Crystal Growth and Water Preparation

Crystal Growth and Water Preparation

The first important process in IC fabrication is the formation of a silicon wafer through the crystal growth. This process can be subdivided into number of sub processes as given below.

1. Silicon crystal ingots growing

2. Ingot trimming and grinding

3. Ingot slicing

4. Wafer etching

5. Wafer polishing

6. Wafer cleaning

Let us consider subprocess one by one.

1. Silicon Crystal Ingots Growing

The important semiconductor material for the fabrication of semiconductor devices and integrated circuits is silicon. Other semiconductor materials are germanium and gallium arsenide. At present 95% of the semiconductor devices and integrated circuits are manufactured using silicon only and for very special applications gallium arsenide is preferred. We have already discussed that the bipolar junction transistor (BJT) was developed first in 1948, with germanium as a basic semiconductor material. But it was observed that silicon is better option than germanium. The comparison between silicon and germanium is as given in Table 1.5.1.

Basically silicon is the element, found in nature in the form of silica and silicates. It is found abundantly in the natrure in the form of silicon dioxide. So this silicon dioxide constitutes almost 20 % of the earth's crust. So obviously one can not start fabricating integrating circuits using silicon dioxide with the earth's crust. The sand can be converted into pure silicon through number of processes. For the fabrication of ICs, the silicon must be in crystalline form. The crystalline form of silicon is the pure silicon with no deflects and no contaminations.

To obtain this purest form of silicon, first Metallurgical Grade Silicon (MGS) is produced in a submerged electron arc furnace. Using this step, MGS is solidified with a purity of about 98 %. Then the silicon is pulverized mechanically. Then it is added with unhydrous hydrogen chloride to form trichlorosilane (SiHCl3). This process is carried out with a catalyst in a fluidized bed at 350 °C. After this reaction, the trichlorosilane obtained is liquid at room temperature. Then by using fractional distillation, purification of trichlorosilane is carried out. After this purified trichlorosilane is applied with chemical vapour deposition process. With the help of chemical reaction, hydrogen is reduced from trichlorosilane. It results in the rods of silicon. This multistep process continues for many hours and finally results in polycrystalline structured EGS rods with 0.2 m diameter and several meter length. This process is advantageous because the costing of the process is low and the byproducts of the reactions are less harmful. The Electronic Grade Silicon (EGS) is also known as Semiconductor Grade Silicon (SGS) which the highly purified form of silicon i.e. polycrystalline silicon. This form of silicon consists many small crystals. However for fabrication of ICs a crystalline silicon is needed. The single crystal silicon can be obtained by using method known as crystal growth. 

The primary method of  CCW the crystal growth is Czochralski (CZ) method.

In practice all the silicon required for integrated circuits is prepared by using this method only. In general, a phase change from solid, liquid or gas phases to crystalline solid phase is nothing but growing crystal. Czochralski method is used for silicon crystal growth from which ultimately silicon wafers are produced. The apparatus used for the crystal growth is called Czochralski crystal growth apparatus or puller.

The puller has four important subsystems namely furnace, crystal pulling mechanism, ambient control and control systems. The simplified Czochralski crystal puller is as shown in the Fig. 1.5.1

The furnace consists a crucible, crucible support, rotation mechanism and heating element housed in a chamber. The crucible is made up of fused silica (SiO2) as this material is chemically unreactive with molten silicon. An EGS block is heated in a fused silica crucible with the appropriate dopant using a heating element. The material in the crucible is heated to a temperature which is greater than the melting point of silicon, i.e. 1417°C. A resistance heating is preferred for large pullers ; while induction heating is used for small melt sizes.

Then a small single crystal rod of silicon is immersed into the molten material. This rod is called seed crystal. This seed crystal is located at the crystal pulling assembly. In this assembly, using rotating mechanism seed shaft and seed chuck are rotated. Lowering the seed crystal in molten silicon allows the crystal ingot to form on the seed by solidification. The main function of crystal pulling assembly, is to control the pull rate of seed crystal and crystal rotation both, with minimum vibrations and precision. During the crystal growth, the crystal is rotated slowly, by stirring the molten and averaging out temperature gradients leading to inhomogeneous solidification. To get the ingots of circular cross-section, the crucible and the seed crystal are rotated in opposite direction. When the seed crystal is pulled out of the molten material, due to solidification, silicon ingot gets formed exactly same as seed crystal.

In general, the diameter is controlled by the pull rate. The standard diameter of the ingot is about 150 mm and the length is about 2 m. The ingot with such dimensions weighs about 60 kg.

The silicon growth is generally conducted in a vacuum or in an inert gas like helium or argon.

To control the process parameters such as temperature, crystal diameter, pull rate and rotation speed etc, control system is used which works under either open loop control or closed loop control.

2. Ingot Trimming and Grinding

First of all, the seed which initiated the crystal growth is separated from the circular ingot. The top and bottom ends are also cut off. As silicon is hard and brittle material, industrial grade diamond is used for shaping and cutting it. This process is called ingot trimming. After completion of the crystal growth, it is generally tested for resistivity and perfection evaluation. So the portions of the ingot failed in the above tests are also cut. Note that these cuttings can be recycled for new crystal growth after cleaning.

After trimming of the ingot, the surface grinding of the ingot is carried out. Actually the ingots are slightly oversized. Hence with the help of lathelike diamond tool, the ingot is ground to a precise diameter.

After grinding the ingot to a precise diameter, generally two flats are ground along the length of the ingot. The larger flat is called major or primary flat and it is positioned relative to the crystal direction. The x-ray technique is used to locate the primary flat. The primary flat is very important as

i) it serves for mechanical alignment of the wafer in automatic processing, and

ii) it serves for orienting ICs on the wafer relative to the crystal.

The smaller flat is called secondary flat which is used to identify the orientation (< 100 > or < 111 >) and conductivity (p or n) of the wafer.

3. Ingot Slicing

After completing ingot trimming and grinding process, the ingot is ready for next process i.e. ingot slicing. The slices of the ingot are called wafers and typically the thickness of wafer may very from 0.4 mm to 1 mm. This process is very important as it is necessary to maintain the flat plane and desired surface orientations. 

The slicing also determines the orientation of the surface. In general, there are two orientations < 100 > and < 111 >. Out of these orientations, the wafers with < 100 > orientations are cut 'on orientation' ; while wafers with < 111 > orientations are cut 'off orientations'. The position of the flats can be identified according to the standards laid by the Semiconductor Equipment and Materials Institute (SEMI) as shown in the Fig. 1.5.2.

The ingot is sliced using a circular cutting blade kept in tension on the outer edge while having the cutting edge on the inner diameter.

The thickness of the wafer is determined by the slicing. It is another important wafer parameter because thicker wafers can easily withstand the stresses of subsequent thermal processes. The higher quality of slicing is achieved by using capacitive sensing device near the blade which helps the blade to be positioned correctly to achieve exactly flat plane cut.

4. Wafer Etching

If the sliced wafers are to be used for VLSI application, then before etching process two sided mechanical lapping process is carried out. Using this process, wafers with uniform flatness are achieved which are mostly required for photolithography. 

Due to the machining operations during trimming, grinding and slicing, the surface and edges of the wafers get contaminated and even damaged. The depth of damage depends on the mechanical operations carried previously. It is observed that the damaged and contaminated regions are not more than 10 pm deep. Even by using chemical etching process, all the damaged and contaminated edges can be removed. Practically mixture of hydrofluoric, acetic and nitric acids is used in chemical etching. This is called acidic etching. The other alternative is to use alkaline etching using potassium hydroxide or sodium hydroxide. By the etching process, typically 10 pm to 30 pm of wafer is removed from both the sides.

5. Wafer Polishing

After etching, the wafer is polished to eliminate the microcracks and debris. The main intension of polishing a wafer is to provide a smooth and perfect flat surface such that the device features can be engraved. The polishing is done with the help of a polishing machine. Note that the polishing process removes further 10 pm to 30 pm of the wafer surface typically.

The three steps of lapping, etching and polishing reduce the wafer thickness by 40 to 150 pm. Thus by considering the typical figures, to have the wafer of 200 pm thickness, the thickness of the substrate required is 560 pm. In practice, the processed 6 inch wafers are typically 250 pm to 500 pm thick.

6. Wafer Cleaning

The silicon wafers are cleaned using chemicals. Generally organic films, heavy metals are deposited on the surface of the wafers. Hence by using HC1 - H₂O₂ aqueous solution, metallic impurities can be removed. First the wafer is cleaned by using HCl - H₂O₂. Then wafer is rinsed in water to deionize. Again the wafer is dipped in hydrofluoric acid. Then again the wafer is rinsed in water.

After cleaning process, the wafer is ready for the formation of the dies.

Review Questions

1. Describe the steps of crystal growth and water preparation in detail.

2. Explain Czocharalski method of crystal growth with the help of a neat diagram.

3. Write a note on ingot trimming and polishing.

4. Explain different orientations in silicon.

5. How is wafer etching carried out ?