1 00:00:05,500 --> 00:00:11,500 We have various types of silicon wafers such as monocrystalline silicon and polycrystalline silicon. 2 00:00:11,790 --> 00:00:18,500 In this block I will give an answer to the question: how do we make these various types of silicon? 3 00:00:18,500 --> 00:00:22,539 How can we make the silicon material pure? 4 00:00:22,539 --> 00:00:27,359 The lowest quality of silicon is the so-called metallurgical silicon. 5 00:00:27,359 --> 00:00:32,010 The source material of making metallurgical silicon is quartzite. 6 00:00:32,010 --> 00:00:36,510 Quartzite is a rock of pure silicon oxide. 7 00:00:36,510 --> 00:00:43,900 In the next animation the process of making metallurgical silicon out of quartzite is shown. 8 00:00:44,309 --> 00:00:49,040 During the production the silicon is purified by removing the oxide. 9 00:00:49,040 --> 00:00:52,750 This happens in a submerged electrode arc furnace. 10 00:00:52,750 --> 00:00:56,619 The quartzite is moved into the furnace, where it is melted. 11 00:00:56,619 --> 00:01:03,170 Using an electrode the quartzite is heated up to a temperature around 1900 degrees Celsius. 12 00:01:03,170 --> 00:01:06,369 The molten quartzite is mixed with carbon. 13 00:01:06,369 --> 00:01:10,530 The carbon source is a mixture of coal, coke and wood chips. 14 00:01:10,530 --> 00:01:13,450 The carbon reacts with the silicon oxide. 15 00:01:13,450 --> 00:01:19,070 I won't discuss the details of the reaction, which is rather complex, but the result is 16 00:01:19,070 --> 00:01:22,730 that the oxygen is leaving the furnace as carbon monoxide. 17 00:01:22,730 --> 00:01:28,080 The molten silicon that is formed is drawn of the furnace and solidified. 18 00:01:28,080 --> 00:01:33,700 The purity of metallurgic silicon is around 98 up to 99%. 19 00:01:33,700 --> 00:01:41,400 70% of the worldwide produced metallurgical silicon is used in the aluminum casting industry 20 00:01:41,570 --> 00:01:47,659 to make aluminum silicon alloy parts which are used in automotive engine blocks. 21 00:01:47,659 --> 00:01:53,520 The other 30% is being used to make a variety of chemical products like silicones. 22 00:01:53,520 --> 00:02:00,520 Only around 1% of metallurgical silicon is used to make electronic grade silicon. 23 00:02:02,030 --> 00:02:06,890 The silicon material with the next level of purity is called polysilicon. 24 00:02:06,890 --> 00:02:16,800 In the next animation you see how out of metallurgical silicon, rods of polysilicon are produced. 25 00:02:17,200 --> 00:02:20,980 The source material is powder of metallurgical silicon. 26 00:02:20,980 --> 00:02:26,670 The metallurgical silicon is then exposed in a reactor with hydrogen chloride at elevated 27 00:02:26,670 --> 00:02:28,730 temperatures in presence of a catalyst. 28 00:02:28,730 --> 00:02:35,080 The silicon reacts with the hydrogen chloride and starts to form trichlorosilane. 29 00:02:35,080 --> 00:02:41,300 This is a molecule that contains one silicon atom, three chlorine atoms and one hydrogen atom. 30 00:02:41,390 --> 00:02:46,030 The trichlorosilane gas is cooled and liquified. 31 00:02:46,030 --> 00:02:52,260 Impurities with higher or lower boiling points are then removed using distillation. 32 00:02:52,260 --> 00:02:59,800 The purified trichlorosilane is evaporized again in a different reactor and mixed with hydrogen gas. 33 00:02:59,880 --> 00:03:08,000 Trichlorosilane reacts with the hot rods which are at a high temperature of 850 up to 1050 degrees Celsius. 34 00:03:08,130 --> 00:03:13,160 The silicon atoms are deposited on the rod whereas the chlorine and hydrogen atoms are 35 00:03:13,160 --> 00:03:16,680 desorbed from the surface of the rod back into the gas phase. 36 00:03:16,680 --> 00:03:21,800 As a result a pure silicon material is grown and this deposition method is called 37 00:03:21,800 --> 00:03:23,709 chemical vapor deposition. 38 00:03:23,709 --> 00:03:29,610 As the exhaust gas still contains chlorosilanes and hydrogen, these gasses are recycled and 39 00:03:29,610 --> 00:03:31,420 used again. 40 00:03:31,420 --> 00:03:34,379 Chlorosilane is liquified and distilled and reused. 41 00:03:34,379 --> 00:03:41,379 The hydrogen goes through a cleanup process and is recycled back into the reactor. 42 00:03:41,920 --> 00:03:49,000 In the animation we have seen a chemical vapor deposition furnace that leads to polysilicon rods. 43 00:03:49,209 --> 00:03:54,400 This is the so-called Siemens process and consumes a lot of energy. 44 00:03:54,400 --> 00:04:01,310 Another method is the production of polysilicon granules in the so-called fluidized bed reactors (FBR). 45 00:04:01,310 --> 00:04:07,780 This process operates at lower temperatures and consumes much less energy. 46 00:04:07,780 --> 00:04:16,500 Polycrystalline silicon can have an purity as high as 99.9999%, or in other words 47 00:04:16,500 --> 00:04:21,349 one out of million atoms is not a silicon atom. 48 00:04:21,349 --> 00:04:28,349 Lastly, I would like to mention an alternative approach, that of upgraded metallurgical silicon. 49 00:04:28,689 --> 00:04:35,259 In this process metallurgical silicon is chemically refined by blowing gasses through the silicon 50 00:04:35,259 --> 00:04:38,669 melt removing the impurities. 51 00:04:38,669 --> 00:04:44,789 Although this processing is cheap, the purity of its silicon is not as high as the Siemens 52 00:04:44,789 --> 00:04:48,409 or the FBR approach. 53 00:04:48,409 --> 00:04:53,419 The next step is making wafers out of the polysilicon. 54 00:04:53,419 --> 00:04:59,620 But first we consider two methods to make monocrystalline silicon ingots. 55 00:04:59,620 --> 00:05:03,089 Ingots are large blocks of crystalline silicon. 56 00:05:03,089 --> 00:05:09,020 The monocrystalline ingots are solids that consist of one big crystal. 57 00:05:09,020 --> 00:05:15,490 In the next animation you will be introduced to the Czochralski processing method. 58 00:05:15,490 --> 00:05:21,600 Let's start with the Czochralski method, as developed by Polish scientist Jan Czochralski in 1918. 59 00:05:21,650 --> 00:05:24,779 It is a method to grow single crystal silicon. 60 00:05:24,779 --> 00:05:30,589 In this method, highly purified silicon is melted in a crucible at typical temperatures 61 00:05:30,589 --> 00:05:33,000 of 1500 degrees Celsius. 62 00:05:33,000 --> 00:05:39,700 Intentionally boron or phosphorous can be added to make p-doped or n-doped silicon, respectively. 63 00:05:39,710 --> 00:05:45,969 A seed crystal that is mounted on rotating shaft is dipped in to the molten silicon. 64 00:05:45,969 --> 00:05:48,779 The orientation of this seed crystal is well defined. 65 00:05:48,779 --> 00:05:54,699 It is either a 100 orientation or an 111 orientation. 66 00:05:54,699 --> 00:06:00,529 The melt solidifies at the seed crystal and adopts the orientation of the crystal. 67 00:06:00,529 --> 00:06:06,719 The crystal is rotating and pulled upwards, allowing the formation of a large, single-crystal 68 00:06:06,719 --> 00:06:09,139 cylindrical column from the melt. 69 00:06:09,139 --> 00:06:13,849 This big single crystalline silicon block is called an ingot. 70 00:06:13,849 --> 00:06:19,099 In this process the temperature gradients, rate of pulling up and speed of rotations 71 00:06:19,099 --> 00:06:21,089 are precisely controlled. 72 00:06:21,089 --> 00:06:26,629 This process is further developed through years of advances and nowadays crystal ingots 73 00:06:26,629 --> 00:06:33,629 of diameters of 200 mm and 300 mm with lengths of 2 meters can be processed. 74 00:06:33,710 --> 00:06:39,409 To prevent the incorporation of impurities this process takes place in an inert atmosphere, 75 00:06:39,409 --> 00:06:41,009 like argon gas. 76 00:06:41,009 --> 00:06:46,180 The crucible is made from quartz, which partly dissolves in the melt as well. 77 00:06:46,180 --> 00:06:52,219 Consequently, Czochralski monocrystalline silicon has a relatively high oxygen level. 78 00:06:52,219 --> 00:06:56,830 The second method to make monocrystalline silicon is the so-called float zone process. 79 00:06:56,830 --> 00:07:03,069 This is a process which results in monocrystalline silicon ingots with extreme low densities 80 00:07:03,069 --> 00:07:06,460 of impurities like oxygen and carbon. 81 00:07:06,460 --> 00:07:09,629 The process is shown in the next animation. 82 00:07:09,629 --> 00:07:16,600 The source material is a polycrystalline rod as processed in the earlier mentioned Siemens process. 83 00:07:16,759 --> 00:07:22,499 The end of the rod is heated up and melted using a radiofrequent heating coil. 84 00:07:22,499 --> 00:07:26,179 The melted part is put in contact with seed crystals. 85 00:07:26,179 --> 00:07:32,069 Here it solidifies again and adopts the orientation of the seed crystal. 86 00:07:32,069 --> 00:07:37,559 Again both 100 and 111 orientations are being used. 87 00:07:37,559 --> 00:07:44,500 As the molten zone is moved along the polysilicon rod, the single crystal ingot is growing as well. 88 00:07:44,500 --> 00:07:49,779 Many impurities remain in and move along with the molten zone. 89 00:07:49,779 --> 00:07:54,990 During the process nowadays intentionally nitrogen is added which improves the control 90 00:07:54,990 --> 00:08:00,229 on microdefects and improves the mechanical strength of the wafers. 91 00:08:00,229 --> 00:08:05,229 The advantage of the float-zone technique is that the molten silicon is not in contact 92 00:08:05,229 --> 00:08:09,199 with other materials like quartz as in the Czochralski method. 93 00:08:09,199 --> 00:08:15,600 In the float-zone process the molten silicon is only in contact with the inert gas like argon. 94 00:08:15,899 --> 00:08:21,459 The silicon can be doped by adding doping gasses like diborane and phosphine to the 95 00:08:21,459 --> 00:08:25,919 inert gas to get p-doped and n-doped silicon, respectively. 96 00:08:25,919 --> 00:08:32,409 The diameter of float-zone ingot is generally not larger than 150 mm, as the size is limited 97 00:08:32,409 --> 00:08:35,900 by the surface tensions during the growth. 98 00:08:35,969 --> 00:08:42,300 Next to monocrystalline silicon ingots, multicrystalline silicon ingots can be processed as well, 99 00:08:42,400 --> 00:08:46,430 as you can see in the next animation. 100 00:08:46,430 --> 00:08:51,710 Multicrystalline and polycrystalline silicon consist of many small crystalline grains. 101 00:08:51,710 --> 00:08:57,310 This can be made by melting highly purified silicon in a dedicated crucible and pouring 102 00:08:57,310 --> 00:09:01,490 the molten silicon in a cubic shaped growth-crucible. 103 00:09:01,490 --> 00:09:06,930 Here the molten silicon solidifies into multicrystalline ingot. 104 00:09:06,930 --> 00:09:09,610 This process is called silicon casting. 105 00:09:09,610 --> 00:09:17,100 If the melting and solidification occurs in the same crucible it is referred to as directional solidification. 106 00:09:17,300 --> 00:09:25,900 The cross-section of a multicrystalline ingot can go up to 70 by 70 cm and the height is typically 25 cm. 107 00:09:25,970 --> 00:09:31,800 Now we know how to produce monocrystalline and multicrystalline ingots. 108 00:09:31,800 --> 00:09:35,190 How do we make wafers out of them? 109 00:09:35,190 --> 00:09:39,280 The answer is sawing as you can see in the next animation. 110 00:09:41,260 --> 00:09:46,730 A disadvantage of the sawing step is that we waste a significant fraction of the silicon 111 00:09:46,730 --> 00:09:48,490 as a kerf loss. 112 00:09:48,490 --> 00:09:54,130 The kerf loss is usually determined by the thickness of the wire or saw used for sawing 113 00:09:54,130 --> 00:09:57,580 and is in the order of 100 microns of silicon. 114 00:09:57,580 --> 00:10:02,990 This is a large fraction of the ingot if we consider that typical crystalline silicon 115 00:10:02,990 --> 00:10:09,990 wafers used in solar cells nowadays are in the order of 150 up to 200 microns. 116 00:10:09,990 --> 00:10:13,000 Sowing will logically damage the surface of the wafers, 117 00:10:13,000 --> 00:10:17,800 so this processing step is followed by a polishing step. 118 00:10:20,000 --> 00:10:27,500 Silicon ribbon is a completely different approach to make wafers as you will see in the next animation. 119 00:10:27,990 --> 00:10:33,330 Silicon ribbon does not face the problem of kerf losses, due to the simple reason that 120 00:10:33,330 --> 00:10:36,270 it does not include a sawing step. 121 00:10:36,270 --> 00:10:40,760 Silicon ribbon is the last processing method I would like to discuss. 122 00:10:40,760 --> 00:10:45,640 Silicon ribbon is based on a high temperature resistant string, which is pulled up from 123 00:10:45,640 --> 00:10:47,180 a silicon melt. 124 00:10:47,180 --> 00:10:52,410 The silicon solidifies on the string and a sheet of crystalline silicon is pulled out 125 00:10:52,410 --> 00:10:54,500 of the melt like this. 126 00:10:54,500 --> 00:10:57,430 The ribbon is then cut into wafers. 127 00:10:57,430 --> 00:11:02,400 The surface is further treated before they are further processed into solar cells. 128 00:11:02,400 --> 00:11:09,400 The electronic quality of ribbon silicon is not as good as that of monocrystalline silicon. 129 00:11:09,400 --> 00:11:19,400 Summarized, we have discussed how out of quartzite we first make metallurgical silicon and then polysilicon. 130 00:11:19,500 --> 00:11:25,900 Monocrystalline ingots are made using either the Czochralski or the float-zone process. 131 00:11:26,500 --> 00:11:31,130 Multicrystalline ingots are made using a casting method. 132 00:11:31,130 --> 00:11:35,500 Wafers are being made by sawing these ingots. 133 00:11:35,600 --> 00:11:42,800 A method which does not have any kerf losses is the so-called ribbon silicon approach. 134 00:11:42,800 --> 00:11:46,530 Now we know how we make the wafers. 135 00:11:46,530 --> 00:11:48,670 Let's make solar cells out of them. 136 00:11:48,670 --> 00:11:52,010 We will discuss the design rules in the next block!