JPH09205083A - Thermal insulation structure and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a thermal insulation structure having higher thermal insulation effect by providing an air layer of a specified shape in a thermal insulation layer of polyimide resin formed on a silicon substrate and setting the thickness of the air layer shorter than the mean free path of the air. SOLUTION: Aluminum 8 is deposited in an arbitrary pattern on a silicon substrate 1 using a patterning and etching technology. The aluminum layer 8 is called a sacrifice layer. The aluminum layer 8 is then coated with polyimide resin 9. Polyimide is not applied to an end part of sacrifice layer 8 in order to feed and discharge etchant and fused aluminum. Subsequently, the patterned aluminum (sacrifice layer 8) is dissolved with etchant and removed and replaced by the air thus forming an air layer 10. Thickness of the air layer 10 is set smaller than the means free path of the air under the pressure of working environment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat insulating structure manufactured by a micromachining technique for heat insulating between a silicon substrate and an electronic circuit element mounted on the silicon substrate.

[0002]

2. Description of the Related Art In an LSI or the like, a silicon oxide film (Si
O ₂ ) and a silicon nitride film (Si ₃ N ₄ ) are often used for protecting the surface of a silicon substrate, separating elements of a transistor, a gate insulating film, and the like. Further, in order to insulate the electronic circuit element mounted on the silicon substrate from the silicon substrate, the above-mentioned silicon oxide film (SiO ₂ ) or silicon nitride film (Si ₃ N ₄ ) is used for the heat insulating layer on the silicon substrate. There is.

In the micromachine technology for handling the above-mentioned microfabrication, the photofabrication technology for collectively transferring the pattern of the photomask by light plays an important role. For the convenience of explaining the present invention, first, a procedure for forming an aluminum thin film having an arbitrary pattern on a silicon substrate by using a conventional patterning and etching technique will be introduced. Here, as an example, a method of forming three aluminum thin films having a narrow width (pattern) on a silicon substrate will be described. FIG. 3 is a diagram illustrating a conventional patterning and etching procedure. FIG. 3a is a plan view showing that an aluminum thin film is formed on a silicon substrate by vapor deposition or the like. 1 is a silicon substrate. A glass substrate may be used. 2
Is an aluminum thin film arbitrarily formed on the silicon substrate 1 by vapor deposition or the like.

FIG. 3b is a sectional view showing that an aluminum thin film is formed on a silicon substrate by vapor deposition or the like. FIG. 3c is a sectional view showing that the photoresist 3 is applied on the thin film of aluminum. Photoresist is a kind of photosensitizer, and the part that receives irradiation light has a property that it is difficult to be polymerized and dissolved in a solvent. (This is referred to as a negative type. Conversely, what is decomposed and easily melted is referred to as a positive type.) FIG. 3d is an explanatory diagram of irradiating the photoresist 5 with a light beam 5 from above the photomask 4. The photomask is a kind of optical filter for forming the aluminum thin film 2 in a desired pattern. FIG. 3e is an explanatory diagram in which the photomask is removed after the irradiation of light rays is finished. Irradiation light 5
The portion (X mark portion) that has received the polymer is difficult to be polymerized and dissolved in the solvent. FIG. 3f is a cross-sectional view after the unexposed photoresist has been dissolved and removed. The photoresist 7 cured by receiving light remains. FIG. 3g is a cross-sectional view of a silicon substrate and a patterned aluminum thin film 6 formed thereon.

FIG. 3h is a plan view of a silicon substrate and a patterned thin aluminum film 6 formed thereon. Next, a method for forming a patterned aluminum thin film will be described with reference to these drawings. Step 1 An aluminum thin film 2 is previously formed on the silicon substrate 1 by vapor deposition or the like (FIGS. 3a and 3b). Step 2 Next, a photoresist 3 is applied on the aluminum thin film 2 (FIG. 3c). Step 3 Next, a light beam 5 is irradiated through the photomask 4 placed on the photoresist 3. As an example, three thin patterns are drawn on the photomask 4, and only the light passes therethrough.

In the central part of FIG. 3d, it is shown that the irradiation light 5 passes through the photomask 4 and reaches the photoresist 3. The portion where the photoresist 3 receives the irradiation light 5 (the X mark portion at the center) is polymerized and becomes difficult to dissolve in the solvent. The horizontal line portion of the photoresist 3 is a portion where light does not pass through the photomask 4 and is not irradiated, and is a portion which is easily dissolved in a predetermined solvent (FIG. 3e). Step 4 Next, the portion not exposed to light (horizontal line portion) is dissolved in a predetermined solvent and removed. FIG. 3f shows a portion (X mark) where the photoresist is cured by receiving the irradiation light 5. Step 5 Next, the exposed portion of the aluminum thin film 2 is dissolved and removed by an aluminum solvent (referred to as an etchant). After that, only the portion (the portion marked with X) that is cured and left because the photoresist 3 is exposed to the light and the aluminum thin film below the portion are left.

Step 6 The photoresist 3 remaining in the operation of the above Step 5 is removed by using a predetermined solvent. Then, three thin aluminum thin films are formed and left on the silicon substrate according to the pattern of the photomask (FIGS. 3g and 3h). In this way, the pattern of the photomask 4 is transferred onto the silicon substrate 1. The above-mentioned solvent that dissolves aluminum is called an etchant and has the following composition and is used at about 60 ° C. _{H 3 PO 4: HNO 3:} CH 3 COOH: H 2 O = 16:
1: 2: 1 A commercially available product can be obtained as the processing apparatus, the photoresist, and the processing chemical solution used here. The process of transferring the pattern of the photomask in this way is called patterning, and the process of removing the unnecessary thin film with a solvent at this time is called etching. The present invention realizes a high heat insulating structure by forming an air layer in a polyimide thin film having a low thermal conductivity by utilizing the above-mentioned patterning and etching technique.

[0008]

Compared to a heat insulating structure in which a general electronic circuit element such as a diode is mounted on a silicon substrate, heat transferred from the silicon substrate is possible in mounting an electronic circuit element such as a heat sensitive sensor. It is necessary to shut off as much as possible. An object of the present invention is to provide a silicon oxide film (S
It is to realize a heat insulating structure having a higher heat insulating effect in place of the conventional heat insulating structure using iO ₂ ) or a silicon nitride film (Si ₃ N ₄ ).

[0009]

A feature of the present invention is that an air layer is formed in a polyimide thin film having a low thermal conductivity by utilizing a patterning and etching technique used in micromachining. There is a focus on realizing a higher heat insulation structure. (1) An air layer having a predetermined shape is provided inside a heat insulating layer formed by using a polyimide resin on a silicon substrate, and the thickness of this air layer is shorter than the mean free path of air. (2) The sacrifice layer of an aluminum thin film having a predetermined shape and thickness is provided on a silicon substrate, a polyimide resin is applied to the surface of the sacrifice layer, and then the sacrifice layer is removed by etching. A heat insulating layer of polyimide resin containing an air layer having a predetermined shape and thickness is formed on the top.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, the thermal conductivity of polyimide is low by one digit as compared with other materials as shown below, and the compatibility with a silicon substrate is good, and it is suitable for semiconductor fine processing. It was done focusing on the points that are suitable. Thermal conductivity of silicon oxide film (SiO ₂ ). 0.014 W / cm. Deg Thermal conductivity of silicon nitride film (Si ₃ N ₄ ) 0.12 Same as above Thermal conductivity of polyimide ... 0.00147 Same as above Also, the temperature of the treatment process when using SiO ₂ and polysilicon is about 650 ° C, but when using aluminum and polyimide, it may be as low as about 350 ° C. There is. Next, a processing procedure which is a feature of the present invention will be described with reference to the drawings. FIG. 1 is a drawing explaining a procedure for forming a heat insulating structure of the present invention. FIG. 1a is a cross-sectional view including a sacrificial layer for forming a heat insulating structure. 1 is
It is a silicon substrate. Reference numeral 8 is a sacrificial layer formed of aluminum. 9 is a polyimide film applied from above the sacrificial layer. FIG. 1b is a cross-sectional view showing that the aluminum in the sacrificial layer shown in FIG. 1a is replaced with air. FIG.
c is a cross-sectional view showing a state in which an electronic circuit element is attached.

Next, a method of manufacturing the heat insulating structure of the present invention will be described with reference to these drawings. Step 1 First, an aluminum thin film 8 having an arbitrary pattern is formed on the silicon substrate 1 by using the patterning and etching technique described with reference to FIGS. 3A to 3H. This layer is referred to as a sacrificial layer because it is later dissolved and removed to proceed with processing. Next, a polyimide resin is applied on the sacrificial layer 8 of aluminum to form a polyimide film 9 (FIG. 1a). At the end of the sacrificial layer, a portion not coated with polyimide is left in order to allow inflow and outflow of etchant and molten aluminum. Step 2 Next, the patterned aluminum thin film 8 (sacrificial layer) shown in FIG. 1a is dissolved and removed by an etchant to form an air layer 10 replaced with air. (Fig. 1b). Figure 1c
Shows a cross section in which the electronic circuit element 11 is attached on the polyimide film. The heat generated in the silicon substrate 1 is the air layer 1
0, it passes through the polyimide film 9 and is transmitted to the electronic circuit element, so that the heat flow is sufficiently blocked. FIG. 2 is a cross-sectional view showing a heat insulating structure in which air layers of the present invention are multilayered. A polyimide resin is applied onto the silicon substrate 1 and the same steps as those described with reference to FIG. 3 are repeated to form a polyimide film 14 including a plurality of air layers 13. When the heat-sensitive sensor 12 for measuring the temperature is attached, a multi-layered air layer is used to sufficiently block the conduction of heat from the silicon substrate 1. Although an example of a silicon substrate is introduced here, a similar heat insulating structure can be formed on a glass substrate or the like.

[0012]

EXAMPLE When the aluminum sacrificial layer 8 shown in FIG. 1a is formed by using the patterning and etching technique as described with reference to FIG. 3, the thickness t can be about 50 nm. Since an air layer having a small thickness t can be formed in this manner, the thickness of the air layer 10 in FIG. 1b is set equal to or less than the mean free path corresponding to the pressure of the environment used, and heat transfer by convection of air is performed. It was noted that a better heat insulating effect can be obtained by suppressing The principle will be described below. The apparent thermal conductivity of the gas layer between the two flat plates occurs as if the thermal conductivity of the gas decreases as the atmospheric pressure of the gas decreases. When the atmospheric pressure is lowered, a state in which gas molecules that collide with one wall surface and are reflected reach another surface without colliding with other gas molecules appears. In such a state, assuming that the distance between the flat plates is t, the thermal conductivity λ becomes the formula (1), and it can be seen that it is related to the distance between the flat plates and the pressure. Assume that t = air layer thickness and 50 nm. (The free path of molecules of air under 1 atm is about 50 nm) α = adaptation coefficient, which is assumed to be 1. (0.5 <α <1), which is the exchange efficiency when gas molecules bounce off the wall surface, γ = 1.4 R = 8.31441 J · mol ⁻¹ / K ⁻¹ P = 1.01325 × 10 ⁸ N / M ² T = room temperature and calculated as 300K. By substituting the above constants into the equation (1), the calculation results in λ≈1 × 10 ⁻⁵ W / cm · deg. Therefore, the thickness of the air layer in the polyimide thin film is t =
When it is set to 50 nm, the thermal conductivity of air under atmospheric pressure (λ
_a = is 2.61 × 10 ⁻⁴ W / cm · deg), and a sufficiently low thermal conductivity can be obtained. These are, for example, [Introduction to heat transfer, written by Yoshiro Katou (Yokokendo, published in 1974)].
There is a reference for reference.

[0013]

According to the present invention, the following effects can be obtained. Silicon oxide film (SiO ₂ ) and silicon nitride film (Si ₃
Since a polyimide thin film having a low thermal conductivity is used instead of N ₄ ), the heat insulating effect is large. Since the air layer is provided in the polyimide thin film, the heat insulating effect from the substrate is great. A plurality of air layers can be formed in the polyimide thin film to obtain a larger heat insulating effect. A large heat insulating effect can be obtained by making the thickness of the air layer in the polyimide thin film less than the mean free path of air.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a procedure for forming a heat insulating structure of the present invention.

FIG. 2 is a cross-sectional view showing a heat insulating structure in which air layers of the present invention are multilayered.

FIG. 3 is a diagram illustrating a conventional patterning and etching procedure.

[Explanation of symbols]

 1 Silicon Substrate 2 Aluminum Thin Film 3 Photoresist 4 Photomask 5 Irradiation Light of Ultraviolet Ray 6 Patterned Aluminum Thin Film 7 Cured Part of Photoresist 8 Aluminum Sacrificial Layer 9 Polyimide Film 10 Air Layer 11 Electronic Circuit Element 12 Thermal Sensitivity Sensor 13 Multilayer air layer 14 Multilayer polyimide film

Claims (2)

[Claims] 1. An air layer having a predetermined shape is provided inside a heat insulating layer formed by using a polyimide resin on a silicon substrate, and the thickness of the air layer is shorter than the mean free path of air. Insulation structure 2. A sacrificial layer of an aluminum thin film having a predetermined shape and thickness is provided on a silicon substrate, a polyimide resin is applied to the surface of the sacrificial layer, and then the sacrificial layer is removed by etching. A method of manufacturing a heat insulating structure, which comprises forming a heat insulating layer of a polyimide resin containing an air layer having a predetermined shape and a thickness on a silicon substrate.