that the cause was mainly due to the hygroscopicity of the bonding layer formed by the anodic bonding method. This was done in consideration of the above, and by controlling the bonding current to form a bonding layer with a thickness of 3 to 6 μm, the upshift under high temperature and high humidity is reduced.

Hereinafter, the reason why the above object is achieved by the present invention will be explained.

First, the principle of the anodic bonding method applied to the present invention will be explained. A gauge chip 1 having the shape shown in FIG. 1 and a glass die 2 are stacked together, fixed with a jig, and housed in a chamber 3. This chamber 3 is placed in an electric furnace and heated to about 300 to 350°C, and a high DC voltage of about 800 to 1500 V is applied, with the gauge chip 1 being positive and the glass die 2 being negative.
At this time, the alkaline component of glass die 2
NaO is polarized, and oxygen ions are released from the bonding interface of the glass die 2 as shown in the following equation (1) and moved toward the gauge chip 1, which is the anode side.

Na ₂ O → 2Na ⁺ +O ^2- ...(1) In addition, the silicon (Si) of the gauge chip is also ionized and becomes Si ⁴⁺ , and the glass die 2 on the cathode side
moved to the side. Due to these ion diffusion,
It is thought that a bonding layer of SiO ₂ is formed at the contact boundary by the reaction of the following equation (2), and the gauge chip 1 and the glass die 2 are bonded by this layer.

2O ^2- +Si ⁴⁺ →SiO ₂ ...(2) Due to this reaction, gauge chip 1 and glass die 2
Junction current I _P flows between them. Therefore, it is considered that this junction current I _P influences the formation of the junction layer.

On the other hand, the cause of upshift in a high temperature and high humidity environment is thought to be due to moisture absorption by the glass die and moisture absorption by the bonding layer. In other words, their absorption of moisture causes a difference in the coefficient of thermal expansion between the gauge chip and the glass die or bonding layer, resulting in an upshift. Moreover, it was found that the upshift due to moisture absorption in the bonding layer was much larger.

Therefore, as mentioned above, since the bonding current I _P is involved in the formation of the bonding layer, we conducted a high temperature and high humidity durability test at 85°C - 85% RH using the bonding current I _P as a parameter, and measured the gauge zero point. Changes in output were measured experimentally. The results of this durability test are shown in Figure 2a.
~c.

In FIGS. 2a to 2c, time h is plotted on the horizontal axis, zero point output change mV is plotted on the vertical axis, and junction current I _P is plotted along the curve in the figures. As is clear from these figures, it can be seen that the zero point output change is greatly influenced by the junction current _IP . However, since conventional anodic bonding is a method that controls the applied voltage at a constant level, the bond may be washed away due to fluctuations in the bonding temperature, variations in the dimensions of the glass die, variations in the NaO content in the glass die between lots, etc. The current I _P was not constant. Therefore, it can be seen that the bonding layer formed was also not constant.

Next, it was specifically confirmed that the upshift in the zero point output is due to the effect of moisture absorption, as described above, and that this causes the diaphragm to become concave downward in FIG. An example of this result is shown in Figures 3a-d.

Figures 3a to 3d show measurements of the unevenness of the gauge chip surface using a surface roughness meter and a laser interferometer during each manufacturing process of the semiconductor pressure sensor. Figure a shows the gauge chip before the diaphragm is formed, and as shown, it has a convex shape of Δh=0.15 to 0.20 μm. When this gauge chip is used to form a thin part by etching at the center of the lower surface, it becomes more convex upward, as shown in Figure b, and Δh=
It became 0.32μ to 0.6μm. When this was anodically bonded onto a glass die, it was deformed in the opposite direction as shown in FIG. When a humidity test (100 hours) was conducted using the semiconductor pressure sensor formed in this way, as shown in Figure d, after the test was completed, the concavity was further increased and Δh = -0.15 ~ -0.2 μm. That is, it was confirmed that the diaphragm part was deformed into a concave shape due to moisture absorption, and was thereby upshifted.

In other words, since the thermal expansion coefficient α _S of the gauge chip 1 is larger than the thermal expansion coefficient α _G of the glass die 2, the thermal expansion coefficient α S of the gauge chip 1 is larger than the thermal expansion coefficient α G of the glass die 2. There is a difference in the
As shown in , a moment indicated by the arrow in the figure is generated in the diaphragm, causing it to concave downward. Furthermore, when the product formed in this way is placed in a high temperature and high humidity environment, the bonding layer 4 absorbs moisture and expands, and when it is taken out to room temperature, a compressive force is applied to the bonding layer 4, as shown in FIG. 4c. However, a tensile force is applied to the gauge chip 1, which causes the diaphragm portion to concave further downward. In addition, after diagonally polishing the joint part, XMA
When SiO ₂ was analyzed using an X-ray microanalyzer, it was found that the bonding layer 4 was a thin SiO ₂ layer formed on the gauge chip side.

In addition, the thickness of this bonding layer and the anodic junction current I _P have the correlation shown in Figure 5, and the zero point output change, anodic junction current, and characteristics shown in Figures 2 a to c. It can be seen from the above that there is an optimal bonding layer thickness. That is, when the bonding layer has a thickness of 6 μm or more, the amount of moisture absorbed increases, and the upshift of the zero point also increases. In addition, when I _P is 10 μA, a bonding layer of approximately 2.5 μm is obtained, but some voids remain that are not bonded, and moisture infiltrates, increasing the apparent amount of water absorption. The amount becomes large. Moreover, it cannot be said that the bonding strength is sufficient.

From these facts, the anodic junction current I _P is set to 15~
By controlling the current to 25 μA and forming the bonding layer to have a thickness of 3 to 6 μm, upshift can be significantly reduced.

As explained above, according to the present invention, even when formed by an anodic bonding method, upshift in a high temperature and high humidity environment can be significantly reduced.
It can have high reliability.

[Brief explanation of drawings]

Figure 1 is a diagram for explaining the anodic bonding method according to the present invention, and Figures 2 a to c show the relationship between zero point output change and junction current obtained by a high temperature and high humidity durability test to explain the present invention. Figures 3a to 3d are diagrams showing the results of surface unevenness measurement in the gauge chip manufacturing process to explain the present invention, Figures 4a to c are explanatory diagrams of the deformation process of the gauge chip, and Figure 5 is an experimental diagram. FIG. 3 is a diagram showing the relationship between the bonding current and the bonding layer thickness obtained by the method. 1...Gauge chip, 2...Glass die, 4...
...bonding layer.

Claims (1)

[Claims] 1. A semiconductor pressure sensor in which a gauge chip consisting of a diaphragm having a thick portion on the periphery is placed on a support made of a glass die, and the thick portion of the gauge chip is fixed to the support by an anodic bonding method, A semiconductor pressure sensor characterized in that the thickness of the bonding layer formed between the thick portion and the support base by an anodic bonding method is set to 3 to 6 μm by controlling the bonding current.