Analysis of common faults of ion implantation technology and equipment
Abstract: The characteristics of ion implantation technology and its principles are introduced. Based on the analysis of the types of equipment, the basic structure of ion implantation equipment is briefly outlined, various factors affecting the injection process are analyzed in detail, common faults of ion implantation machines are summarized based on years of experience in equipment maintenance, and measures for handling various faults are proposed.
Ion implantation is a very important doping technology in modern integrated circuit manufacturing, which injects doping elements into the interior of semiconductor wafers in an ion-accelerated manner to change their conductive properties and eventually form the desired device structure. The ion implantation process has many advantages and has replaced the diffusion process on a large scale as the most common doping technique in semiconductor processes. The working process of an ion implanter is as follows: the ion source ionizes the doped element into positively charged ions, the ions are extracted from the source by a suction electrode, separated by a magnetic analyzer to separate the desired ions from other ions, then the doped ions are accelerated to the required energy by a gas pedal, and finally the ions are injected into the semiconductor material by a focused scanning system. Ion injection machine is one of the most complex equipment in the semiconductor process, its equipment maintenance involves physics, electrical, mechanical, mechatronics, automatic control and other multidisciplinary knowledge.
1 ion injection technology features and principles
1.1 Ion injection technology features
Ion injection technology is a pure surface treatment technology, it does not need to be carried out in a high-temperature environment, so it will not change the shape and surface finish of the processed workpiece. Its main features are as follows.
(1) High purity of injected ions, single energy, clean and dry injection environment, and very low contamination of impurities.
(2) The dose of injected ions can be precisely controlled, so the uniformity of doping is high.
(3) The ion injection can be carried out at room temperature, so the requirements of the ion mask layer is not demanding, silicon dioxide, photoresist, etc. can be used as a mask layer.
(4) The doping concentration of ion injection is not limited by the solid solubility of impurities in the substrate. It does not change the composition of the compound semiconductor material.
(5) The lateral doping effect of ion implantation is small, which is conducive to reducing the area of the chip and lowering power consumption.
(6) The disadvantage of ion implantation is that the bombardment of high-energy ions can cause damage to the lattice structure of semiconductor materials.
1.2 Principle of ion implantation technology
Ion injection is a technique in which ions generated by an ion source are separated and purified, accelerated and then injected at high speed into the material surface and into the material body. The injected ions enter the material surface and collide with atoms in the material, squeezing them into the interior. These knocked-out atoms then collide with other atoms, and within a few hundred nanoseconds, a region with hundreds of interstitial atoms and vacant sites is formed in the material (as shown in Figure 1). This cascade of collisions creates a concentration peak of the injected element on the material surface with a Gaussian distribution. The depth of ion injection is a function of the energy and mass of the ion as well as the mass of the substrate atoms; the higher the energy, the deeper the injection. In general, the lighter the ion and the lighter the atom of the substrate, the deeper the injection. When ions are injected into the material, the ions are absorbed by the material and become part of the material, thus the injected layer does not fall off or peel off. The injected ions can interact with solid atoms, or with each other in a series of physical and chemical interactions, and the incident ions gradually lose energy and finally stay in the material and cause changes in the surface composition, structure and properties of the material, thus optimizing the surface properties of the material and obtaining some new excellent properties. obtain certain new excellent properties.
2 ion implantation machine classification and equipment structure
2.1 Classification of ion implanters
Ion injection equipment is an important process equipment in semiconductor manufacturing, according to the use of process and ion energy and ion beam current size can be divided into high energy, high current, medium current three [1].
(1) The ion beam of high-energy injectors has a high energy, generally reaching more than 500 keV, but its doping concentration is low and is mainly used for deep-trap doping of material substrates.
(2) High-current injector can obtain a larger ion beam current, its doping concentration is large, but the injection depth is shallow, mainly used for doping of the source-drain region and LDD region doping of devices.
(3) Medium current injector is widely used, its ion energy and beam current size is much smaller than the high energy and high current injector, is widely used in addition to deep well doping and source leakage doping almost all the ion doping process.
2.2 Ion injector equipment structure
The equipment structure of ion implanters includes ion source, magnetic analyzer, scanning system, focusing acceleration system and injection system.
2.2.1 Ion source
The ion source is one of the key components of the ion implantation machine. Its function is to ionize the doping gas into ions to form an ion beam. The doping gas enters the arc reaction chamber of the ion source through a pipe and is bombarded by hot electrons excited by a filament in the reaction chamber to form plasma. These plasma in the reaction room under the action of negative voltage electrode, the positive ions from the plasma will be separated from the arc reaction chamber, leaving the ion beam with a certain energy. The two main types of ion sources widely used are BERNAS and IHC.
2.2.2 Magnetic analyzer
The ions produced in the ion source are usually multiple ions, while only one impurity ion is required in the actual process processing. Therefore, a magnetic analyzer must be adopted to separate the ion beam and select the desired single ion. In the magnetic analyzer, the ion beam stream moves in the vacuum at a constant velocity in a plane perpendicular to the magnetic field, and the particles in the ion beam move in a uniform circular motion under the Lorentz force. Since the radius of uniform circular motion is completely different for different masses of ions, the magnetic analyzer separates the different masses of ions one from the other and selects only the desired impurities. Usually the magnetic analyzer is made into a curved cavity of 70° to 120° with graphite baffles on both sides of the inner wall. In the direction perpendicular to the ion path up and down, there is a pass electromagnet, the role of which is to regulate the strength of the analysis of the magnetic field size.
2.2.3 Scanning system
The ion beam is a linear, high-speed ion stream that must be scanned to cover the entire injection area. Two common scanning methods are fixed sample moving ion beam and fixed ion beam moving sample. There are four types of scanning systems for ion injectors, namely electronic scanning, mechanical scanning, hybrid scanning and parallel scanning, and the most commonly used system at present is electrostatic scanning. The electrostatic scanning system consists of two parallel sets of electrostatic deflection plates, one set for transverse deflection and the other for longitudinal deflection. By applying an electric field to the parallel electrode plates, the positive ions are deflected toward the electrode plate on the lower voltage side, and the deflection angle of the ion beam can be changed by changing the voltage magnitude. The advantage of this scanning is that electrons and neutral ions are not deflected and can be eliminated from the beam stream. The disadvantage is that the ion beam cannot bombard the sample vertically, which can lead to shadowing effects on the injection mask pattern and hinder the injection of the ion beam.
2.2.4 Focusing and Acceleration System
Focusing system: The ion beam accelerated from the magnetic analyzer is mutually repulsive because it is all positive ions, so the ion beam will have a tensor angle. In order to reduce the beam loss and enable the ion beam to be uniformly distributed on the surface of the injected sample, it is usually focused by an electromagnetic lens. The focuser of the ion beam usually consists of several pairs of polar plates with the same potential, which are loaded with a certain voltage to regulate the gathering effect of the ion beam.
Gas pedal: The impurity ions drawn from the ion source must be accelerated or decelerated by a gas pedal with a strong electric field to obtain the ion energy required for the process. The acceleration of the ions before the magnetic analyzer is usually called "pre-acceleration", and after passing through the magnetic analyzer, the ions can again be accelerated or decelerated for another period, usually called "post-acceleration or deceleration". The common post-acceleration or deceleration methods of ion injectors are DC and RF, and the gas pedal is mainly composed of a vacuum chamber and a high-voltage power supply.
2.2.5 Injection System
The internal chamber of an ion implanter needs to be maintained under a very low vacuum, usually with a chamber pressure of less than 5 × 10-7 t. This is to avoid interference of the ion beam from ionization generation to the final scan injection by other particles in space. The vacuum system of the ion injector consists of a dry pump, a molecular pump and a cold pump. In addition, the vacuum of the ion source also needs to be kept at a very low pressure. If the pressure is higher than required, it is extremely easy to discharge between the arc cavity and the suction pole, causing ion beam instability. If the vacuum pressure of the internal cavity of the ion injection machine is too high, will cause the collision of ions and residual gas molecules, resulting in lower ion beam current, will also cause charge exchange to form energy pollution.
3 affect the ion implantation process factors and equipment common troubleshooting
3.1 Factors affecting the ion implantation process
Many factors affect the uniformity of the ion injection process, of which the most important are four.
3.1.1 Vacuum degree of the injection system
The vacuum degree of the injection machine system has a very strong influence on its beam quality. If the vacuum level is too low, on the one hand, the beam flow of the injector will be smaller, resulting in a lower injection speed; on the other hand, it will lead to a poor focus of the ion flow and the purity of the injection will not meet the requirements. In addition, the vacuum level also has an effect on the uniformity of injection, because the ion beam is blocked by some stray gas molecules in the path of acceleration, resulting in poor uniformity of ion injection depth in the same sample.
The ion beam has a long path from the ion source to the injection target, which places high demands on the vacuum level of the whole system. If the vacuum level is not sufficient, the ion beam will collide with the residual gas molecules in the system in a cascade, generating many low-energy electrons and derived ions, resulting in a chaotic direction of ion motion. This is not only a kind of pollution for the ion beam itself, but also makes the energy statistics of the ion beam deviate and exhibit high energy dispersion because of the randomness of the collisions.
3.1.2 Cleanliness of the sample surface
Ion injection is very sensitive to particle contamination, and particles on the sample surface can hinder the injection of the ion stream. Usually the larger the injected beam flow the more particles are likely to be generated, and although these particles can be cleaned off after injection, their obscuration of the injection will produce some fatal defects in the injected layer of the sample. Most of the particles are caused by other processes or environmental factors. In addition, the standardization and correctness of various operations in the actual process are also very critical. Incorrect sample pickup, incorrect evacuation procedures, and the use of unfiltered gas when filling the system all produce more particles [3].
3.1.3 Focusing and scanning
The ion flow from the ion source accelerator tube is divergent and its beam density is very heterogeneous. Focusing and coaxial processing must be performed to provide fine, high-quality beam spots for subsequent scanning, so that the beam signal in the scanning plane is symmetrical and consistent and the impurity distribution in the injected layer is uniform.
3.1.4 Selection of beam size
When the ion beam is incident on the wafers of insulating materials, a charge accumulation layer is formed on the surface of these materials, a phenomenon called wafer charge accumulation. The accumulated charge on the wafer surface has a scattering effect on the injected ions, which affects the uniformity of the injection. The larger the injected beam current, the more serious the charge accumulation on the wafer. Therefore, in the process processing, a smaller beam current should be chosen as much as necessary.
3.2 Common faults of the injector and handling
There are four main types of common faults in ion implanters, namely, aging of components, vacuum problems, power supply problems and system contamination.
3.2.1 Aging of components
Most of the domestic injectors are used equipment from foreign production lines, so the aging of components is a frequent failure problem. In the maintenance of the injector system, we should carefully analyze and judge its vacuum parts, high and low potential control fiber and some moving parts that often move, etc. We should conduct regular inspection and maintenance, replace aging parts in time and tighten loose screws, etc.
3.2.2 Vacuum problem
The good or bad vacuum of the injection machine equipment system is an important factor affecting the quality of injection. The maintenance of the vacuum of the injection system usually has the following measures.
(1) regular maintenance, to often replace the seals at the connection of each chamber regularly. For example, the vacuum seal ring of the injection target chamber, often due to debris or debris scratch and lead to air leakage, should be replaced in time.
(2) Regular maintenance of vacuum pump set. Often check whether the oil level of the mechanical pump is appropriate; observe whether the pressure of the cold pump compressor is low, replace the adsorption barrel in time, and whether the normal temperature of the cold pump is maintained below 13K; regularly maintain the molecular pump to ensure the vacuum degree of the body.
3.2.3 Power supply problem
The power supply of the injection machine mainly includes high-voltage power supply, suction pole power supply, filament power supply, magnetic analysis power supply and scanning power supply. When carrying out the overhaul of the power supply, pay attention to whether the load of the power supply is normal, first analyze and locate the power supply module where the failure occurred, then gradually investigate the causes leading to the failure, and after understanding the working principle of each unit circuit, then carry out the analysis of DC and voltage and current stabilization process.
3.2.4 System contamination
The causes of contamination of the injection system are mainly the following.
(1) ion source contamination. Measures: check the ion source vacuum system for air leakage; check the purity of the source material used to meet the requirements.
(2) Contamination of ion beam in mass analysis. Measures: check whether the system vacuum leakage; check whether the narrow peak of the mass analyzer is appropriate; check the ion energy filtration system; check whether the system is contaminated by metal.
(3) System components contamination. Mainly: the ion beam hit the motor generated by sputtering metal; sample surface photoresist alkaline element staining; Faraday cup of aluminum; the same injector injection of different elements of the interaction contamination.
4 Conclusion
As one of the most complex equipment in the semiconductor process, the maintenance of ion injectors is a complex task combining knowledge from multiple disciplines. With the further development of semiconductor process technology, the ion injection technology has put forward higher requirements for precision and uniformity, and the complexity of its equipment is getting higher and higher, maintenance work will also become more and more complex and difficult, for each of us maintenance personnel, we must clarify the thinking, follow the vine, so that any problem will be solved.
