Analyzing Sensors Silicon Crystals Defects and Production Technologies
Annual sales volumes of microelectronics products have recently exceeded 250 bln dollars, and those of components on the basis of MEMS are over 10 bln dollars. Today the proceeds of the sales of electronics products are more than 250 billion dollars, including 15 billions of MEMS sales.
According to the data of Yole Development, a French organization that monitors and analyzes the current state and prospects of the world innovatics market, the world MEMS market including the equipment for its production, will soon be over 15 billion dollars.
Jean-Christophe Eloy (the president of Yole Development) has explained that in the next 5 years average growth rates of MEMS sales volume will exceed 12 %. In 2018 the sales volume will reach 20 bln dollars. Today the components sold in the market are characterized by high technological level. The production of MEMS components has been steadily extending, new spheres of application appear, both in medicine and in industry.
The accelerated introduction of MEMS-technologies is mainly explained by their principal pluses, such as compactness, functionality, durability, low energy consumption, simplicity of introduction, a strong demand by actually whole markets of electronics products. And global demand for MEMS is still growing along with their new applications: cars, common gadgets (cellular phones, laptops, netbooks, etc.), the devices of special function which are manufactured in small lots: medical equipment, army and space technical usage, equipment for the weather forecast.
Thanks to their pluses MEMS have every chance of becoming a unique, easy product for almost all difficult problems of diagnostics and control that were unsolvable before. Currently, the formation of manufacturing and testing techniques, as well as a possibility of local production of MEMS, allows the client to create personal, unique products within their own scientific and technical base. Globally, MEMS are understood as different mechanic-electric (perceiving) or electromechanical actuators, whose size is about 1 micron. The shift of replaceable parts of MEMS does not exceed 1 micron.
The MEMS technology is built on introducing into a silicon substrate the microproduction of micromechanical structures of sensors, actuators (operating parts) and electric devices that collect, analyze, monitor and form the operating signals. These technologies have much in common with those used for the production of chips. That is why MEMS devices can guarantee the highest degree of functionality, durability, and low cost. These features explain their extensive use, as well as microelectronics products popularity. Though the equipment of special function allows setting comparatively high costs, due to the fact that MEMS-schemes allow achieving low cost per piece, it is possible to significantly expand touch networks, even if the integrators consider it superfluous. MEMS-detectors of pressure and movement (accelerometers, gyroscopes) create movements whose characteristics are monitored by MEMS-design. The moving design of these devices is not interfaced in any way with the object, so they are marked as contactless devices. If the mobile structure is qualitatively designed and is sufficiently protected by the cover, the small sensor is highly reliable and has an opportunity to work in different conditions of the environment and states, with quickly changing temperatures, accelerations, blows, moisture, pollution, electromagnetic violations, radiation. These conditions guarantee the reliable and accurate operation in dynamic system, i.e. in case of near prompt, spasmodic, periodic or aperiodic change of characteristics.
ANALYZING THE REQUIREMENTS FOR USING STRUCTURES OF SENSITIVE ELEMENTS OF INSTRUMENTATIONS WHILE PRODUCING MEMS-SENSORS OF PRESSURE
Regretfully, Russia’s companies engaged in design and creation of integrated structures in single-crystal silicon while researching and adjusting the release of converters of pressure, or transferring directly the experience of one technology to another, fail to take into account one characteristic feature of crystals operation: micro circuitry crystals work in the system of information transfer without any additional loadings, and the quality of their work is not coordinated with the correctness of measurement of this or that concrete parameter in any way [1–3].
The original qualities of elasticity of single-crystal silicon are caused by the structure of a crystal lattice. This is why microdefects of a crystal grid caused by impurity in the form of O2, carbon and N2 are sensitive to the introduction of single atoms into interstices of a crystal grid (Fig. 1a). These atoms directly affect qualities and properties of the semiconductor, making 3D-microdefects (Fig. 1b), have a spiral form and are called a swirl-tape.
During the whole cycle of production of devices based on plates from single-crystal silicon, initial plates are exposed to high temperatures about 1200 °C, and as a result in some turns of the spiral the disintegration of solid solutions is faster, forming various types of impurity complexes, clusters of dot defects, defects of packing and other structural imperfections. The arising defects have every chance of being the centers of sedimentation of excess solutions and other inclusions capable of forming the centers of single drifting microdefects. The error of measurement increases by 1–2 % and there appears the so-called “deliquescence of a signal” defect, i.e. after load dumping the electric signal of the measuring bridge disbalance constantly changes its value.
Above-mentioned defects  unambiguously influence elastic properties of a plate of single-crystal silicon as in dislocations the interatomic distance considerably increases and the damaged areas destabilize linearity of an electric signal depending on the size of deformation of Si layer in the beginning and at mechanical load removal. Besides, due to temperature influences the atoms’ motive ability in the field of dislocations increases, and these areas acquire a new ability to change their configuration and size, and as a result, the structure of a crystal lattice changes (Fig. 2), as well as the values of elastic deformation and electric signal. As a result, the error of measurement increases within 1–2 % and there appears the so-called “deliquescence of a signal” defect, i.e. the electric signal of the measuring bridge disbalance after load dumping constantly changes its value. After that the accuracy of measurement and the stability of indications of measurements results of new devices, on this crystal, depend on the properties and structure of a silicon plate.
The design of SE consists of a cover (1), a tube for supplying pressure (2) and a ceramic substrate (3). The substrate includes one or two operational amplifiers (4) executed on separate semiconductor plates and membrane SE (5) made in the form of a separate element, microcontroller (6) and passive elements (7) that are necessary for the circuit work.
The quality of a plate surface depends on the degree of its roughness, purity from inclusions of alien elements and the remains from masks and solvents, as well as on crystals perfection of blankets. Both during mechanical and chemical processing of a silicone surface top layer, the crystal lattice is exposed to partial destruction and introduction of impurity atoms [4, 5]. Researches of surface by methods of electronic microscopy have shown that the damaged layer consists of at least four layers: the first being a relief layer having ledges and dents, characteristic roughnesses after polishing, the second layer — dislocation loops and grids, microcracks, dot defects in a zone of cracks, exits of dislocations. The existence of cracks in the second layer leads to the formation in the third layer of tension and dislocations whose density is becoming less with depth. The third layer, in fact, is transitional from amorphous structure to a pure monocrystal, and may also contain elastic or plastic deformations. In the course of production of integrated circuit on a silicon surface the plate is exposed to repeated influence of various temperatures and chemical reagents, and as a result the number of violations in the second layer increases, and then it is possible to observe an exit of dislocations to the surface of the first layer and an increase in the dislocations formations in the third layer, i.e. in the plate volume there is an increase of the internal unevenly distributed tension that tending to an equilibrium state will start affecting crystal structure of a measuring membrane and influencing instrument readings. It is necessary to remember that the plate is cut from an ingot and therefore its surface is characterized by non-planarity and non-parallelism, and therefore the requirements to the reverse side of a crystal are strict. Firstly, different processing of both sides of the plate leads to additional residual mechanical tension and crystal’s deformation that is caused by a plate bend. Secondly, on the reverse side the working measuring membrane is etched, the membrane thickness being 10–250 microns, and that of the crystal — 430 microns, which contributes to residual mechanical tension.
Generally, parameters of quality control for single-crystal silicon plates and for production of semiconductor devices and crystals for instrumentations correspond to the same criteria: plate thickness, its non-parallelism, a deflection, non-planarity, quality of surface processing, specific resistance, macro and micro inhomogeneity by specific resistance, crystal lattice defects [6–8].
MEMS production technologies are part of a range of production technologies which also include traditional processes of exact machining. Traditional processes of machining can use a large variety of materials and make difficult three-dimensional and high-precision devices. As a rule, MEMS production processes are more limited in the materials being used, but they can make functional devices with sizes less than a micron. Table 1 presents a comparison of processes of MEMS production and traditional processes of machining.
The assessment of the production process demands interaction of many factors:
critical minimum size that can be made;
the process accuracy (i.e. dimensional accuracy or nominal size of the device);
materials available for production;
requirements to an assembly line for making the functioning device;
the process scalability (i.e. can large numbers of devices be made?);
integrability with other processes of falsification (for example, microelectronics).
The wide range of MEMS production processes has been developed, and they can be grouped into three big categories that are described below:
Lithographie, Galvanoformung, Abformung (LIGA), that is Lithography, Electroplating, and Molding);
sacrificial surface micromachining.
Fig. 3 shows fundamental concepts of each category of production.
Bulk micromachining and sacrificial surface micromachining are often based on silicon and are usually interfaced with a branch of production of microelectronics as they use the same equipment.
Bulk micromachining uses processes of damp or dry etching to make isotropic or anisotropic profile in material. Bulk micromachining can create large structures of MEMS (tens of microns) which can be used for measuring the consumption of liquid.
Commercial application of group micromachining has been available since 1970s. These decisions include pressure sensors, inertial sensors and printer heads. Sacrificial surface micromachining (SSM) is a direct offspring of the microelectronic industry processing, silicon being the most widely used material. This technology had some commercial achievements in the last decade, including optical massifs of mirror and inertial sensors. Both of these decisions include integrated microelectronics for measuring and managing functions. As a rule, this technology is limited to the film thickness of 2–6mm. This gives the SSM technology a considerable advantage of making decisions that involve a large number of devices. Besides, the SSM technology has a way to integration from electronics with MEMS structures which will be used for control or measurement functions.
The LIGA technology appeared in the 1980s. This technology can make devices with small critical measurement and a high format of thickness or width of metal materials that allow applying a metal layer by electroplating. This provides advantages as regards applications requiring a wide set of materials. However the assembly of big massifs of devices is a real challenge.
The LIGA technology is capable of creating complicated structures of electroplated metals with very high formats of proportions and thicknesses of several to hundreds microns. The LIGA process uses X-ray lithograph, thick resistive layers, and electroplated metal layer. Since the radiation of an X-ray is used as an exposition source for LIGA, the mask base is made of X-ray transparent materials (for example, silicon nitride, polysilicon). The corresponding layer with a mask drawing would have to be made of heavy material (such as gold).
Bulk micromachining uses damp and dry methods of etching to obtain isotropic and anisotropic features of etching in materials. To make products for practical application, many various aspects of the process have to be taken into account:
the mask drawing;
selectivity of etching depending on various crystallographic orientation;
determining the moment of the etching start and end.
Damp etching is a chemical process that can be isotropic in amorphous materials, such as silicon dioxide, and directed in crystal materials, such as silicon. Pollutants and macroparticles in this type of process are just a functional of chemical purity. Agitation of a damp chemical bathtub is often used to help the movement of reagents and by-products to and from the surface. Agitation will also help get the uniformity of etching since by-products can be in the form of firm particles or gases that have to be removed. Besides, modern damp and chemical processing usually has agitation, temperature and time control devices as well as filtration to remove macroparticles.
Sacrificial/Donor surface micromachining. The fundamental concept of surface process of micromachining has its roots in 1950s and 1960s with electrostatic massifs of locks and the resounding transistor source. However only in 1980s surface micromachining and the use of a set of microelectronics tools received considerable attention. Howe and Mahler gave the principal definition of functions of polycrystalline silicon surface for micromachining. Surface micromachining is a production technology based on deposition, patterning, and etching of a set of materials on a wafer. Sacrificial material is removed in the end of a production cycle that is developed as the assembly mechanism.
Fig. 4 shows the sequence of production of a cantilever on structural layers and one sacrificial layer.
The research analyzes designing methods taking into account technological restrictions of integrated microstructures for ultraprecise instrumentations of physical quantities measurement.
The authors consider that the following provisions and results of the research are novel:
While implementing crystals of measuring elements it is necessary to take into account that they are exposed to external factors, which results in clusters of dot defects, defects of lattice packing and other structural imperfections. The formed defects in turn can serve as centers of sedimentation of excess solutions of other impurities capable of forming the centers of single drifting microdefects. As a result, this leads to an increase in an error of measurements within 1–2 % and the formation of the so-called “deliquescence of a signal” defect, i.e. after load dumping the electric signal of the measuring bridge disbalance constantly changes its value. The specified microdefects unambiguously influence elastic properties of a plate of single-crystal silicon as in places of dislocations the interatomic distance increases and defective areas destabilize rectilinear dependence of an electric signal on the size of deformation of silicon layer under mechanical loading and at its removal.
The recommendations of the report can be useful to developers of SE crystals for control and measuring sensors and to the enterprises of contract semiconductor manufacturing for adapting their technological processes to requirements imposed on sensitive elements of control and measuring sensors.
The work was carried out with partial
financial support under Agreement
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