Solar cell silicon material

Focus on wafer grinding and heat treatment

admin on June 1, 2022 0 Comments • Tags: #wafergrindingandheattreatment

Focus on wafer grinding and heat treatment

For the sliced ​​silicon wafers, very few can be used directly, and most of them need to be processed on the corners of the silicon wafers. In order to make the corners smooth, they need to be ground, or even heat treated to change the stress structure.

  1. Chamfering the edge of the silicon wafer

Before grinding the silicon wafer, it is necessary to perform edge chamfering treatment first to improve the qualified rate of the cut silicon wafer. In the production, special equipment and technology are used, and a high (7999~9999r/min) diamond grinding wheel is used to work with the silicon wafer. The frictional motion grinds the edge of the silicon wafer. Chamfering is often applied after wafer dicing to eliminate stress concentration areas on the edge of the wafer and reduce damage to the wafer during subsequent processes. For example, edge chamfering greatly improves the crushing strength of silicon wafers during grinding. R-shaped chamfering is common for silicon wafer chamfering, and T-shaped chamfering is also used under certain conditions. When the silicon wafer is chamfered, the silicon wafer is usually fixed on a rotatable support, and there is a diamond grinding wheel rotating at a high speed at its edge, and the rotation speed of the grinding wheel is 5999~8000r/min or higher.

Silicon wafer chamfering operation steps: preparation work → calibration adjustment (parameter input) → automatic grinding → end work.

(1) Preparations
1 Check whether the vacuum pressure, nitrogen pressure and water pressure meet the requirements.
2 Select the cam, select the cam that matches the shape of the silicon wafer according to the shape of different silicon wafers, and install it on the main shaft of the equipment.
3Select the grinding wheel. Select the appropriate grinding wheel according to the thickness of the silicon wafer and its edge requirements.
4 Replace the corresponding suction cup according to the size of the silicon wafer, and adjust the cooling water nozzle.

(2) Calibration adjustment
1 Calibration test (diameter and thickness) with standard specimens.
2 Enter the wafer geometry parameters into the computer.
3 Adjust the concentricity of the silicon wafer, which should be less than 0.025mm.
4 Adjust the high and low position of the silicon wafer.
5 Select the appropriate spindle speed according to the size and shape of the silicon wafer.
6 Use the single-chip mode to try pouring 1 film, test and observe, and adjust as necessary until it meets the requirements.

(3) Automatic grinding
Use automatic grinding mode to perform batch chamfering on silicon wafers, and the processed silicon wafers will return to their original position in the flower basket.
1 Place the flower basket with silicon wafers on the wafer feeder and the empty flower basket on the receiver.

  1. Monitor the operation of the equipment, and put and take out the film in time.
    (4) End the work
    After batch processing is completed, count the number of pieces and end the work.
  2. Wafer grinding process

Silicon wafer grinding is to grind the upper and lower planes of the silicon wafer, remove the knife marks or line marks on the surface, and improve the surface flatness of the silicon wafer, that is, a uniform surface damage layer is generated, and the thickness deviation of each batch of silicon wafers is reduced. near.

Wafer grinding can be regarded as the weak cutting of the workpiece between the upper and lower grinding discs rotating in opposite directions by free abrasive particles, which contains complex physical and chemical effects. Before grinding the abrasive sheet, attention should be paid to: ① The surface precision of the abrasive tool is high; ② The workpiece should perform complex compound motions: ③ The concentration, viscosity, flow rate, etc. of the abrasive should be moderate; ④ It should have appropriate pressure and grinding speed; ⑤ During grinding Double-sided cutting is adopted to uniformly and effectively grind the upper and lower surfaces of the silicon wafer and improve the stability of the grinding disc during the grinding process. The main process of grinding is shown in Figure 1.

Figure 1 - The main flow of grinding
Figure 1 – The main flow of grinding

The grinding process mainly includes the following steps:

  1. Wafer thickness sorting:
    There are differences in the thickness of the silicon wafers after cutting. In order to make the thickness of the ground silicon wafers more consistent and the total thickness variation is small, the thickness lines of the cut silicon wafers are sorted, and the silicon wafers with the same thickness are ground on the same disk. Greatly improve the grinding efficiency.
  2. Preparation of grinding liquid:
    The commonly used abrasives are emery, pure water, grinding aids (usually weak alkaline, which can enhance the suspension of sand particles and increase the grinding rate), and metal detergents. Appropriate proportions should be used depending on the material. When preparing, always pay attention to keep it clean and not be polluted by sundries.
  3. Grinding disc:
    Just like measuring the angle, the plane must be level, so the plane needs to be corrected, and the same is true for the grinding table. When correcting the grinding table, 3~5 correction wheels with similar specifications are usually placed on the grinding table evenly, start the mortar, and start the correction. The correction time is 20min or so.
  4. Settings:
    According to the type of polished silicon wafer, the amount of de-layering in diameter and the number of wafers per disc, the number of revolutions for light grinding, medium grinding and grinding is properly set.
  5. Grinding:
    Clean the flat grinder that has been corrected, place the silicon wafers corresponding to the size of the silicon wafers to be ground evenly on the lower grinding disc, and lightly grind about 20 revolutions to correct the carrier. Then put the same-grade silicon wafers with similar thickness into the carrier holes and evenly distribute them on the entire grinding disc. Check whether the alignment is correct. After confirming that it is correct, operate the grinding machine to slowly lower the upper grinding disc to contact with the silicon wafers. Turn the time relay switch to the automatic position, start the sand pump to pump in the prepared grinding liquid, turn on the grinding liquid switch, adjust the flow, and start grinding.
  6. Cleaning:
    After the silicon wafer is ground, it needs to be cleaned to remove the grinding waste liquid on the surface.
  1. Back injuries

Damage to the backside of the silicon wafer causes defects such as lattice dislocations and heavy metal impurities to sink into these high-stress defect sites. If back-damaged silicon wafers are treated at high temperatures, a large number of mobile metal ions will move in all directions and eventually trap on the backside. Dislocations also attract point defects in the silicon lattice, which occur during single crystal pulling and move at high temperatures (>1000). When they reach the dislocation position, they are immobilized. The disadvantage of this process is that if the wafer is exposed to high temperatures for a long time, some of the damage may anneal, allowing the metal to re-enter the wafer.

The backside can be damaged in a variety of ways. These mechanical means such as sandblasting, laser irradiation with appropriate power, and rubbing the backside with a roller brush or stylus are used to evaluate backside damage by mechanical means. One technique for measuring the amount of damage is to use a reflector to measure the reflectance of the colored surface. This type of damage should be done in an isolated area, as the process generates a lot of dirty particles.

Laser back damage is a more controlled and clean process. However, this is a slow process as the laser beam scans the wafer surface to achieve a good coverage. The irradiation of the laser will melt the surface of the silicon wafer, the melt will flow into the silicon wafer, and when it recrystallizes, the thermal stress will be released due to the generation of dislocations. If these dislocations are thermally stable, they will be deposited as metal impurities during subsequent high temperature processing such as oxidation. Whether the dislocations are thermally stable or not depends on the depth of laser damage, which is directly related to the laser power. . It has been observed that for successful gettering, the laser power needs to be 15J/cm2 or more.

Another method of back injury is ion implantation. When the ions pierce the surface of the silicon wafer, damage is introduced into the grid. This lattice damage acts as a gettering point for metal ions.

  1. Edge polishing

The purpose of wafer edge polishing is to remove the remaining etch pits on the wafer edge. When the edge of the silicon wafer becomes smooth, the stress on the edge of the silicon wafer also becomes uniform. The uniform distribution of stress makes the silicon wafer stronger. The polished edge can minimize the adsorption of particulate dust, and the polishing method of the edge of the silicon wafer is similar to the polishing of the surface of the silicon wafer. The silicon wafer is sucked by a vacuum suction head and rotated in a rotating barrel at a certain angle without hindering the vertical rotation of the barrel. The barrel has a polishing pad and through which the mortar flows, chemical/mechanical polishing is used to remove etch pits from the edge of the silicon wafer. Another method is to acid etch only the edge of the silicon wafer.

  1. Preheat cleaning

Before the silicon wafer is stabilized, it needs to be cleaned to remove organic and metal contamination. If there is metal remaining on the surface of the silicon wafer, when it enters the resistance stabilization process and the temperature rises, it will enter the silicon body. The cleaning process here is to immerse the wafer in a cleaning solution (H2so4+H2O2) that removes organics and oxides, and many metals are dissolved in the chemical cleaning solution in the form of oxides: Then, the surface of the wafer is treated with hydrofluoric acid The oxide layer dissolves to remove contamination.

  1. Silicon wafer heat treatment

In the process of crystal ingoting, the crystal will inevitably contact the quartz crucible, so the distribution of oxygen will inevitably be formed in the crystal, and the presence of oxygen will produce a donor effect, which will lead to the change of the electrical properties of the silicon material and the failure of the device. Rapid cooling after heat treatment can remove most of the donors from danger, thereby weakening or even eliminating the donor effect of oxygen in silicon.

In the early stage, the diameter and thickness of single crystal silicon were small. Small heat treatment furnaces could meet the requirements of silicon wafer processing technology. Single crystal silicon rod heat treatment was the main process. With the industrial development of semiconductor devices, the demand increased. The diameter and thickness of crystalline silicon are also getting larger and larger, which in turn leads to larger and larger heat treatment furnaces. But here comes the problem. With the increase of the crystal volume, the cooling time is correspondingly prolonged, especially since the temperature of the crystal center cannot be rapidly cooled, so the direct heat treatment of the silicon ingot cannot achieve the expected effect, but the silicon wafer is processed after the silicon crystal is cut and ground. Heat treatment effectively solves this problem. At the same time, the heat treatment of the silicon wafer can also release the stress in the silicon wafer after grinding, which is more beneficial to the subsequent processing.

Through the explanation of heat treatment reasons, it can be seen that necessary heat treatment is an important step to ensure the quality of silicon wafers, but not all silicon wafers need heat treatment. heat treatment. For the single crystal silicon obtained by regional smelting, the oxygen content is extremely low, and the behavior of oxygen after heavy doping is different from that of ordinary doping, so heat treatment is not performed.

The processing temperature of silicon wafers is generally around 650°C, and the time is 30~40min. A general diffusion furnace can be used to place an appropriate number of silicon wafers into the constant temperature area in the furnace. During the treatment process, it needs to be kept in an inert gas such as high-purity ammonia gas to reduce the oxidation of the silicon wafer.

The heat treatment system includes quartz boat, silicon boat, quartz tube, etc. They were first soaked in hydrofluoric acid solution for 2 hours, rinsed and then calcined at 650°C for 2 hours before they could be used by the treatment system.

The main steps of heat treatment of silicon wafers:

  1. Preparations
    First check the incoming materials and processing instructions, then turn on the indoor exhaust air and cooling water, turn on and adjust the nitrogen gas flow, and heat the heat treatment furnace to 650 °C for constant use.
  2. Loading
    Wear clean wire gloves and PVC gloves, select a suitable quartz boat according to the diameter of the silicon wafer, and carefully load the silicon wafer into the quartz boat to avoid scratching and contaminating the surface of the silicon wafer.
  3. Into the furnace
    Put the quartz boat loaded with silicon wafers on the furnace mouth and push it to the constant temperature zone with a quartz rod.
  4. Constant temperature
    The temperature of silicon wafer heat treatment is 650℃±20℃, and the constant temperature time is 30~40min.
  5. Out of the oven
    After the constant temperature is over, the quartz boat is pulled to the mouth of the furnace with a quartz rod and rapidly cooled, and the heat treatment of the next furnace is carried out at the same time.
  6. End work
    After heat treatment, the number of clean sheets is sent for inspection. After the work is completed, turn off the water, electricity, gas and exhaust.
  7. Back seal

For heavily doped silicon wafers, there will be a high temperature stage where a thin film is deposited on the back of the silicon wafer to prevent the dopant from diffusing outward. Usually, three kinds of thin films are used as the back seal material: silicon dioxide , silicon nitride, polysilicon. If an oxide or amide is used for back sealing, it can be strictly regarded as a sealant, while if polysilicon is used mainly as a sealant, it also plays an external impurity role. Figure 2 is a schematic diagram of preheat cleaning, resistance stabilization, and back sealing.

Figure 2 - Schematic diagram of preheat cleaning, resist stabilization and back sealing
Figure 2 – Schematic diagram of preheat cleaning, resist stabilization and back sealing
  1. Adhesive sheet resistance and stability

Before the silicon wafer goes into polishing, it needs to be bonded first. The adhesive sheet must ensure that the silicon wafer can be polished and flat. There are two main ways of sticking the sheet, namely, the wax sticking sheet or the template sticking sheet. The wax sticking sheet is bonded to the silicon wafer with a solid rosin wax, and a schematic diagram of a back seal is provided. , which provides a solid reference plane for polishing. Rosin wax prevents the wafer from moving when the wafer is polished under a carrier on one side, and the wax stick is only useful for wafers polished on one side.

There are two different methods of stencil sticking. One is only suitable for single-sided polishing. In this way, silicon wafers are fixed on a circular template and placed on a soft pad. This pad provides enough friction so that during polishing, the edge of the wafer is not fully supported against the side carrier, and the wafer is not in hard contact, but “floats” on the object, when the front side is removed. , the single-sided sticky sheet protects the back of the silicon wafer. Another method is suitable for double-sided polishing.

In this method, the upper and lower sides of the template on which the silicon wafer is placed are open, and the template with both sides open is usually called a carrier. This method allows both sides to be polished simultaneously on a single machine, operating similar to a tablet grinder. The wafer’s two polishing pads are placed in opposite directions so that when the wafer is pushed toward the top in one direction and toward the bottom in the opposite direction, the resulting stresses cancel each other out. This is beneficial to prevent the silicon wafer from being pushed against the rigid carrier and causing damage to the edge of the silicon wafer. Aside from the many loads placed on the edge of the wafer, it is unlikely that the edge will be damaged as the wafer travels with the carrier.

  1. Wafer polishing

Silicon wafers for integrated circuits need to be polished after grinding to obtain a perfect wafer surface.
Usually, the silicon wafer is chemically thinned before polishing. On the one hand, it can reduce the residual stress of the silicon wafer in the previous process, and at the same time, the surface of the silicon wafer can be peeled off once to remove the impurities attached to the surface of the silicon wafer; on the other hand, it can also reduce the Polishing workload in turn increases productivity. Chemical thinning usually uses acid or alkali to react chemically with the surface of the silicon wafer under certain conditions.

Acid corrosion: The corrosion liquid is prepared by HF, HNO3, HAc according to a certain proportion, usually the ratio is HF: HNO3:HAc=(1~2):(5~7):(1~2), due to acid corrosion The corrosion rate is fast and the layer scene is difficult to control, and the surface of the silicon wafer is often rough due to the rapid reaction. Therefore, the effect of acetic acid is shown. Acetic acid is a weak acid, which can slow down the corrosion rate and make the surface of the silicon wafer as smooth as possible, but it cannot reduce the possibility of the slump of the silicon wafer. In general, chemical attack has speed but poor edge preparation.

Alkaline corrosion: Alkaline corrosion is mainly vertical corrosion, and its corrosion effect is related to the crystal orientation of the silicon wafer. In actual production, different alkali ratios are used depending on the type of silicon wafer to be corroded. Other directions are more vigilant, the overall efficiency of alkali corrosion is slow, the thickness of the alkali rot silicon wafer is easy to control, and the waste liquid is easy to deal with, but it is easy to form rough corrosion pits and laces on the surface of the silicon wafer, so it is necessary to be in the process of alkali corrosion. Pay special attention.

The polishing method commonly used in the industry is the alkaline silica polishing method. The polishing principle is: firstly, the alkali reacts with the surface of the silicon wafer to generate Na2sio3. Na2SiO3 further reacts with water to generate H2SiO3, and the polymerized part in the H2SiO3 is partially ionized to form a colloid, which adheres to the surface of the silicon crystal and adsorbs the metal impurities and ions on it. Easily hydrolyzed compounds. At the same time, silica colloidal particles have a strong adsorption effect, which can adsorb the generated colloidal particles. Therefore, in polishing, silica gel particles are used to rub and remove the reactive colloid to achieve polishing effect.

The polishing application of silicon wafers is divided into three categories, mechanical polishing, chemical laser lightening and chemical mechanical lightening, and mechanical polishing is the surface processing of silicon wafers realized by mechanical friction between the abrasive in the polishing liquid and the surface of the silicon wafer. The mechanical polishing speed is fast, the processed silicon wafer has high flatness and strong adaptability. However, due to the high hardness and uneven thickness of the abrasive particles, the surface of the silicon wafer is rough, and there is a new surface damage layer. Dressing, chemical mechanical polishing is a combination of chemical action and mechanical action to make chemical reagents chemically react with silicon wafers, and then remove the reactants by the friction of polishing pads and abrasives, so that chemical and mechanical polishing are cycled to achieve the purpose of polishing.

Chemical mechanical polishing is usually used in production, and the above-mentioned alkaline silica polishing method is the mainstream process, mainly including preparation before polishing, thickness sorting, loading, rough polishing, fine polishing, chip removal, and finishing polishing Seven steps.

(1) Preparation before polishing
1 Power conditions: voltage requirement 380V, 50Hz; cooling water pressure requirement 0.10~0.55MPa; air pressure requirement above 0.2MPa; pure water flow rate requirement 4.55L/min.
2 Wafer loading.
3 Polishing cloth pad. Coarse throwing pads use polyester pads containing polyurethane, and fine throwing pads are based on non-woven fabrics, on which a plush structure is grown.
4 Prepare the polishing liquid, then filter it, and adjust the pH with NaOH and KOH. The polishing liquid is usually filtered with 180-mesh and 240-mesh asbestos.

(2) Thickness sorting
The thickness of silicon wafers is sorted by 2µm/block and marked. Select the appropriate crystal orientation for polishing, and then prepare the wafer for waxed and wax-free polishing.

(3) On-machine polishing of silicon wafers
On-machine de-glazing of silicon wafers is usually done by rough polishing first and then fine polishing. Rough polishing is to remove the layer on the surface of the silicon wafer to achieve the required thickness and flatness, while fine polishing makes the surface have better cleanliness.
To polish the silicon wafers with wax, first measure the thickness of the silicon wafers on the ceramic plate with a micrometer to ensure that the thickness difference of the silicon wafers on the same ceramic plate is within 5µm, otherwise, reattach. Then check whether the clamping block on the polishing head is loose, and install the glued porcelain plate on the polishing head and clamp it well, turn the manual rotary valve, the cylinder presses the polishing head to make it lower, close to the polishing table, and then the machine can be turned on. The silicon wafer is polished according to the required specifications, first rough polishing and then fine polishing. After the polishing time is up, reduce the polishing pressure to zero, turn off the polishing liquid, and soak in water for 3~10s.

(4) Take the film
Absorb the surface of the silicon wafer and the ceramic plate in water with clean filter paper, and then place it on the sticker for heating (control the temperature at 115~145°C). After the polishing wax is melted, gently push the edge of the silicon wafer to take the piece. On the filter paper, there should be no traces of tweezers and polishing wax on the surface of the silicon wafer.

Read more: What is a polysilicon ingot furnace?

Comments are closed.