Dr Shi 珺 on crystal growth of polycrystalline silicon ingot processes

Polaris solar PV net news: after the vacuum melting, but after a cool and stable, enters the stage during Directional solidification. This process is silicon crystal growth process, are able to recycle materials and metallurgy process, impurities in polycrystalline silicon material is further purified.

(A) Directional solidification and segregation phenomena

Impurities in silicone fluid silicone fluid solidification from the bottom when the impurities tend to in liquid, but will not stay on solid. This phenomenon is called segregation phenomena.

When in the solid-liquid interface stability, the number of impurities in solids and in liquids in the ratio of the number, called the segregation coefficient. Impurities segregation coefficient is less than 1, directionally solidified when top tends to enrichment. The amount and degree of enrichment, depending on how many the segregation coefficient. In General, the segregation coefficient of metallic impurities below 10-3 (aluminium is approximately 0.08), so that Directional solidification modes and degradation, is more effective for metal impurities and boron and phosphorus segregation coefficient and 0.8 respectively 0.36 and, therefore, boron and phosphorus segregation phenomenon is not too obvious.

Purification of directionally solidified at the same time, taking into account the Silicon crystal growth technology, allows Directional solidification of silicon to polycrystalline silicon ingot slices directly, which is purified and United finished in one process of ingot. This General purified ingot furnace is an important means of purification. Due to the presence of impurities of silicon materials and crystals of high purity material and molten nature is different and therefore purified ingot furnace the temperature field and thermal field there is a difference between pure ingot furnace.

Development General company currently using their own patented design of a purified ingot integration, successfully resolved the problem, vacuum melting and casting is done in one process, purification can be solved both problems, also successfully completed the casting requirements.

(B) the process of crystal growth

Directional solidification is divided into the following four stages, including: formation and polycrystalline growth of embryos, the top top collection, annealing and cooling.

Embryos formed

After the melting, temperatures decrease to around 1440 c of silicon solutions and stay there for a while, and then make the bottom of the Crucible begins to cool, cooling below the melting point ℃ 6-10 or 1404-1408 degrees Celsius.

RDS4.0 method of decreasing the temperature at the bottom of the furnace body is to reduce power at the bottom, and gradually open the thermal switch at the bottom. With conventional ingot furnace upgrading insulation and heating element compared to because there was no cooling then gradually started around the central process, therefore, much more uniform temperature in the bottom.

When casting, bottom infrared temperature measurement data is not exactly Silicon liquid temperature at the bottom, because the measurement points and the Crucible silicone fluids are separated by at least one layer at the bottom of the Crucible, so the infrared temperature can reference only, and is based on the empirical data for each stove. At this time, the bottoms will form a supercooled liquid below the melting point, due to the bottom of the crucible of the fine structure of uneven on some particles will form a nucleus, namely the first solidification of particle, forming crystals. These particles may be prominent uneven points on the Crucible, is probably the crucible of SAG, because of its low position than any other position, so the time to cool down, the temperature will be lower.

Nucleus is formed, due to the large size of solar cells need to radial columnar, so best not to let grow upwards as soon as nucleation, once formed, this can lead to too much grain but after you first want to nucleation, first in the bottom of the Crucible lateral growth, and grow to a certain size, and then upward growth. In this way, requires the temperature at the melting point of the bottom of the Crucible after slightly lower, remained stable, no longer declining. In this way, bottom of the Crucible after nucleation, due to an upward growth, temperature is too high, it cannot, therefore, only horizontal growth.

When you begin to form the nucleus, because the bottom of the Crucible is not uniform, nucleating the formation is not uniform, some secret, some places are sparse. At the time of the nucleus lateral growth, grows to a certain level, will meet encounters due to a growing momentum in the face of other chips, it encountered resistance, resistance when the chip is too large, it will stop growing. Sometimes, this resistance may cause loose chips off combined with the bottom of the Crucible, so that the firmer chip will fall off leaving gaps continue to grow until after the entire bottom is covered with chips, and crush all the chip can start growing up. At this point, each Flake crystals began to grow up, it’s called embryos. This is the process of formation of embryos.

Nucleation in the pots start competition lead chips falling off, these chips due to lighter, will float above until the float to the Silicon surface. Due to the chip temperature is lower, therefore, leads to low temperature of the infrared thermometer, but usually melts quickly, so the temperature will reply. In this way, will appear in the temperature curve downward spikes.

Growth of polycrystalline

After the formation of embryos, began to grow up. Growth of polycrystalline silicon Mono-crystal growth of some different places. First, polysilicon Silicon of growth is many of Crystal column common growth, and mutual Zhijian also has competition and met; and Crystal is only a Crystal, not exists grain Zhijian of competition problem; second, more crystal silicon of growth is due to temperature field of role, bottom temperature constantly declined, led to solid liquid interface constantly rose; and crystal of liquid surface temperature basic not variable; third, polysilicon cast ingots of Silicon liquid relative still, and Silicon of Silicon liquid and Crystal is rotating of. These differences, resulting in polycrystalline silicon crystal growth has its advantages and disadvantages.

Once embryos form, it should be up to a certain speed is crystal growth. It needed to make Crucible bottom surface temperature drops slowly, which led to the melting point of Silicon liquid surfaces, in the slowly rising, increasing at a rate consistent with the Silicon crystal growth rate. Silicon crystal growth rate is not constant, has a range of between about 6~20mm/hours. Therefore, control the speed of the solid-liquid interface, so that it remains in this speed range, you can.

While in conventional ingot, many people think of the thermal mass speed is the speed of solid-liquid interface, which is completely wrong. Speed of solid/liquid interface and holding fast the body concerned, but also with the bottom surface temperature and heating power. Sometimes, the solid-liquid interface movement about insulation body moving speed of, at times, speed is less than the thermal insulation of the solid-liquid interface of movement speed. If the underside of the temperature is too low, when there’s not much heating element power, insulation represents a slight increase, within a region may cause the entire bottom temperature fell relatively quickly when moved faster than the solid-liquid interface insulation can be faster, leading to growth soon.

Most of the time, if the insulation is creeping up, and heating element power is large, when the insulating body at a certain level, because insulation body part below still has radiation so as to maintain a high temperature, then, the solid-liquid interface speed is much slower than insulation body movement. In short, Crucible also needs to constantly change in temperature at the bottom, and the Silicon liquid temperature at the top of, and is constantly changing. Is more complex, these two parameters change over time is not linear.

Taking into account the role of liquid silicon due to convection, temperature difference is small, solid silicon because Silicon cannot be moved, heat only by way of heat conduction, and the low thermal conductivity of silicon, with poor thermal conductivity and, therefore, can form a larger temperature difference within the solid silicone. In this case, embryos, once formed, can make to temperature gradient at the bottom and at the same time, can keep a relatively higher temperature at the top, so that the bottom of the column.

When columnar crystals grow to 80-120mm high, because of the poor thermal conductivity of solids and, therefore, decline in temperature based on a linear speed at the top, this can be uniform in favour of solid-liquid interface is up.

Dang columnar long to 160-220mm high Shi, due to most has is solid, so, bottom temperature on solid liquid interface of temperature effects has is unlikely to, but due to solid liquid interface in Crystal Shi of latent heat, so, produced of heat also is needs constantly to from bottom was away, so, bottom temperature must enough of low, to in solid silicon internal formed enough big of temperature gradient, put solid liquid interface of temperature away. At this point, prompting rose’s main power is the solid-liquid interface on top of the temperature dropped, and bottom temperature drops. When the temperature of the surface becoming the solidifying point of silicon, crystal growth, near-surface, closer to completion stage of crystallization.

Typically, ingot to be successful, to guarantee two basic conditions, low under a temperature gradient is always high, and crystal growth of solid-liquid interface we want to speed up and, secondly, to ensure the solid/liquid interface levels as much as possible. If solid-liquid interface are not level, it will inevitably lead to grow faster in the Middle, or next to grow faster. In this way, is not conducive to removing impurities during Directional solidification and Crystal Cross in the Middle, could lead to polycrystalline silicon ingot internal stress increases, makes the ingots easily breakable.

To ensure that the solid-liquid interface levels is no easy thing. Main reason is that the silicone fluid in after solidification, thermal conductivity is small, so silicone fluid and the internal temperature of the ingot heat of solidification and crystallization time release of latent heat of crystallization of dissemination is not very easy. Because heat during the four weeks, bottom heat dissipation, so that horizontal temperature differences will always exist. Especially in the second half of the Crystal time with increasing Crystal thickness, temperature and greater complexity.

Now casting-thermal field calculation model of DSS, HEM, RPDS, etc; these ingots with the actual real thermal distribution has a large gap. Impurities in Silicon-vapor pressure, free energy, entropy change of the Crystal, has an effect on crystallization process, and these factors, when Virgin polysilicon ingot is used does not need to be considered.

General solution to temperature gradient is, keep the bottom of the crucible of isothermal surface level, namely temperature difference, then, as the top surface is liquid, can be approximated as isothermal surfaces, so, in the upper and lower surfaces between, roughly the level of assurance is located in the middle of the solid-liquid interface.

Crucible bottom stay isothermal methods mainly from two channels, one was placed platform platform with good thermal conductivity graphite Crucible, graphite heat conductivity is good, therefore, when outside temperature difference exist in and around the Middle, given enough time, will be able to bring the entire surface temperatures are roughly tied. To do this, you also need to work on the platform of graphite structure, there is the General patented technology, basically guaranteed that.

Also keep the solid-liquid interface levels are a necessary condition, is the bottom heating element and thermal structure of bottom temperature uniformity. Shanghai General ingot furnace is currently able to ensuring international market only maintaining even heat heating and cooling in the bottom field and the furnace structure.

Top your top

When crystals grow to close to the top, top the final process is very important. Due to infrared temperature measurement of problem, often measurement of temperature does not necessarily is accurate, so, set of Crystal temperature may no crystal, when Department of Silicon liquid in the contains impurities of when (this is must of), Silicon liquid of melting point also will occurred changes, impurities more more, melting point more low, and impurities of content is to determines to of, so, hard control to temperature just top Department Crystal Shi ended. In addition, in Silicon in metallurgy, there will be a top layer of slag, the existence of these residues can lead to deviation of the temperature. Also, when Crystal long top, due to the difference in temperature in four weeks and Central, Central temperature even if accurate, the surrounding temperature may vary and, therefore, end in crystal growth processes, it is necessary to consider the entire ingot.

If ending malnutrition, can lead to more serious consequences, it may even lead to naught of the ingot process. First, if temperature control bad, closeout of when, joined Crystal distance top Department also has a distance, so, in closeout of when, due to always exists a fast cooling of process, this process will led to top Department of Silicon liquid from surface first solidified, such, in silicon ingots lower of Crystal and top Department of solidified into of solid shell layer Zhijian, will residues some liquid, these liquid in then of cooling process in the also will solidified, due to Silicon liquid solidified Shi volume will expansion, such words, light is led to silicon ingots surface “bulge” phenomenon of produced, Heavy then causes the upper partial rupture has been frozen. Due to the stress of the strength is very large, the rupture is not just part of the final set, but will also extend to the lower part has solidified silicon ingots.

However, the entire top of the ingot while crystallization is only a kind of ideal conditions, it is practically impossible to achieve. Under normal circumstances, ingots were the first to grow up to a certain part of the Crystal top of silicone fluid, the first complete crystallization. At this point, should the temperature is maintained for some time, slowly lower the heating power, the ingot slowly upward growth at other parts, usually about 120-180 minutes later, the entire top of the ingot growth could be completed.

Annealing and cooling

Crystal growth is completed, cannot be cooled immediately, because the crystallization is complete, at 1410 degrees Celsius on the top and bottom temperature 900 ° c or so, as much as 500 degrees Celsius temperature difference between up and down, and such a large temperature difference will produce a lot of heat stress and, therefore, must go through the annealing process.

Provide an environmental temperature of the ingot annealing refers to converge at a slow pace, and remain so for some time. Annealing ingot internal stress can be eliminated also defects such as dislocations in the crystal growth process with some degree of elimination, so crystal is not easy to break. Even if growth really well, improper annealing can also cause a breakdown of silicon ingots. For the next step for the ingot to be sliced, cracked was equal to scrap. If only purified without ingot, a complete silicon ingots are easier to work with than broken, such as the tops and tails. Therefore, the annealing process, should not be overlooked, not because growth has been completed, to the effect that by this time.

Crystal growth is complete, due to the high temperatures at the top and, therefore, need to keep the temperature cool slowly. Typically, power from the Crystal of power according to a fixed rate, meanwhile, will drop insulation body and heat hit bottom when power drops below a minimum value.

There are different versions of the annealing temperature. At 1300 ° c for thermal insulation, it is called high temperature annealing; annealed at 1100 ℃, known as low-temperature annealing.

Optimum annealing temperature is controversial. Generally considered long Crystal should be completed, middle of top and bottom of ingot temperature value, this argument seems reasonable, but also consider Silicon annealing from cold to heat up to temperature and annealing temperature from cool to hot, are subject to thermal stress is different. Cold temperature, thermal stress seems to be bigger, from this perspective, the annealing temperature should be around 1100 ℃ appropriate.

The annealing time, based on theory and experience, in 3-4 hours, after which the annealing temperature, then heat for 3 hours or so, and then, you can supplement the natural heating and cooling.

Natural cooling stage of so-called supplementary heating, cooling the ingot insulation inside were slow, and the temperature of the heating element should also be given a small amount so that temperatures do not drop too fast. Since at temperature up to 900 ° c, you can turn off the power, so its completely natural cooling.

Often think of temperature below 400 ℃, can turn on the stove, and then in about 100 ℃ Crucible even trim out, but kept in a closed, airless room, wait until more than 12 hours, remove shielding and the crucible.

Original title: Dr Shi 珺 on crystal growth of polycrystalline silicon ingot processes

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