RU2141649C1 - Integrated thermal cell - Google Patents

Abstract

FIELD: heaters for electronic devices. SUBSTANCE: thermal cell used for heating semiconductor integrated-circuit gas lasers, infrared sources of adsorption-type optical gas analyzers, activators of ink-jet printer heads has thin layer of single-crystalline silicon applied to thin-film multilayer insulating diaphragm; naked surface of silicon layer is covered with multilayer insulating film whose layers are identical to those of diaphragm and are arranged in mirror-like manner and symmetrically relative to silicon layer; film is, essentially, combination of SiO₂ and Si₃N₄ insulating layers their thickness ratio being 3.5 -2.7:1.0; SiO₂ layer is applied to silicon layer. EFFECT: improved reliability and service life of thermal cell. 2 dwg

Description

 The invention relates to the field of microelectronic and micromechanical devices and can be used as a heater of an integrated semiconductor gas sensor, an infrared emitter of an adsorption optical gas analyzer, an activator of the print head of an inkjet printer.

State of the art
A known thermal element is an infrared emitter made of molybdenum wire silicide [1] using the technology of discrete components of electronics, which limits the scope of its application only as an emitter of infrared radiation and causes its low reliability and durability when used in appropriate gas analysis systems.

 Closest to the present invention is a thermal element of a gas sensor made by the integrated technology of microelectronics by applying a thin layer of single-crystal silicon on a multilayer dielectric thin-film membrane [2]. This thin layer of single-crystal silicon is heated by an electric current passing through it.

 The disadvantage of this element is that the current-heated silicon layer is protected from external environmental influences only from the side of the membrane on which it lies, while the other silicon surface, in contact with the analyzed gas medium, is exposed to these media. In addition, the asymmetry of the membrane and silicon layers causes, due to the difference in the coefficients of thermal expansion of the layers (especially at high operating temperatures and impulsive operation), mechanical bending stresses and deformations, which lead to rapid fatigue of materials and destruction of the thermal element. These disadvantages reduce the reliability and durability of the integrated thermal element.

 Also known is a heating element, the surface of which is protected by a layer of dielectric material of the same as the base material [3]. The disadvantage of this design is that a homogeneous single-layer material of protective films on both sides of the heating element narrows the technological capabilities in the implementation of selective engraving of surfaces that are necessary in the manufacturing process.

Disclosure of Invention
The purpose of the invention is to increase the reliability and durability of the thermal element. The goal is achieved by the fact that a multilayer dielectric film is deposited on the protected surface of the silicon layer, the layers of this film being identical to the membrane layers and arranged mirror symmetrically with respect to the silicon layer.

 In this embodiment, the heating silicon element is completely protected from environmental influences.

As the film, a combination of layers of dielectrics SiO ₂ and Si ₃ N ₄ with a ratio of thicknesses of 3.5-2.7: 1.0; moreover, the SiO ₂ layer is adjacent to the silicon layer. This combination allows selective engraving of the layers during the manufacturing process of the heater.

 Tangential mechanical stresses that occur during heating and cooling of an element, due to the symmetry of the proposed structure of a thermal element, are realized in tensile and compressive stresses. And since the compressive and tensile strength of all materials used is 2-3 orders of magnitude higher than bending, this practically leads to a corresponding increase in the reliability of the thermal element.

Brief Description of the Drawings
The invention is further illustrated by the description of specific, but not limiting, embodiments of the invention and the accompanying drawings, in which:
In FIG. 1 shows an integral thermal element.

 In FIG. 2 shows a section through the active part of an integral thermal element.

The integral thermal element of FIG. 1, contains a resistive heating element - a silicon layer 1 located on the surface of the membrane 2, made of several layers of dielectrics SiO ₂ and Si ₃ N ₄ , a protective film 3 made of analogous membrane layers of dielectrics SiO ₂ and Si ₃ N ₄ . The membrane 2 is located on a silicon frame 4, on two opposite sides of which are made contact pads and electrical leads 5 from the silicon layer 1.

In the context of the active part of the integral thermal element, FIG. 2, the arrangement of the layers of the membrane, silicon and protective film is shown. The dielectric layers 6 - Si ₃ N ₄ and 7 - SiO _{2 of the} membrane 2 and the protective film 3 are identical and form a mirror-symmetric structure with respect to the silicon layer 1. The 7 - SiO ₂ layers are adjacent to the silicon layer 1.

The thermal element is performed by standard technological methods of microelectronics. A functional layer of silicon is formed by the method of ion implantation of boron through open areas of the protective mask. By the pyrolytic method, the SiO ₂ and Si ₃ N ₄ layers forming the membrane are deposited on the silicon surface. By deep etching of silicon, the membrane is released from the back of the plate and a silicon layer is formed - a thermal element.

In the same way, membrane layers were made — SiO ₂ and Si ₃ N ₄ layers are deposited by plasma pyrolysis onto a silicon frame from the side of the etched well.

 Then open the windows in the membrane layers to the silicon layer. By sputtering, electrical leads and contact pads lying on a silicon frame are formed.

 When a thermal element is brought into operation, voltage is applied to the contact pads from an external source. The current passing through the silicon layer heats it to the desired temperature in the range of 500-900 degrees Celsius.

 Depending on the purpose of the described thermal element, either as a heater of a gas absorption sensor, or an infrared emitter of an optical absorption gas analyzer, or an activator of an inkjet or print head of a printer, additional layers of either gas absorption films or films forming interference optical filters can be applied to its surface, or increasing thermal radiation, or not applied any.

The experimental sample was made in the form of a silicon crystal 3x4x4.6 mm ³ in size, the silicon layer forming the heating element had a resistance of 700 Ohms, the membrane and protective layer each had 3 layers of SiO ₂ and Si ₃ N ₄ , respectively, the total thickness of the active part of the thermal element FIG. 2 is 3.7 microns. Power consumption at a temperature of 700 ^o C amounted to 210 mW.

 The proposed design of the thermal element provides increased reliability and longevity of its work, creates a condition for compatibility of its manufacturing technology with the manufacturing technology of CMOS ICs.

Industrial applicability
The described integral thermal element can be used as a pulsed source of near infrared radiation in miniature optical absorption gas analyzers for gases containing CO _x and hydrocarbons having strong absorption lines in the spectral range from 2 to 14 μm.

 Also, said integral thermal element can be used as the basis of a gas analyzer sensor for depositing electrically conductive films on it that absorb integrally or selectively various gases and change their electrical resistance. Heating an integral thermal element to a high temperature allows gas desorption from the film when it is saturated.

 It can also be used as an ink evaporator or other working fluid for an inkjet printhead, or a heater for a thermal printhead.

Literature
1. Japan patent N 05296833A, published 12.11.93
2. C.L. Johnson, K. D. Wise, JW Schwauk A Thin - Film Gas Det ecfor For Semiconductor Proctss Gases. Digest inf. Conf IEDM88, p. 662 - 665.

 3. SU 1805372 A1, 03/30/93, G 01 N 27/12.

Claims (1)

An integral thermal element containing a heating element made of a single-crystal silicon layer lying on a dielectric membrane formed by several layers of a dielectric film, and the second surface of the heating element is protected by identical layers of dielectric films, characterized in that the layers of dielectric films repeat the membrane layers mirror symmetrically relative to the layer silicon, and as a film, a combination of layers of dielectrics SiO ₂ and Si ₃ N _{4 is used} with a ratio of thicknesses (3.5 - 2.7): 1.0, p In fact, the SiO ₂ layer is adjacent to the silicon layer.

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