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Etching process is done on the chip to get the desired pattern. It is done after thin films are deposited on the surface of wafer and can be done in any direction, vertically or laterally. Etching is done at the end of lithography process to flattern the wafer. It is done continuously until a new layer comes underneath . In isolation structures etching is done to create trenches. There are some chemical reactions included in etching process in order to remove unwanted material from the surface . There are mainly two types of etchants used to etch: liquid etchants and plasma etchants. Liquid etchants are also known as wet etchants and plasma etchants are also known as dry etchants. That's why dry etching is called as plasma etching and wet etching is called as liquid etching. Among these dry etching is used when the etching area is small and also it is slow for the etching of whole wafer. While for the whole wafer, wet etching is fast and effective . Moreover, wet etching can be done at room temperature while for dry etching high temperature is required. By these two techniques we can reduce the thickness of actual silicon wafer and we can get the desired flexibility for applications like car sensors, space applications etc. Here the focus is on wet etching as well as dry etching. There are also some other methods to etch a wafer . The work is done all of these techniques for silicon etching and particularly with different etchants, combinations and etching speed. At the end of the etching process, the silicon wafer will be thinner than before and more flexible for expected applications. General process of integrated circuit fabrication is shown in figure 1. So, it is clear that after which process etching takes place. Etching process is shown in figure 2 . In addition to these, there should also be a process to stop etching. So, at last some information of stop etching is included.
In industry, this is the first process used for etching. It is isotropic because it etches in all (laterally and vertically) directions equally. This is shown in figure 3 . In wet etching, etching is done by chemical etchants. So, it is also called as chemical etching. To do this process wafers are immersed into the chemicals to etch away unwanted material from wafer and leaving others. Wet etch process for SiO2 is known as hydrofluoric (HF) . Hydrofluoric acid plays an important role in etching. It is very dangerous to use but if it is used with the concentration of 49% than it increase the safety and it can be stored at room temperature . Now, let's take an example of one etching process through this process. In this method, a silicon wafer is submerged into the “chemical solution,” which consists of different etchants used for wet etching, and the concentration of that etchants is decided on the basis of the required etching rate.  The overall reaction for HF process is as follows:
SiO2 + 6HF -----------à H2SiF6 + 2H2O
In this reaction H2SiF6 is water soluble byproduct. In some cases, first oxidizing of surface wafer is done and then dissolves it into the chemical etchant. Thus in continuation of above equation the new reaction is:
Si + 2NO2 + 2H2O -------------à SiO2 + H2 + 2HNO2
So, the overall equation becomes:
Si + HNO3 + 6HF --------------à H2SiF6 + 2HNO2 + H2O + H2
There are various kinds of chemicals are used as an etchants . Previously, etchants like, TMAH (Tetra Methyl Ammonium Hydroxide) and KOH (Potassium Hydroxide) were used. The etch rate in this process depends on the total no. of wafers that were used in the wet etch bath. Moreover, more no. of wafers added complexity to the process development. Also, the etchant KOH that was used in the etching previously is highly toxic and undesirable in traditional IC processing. Compare to these etchants, HF (Hydrofluoric acid), CH3COOH (Acetic Acid) and HNO3 (Nitric Acid) are preferable . Compare to the etchants used earlier, these etchants have high etching rated. So, new etchants have taken place of older ones. There is another way to spread etchants on wafer. Sometimes it is known as spinning method. Here, the silicon wafer is fixed on the spinner and etchants are sprayed on wafer using tubes or valves. Wafer rotates at a constant speed, and the etch rate at which etchants are sprayed is depending on the requirement of the etching rate and amount of etch needed on the wafer .
Now, as name indicates, most wet chemical etchants etch isotropically due to its crystalline structure. If etchant etches in <111> directions then it is slower compare to other directions . Other directions are <001>, <010>, <100>, <011>, <101>, <110>. In these directions etchant has to etch in either X or Y or Z or two of them but not in all directions. While in <111> direction etchant has to etch in all X, Y and Z directions. So, it is slower than other directions. In etching process one more term exist is “etch bias”. It's the amount of etch underneath the edge of mask .
This etching process is known as “Wright etch” because it is invented by Wright Jenkins . Wright etch process is very important because it solves the problem of stacking faults, dislocations, swirls etc. This process also give some good results for <100>, <111> orientation. It also solves the problem of p type and n type crystals . Sometimes in normal etch process it happens that surface defects cant be removed properly but Wright etch process solves surface defect problem also. Materials which are used for this process are as follows: 60 ml of HF, 30 ml of HNO3, 30 ml of CrO3, 2 grams of Cu(NO3)2*3H2O , 60 ml of Acetic Acid and 60 ml of H2O . With these concentration of components etchant of Wright etch is made and then after the process of etching starts. In Table 1 it is mentioned that for which type of defects which etching method is used .
ION BEAM ETCHING
Ion etching is also one of the methods to etch wafer. This process is shown in figure 5. First of all, photoresist is spread on the wafer especially the portion of wafer which wants to be etched away. Now, ion beam is thrown on the wafer. Ion beaming is usually deep about 1300 Angstrom. If beaming is continued beyond this stage, then substrate will also be penetrated. After this process we can get perfect sidewalls. So, this process is used to etch away other material than sidewalls. These all are shown in Figure 5 . The whole system of Ion-beam etching is shown in figure 6 .
There are also some other etching methods like Sirtl Etch, Secco Etch etc.
DRY (PLASMA) ETCH
One of the mostly used and important etching processes is Dry etching. It is also known as Plasma etching. In early days, wet etching was used very widely but now, it has been taken place by plasma etching . There are mainly two reasons behind that. Those reasons are as follows:
The first reason is that in Plasma etching process very reactive chemicals are produces for etching process and those are very effective in plasma . Plasma etching is widely used for etching the passivation layer of wafer which is made up of Silicon Nitride. For this, wet etching is not preferable because it is very slow and for SiO2 it is not good. One can say that, this problem can be solved out by using phosphoric acid but why not to use a simple method instead of making it complicated and time consuming. In dry etching, photoresist mask is not lifted off which is a problem in wet etching .
The second and most important reason for using Dry etching is that it is anisotropic and it is shown in figure 4. If we do etching in only vertical direction then it is called Dry Etching . Because directional etching is needed to minimize underetching of wafer. By using this process we can get more tightly packed structure .
“Dry etch system is designed such a way that either reactive chemical component or the ionic components dominate.”  In some cases combination of both components are used. But individuals are much faster in process than the combined one .
The basic plasma system is shown in figure 7. In this system the energy is supplied by RF generator at 13.56MHz operating frequency. A low pressure gas has already filled out. Then a high electric field is applied to it. Due to that free electron and positive ions are generated. Due to difference in mobility of electrons between free electrons and positive ions a voltage difference occurs between plasma and two electrodes . But, more free electrons are lost to electrodes compare to ions, so plasma becomes positively charged. To reduce this unbalanced electron distribution it is recommended to use a “sheath” near electrodes . In figure 8 the steady-state voltage distribution in RF-powered plasma system clearly shows that unequal area of electrode generates an unequal electron loss . The sheaths are used to slow down the electron loss. It means sheaths compensate a loss of ion per RF cycle. The lack of electrons near sheath also increases the impedance, so most of the voltage drop occurs across the sheath. In figure 8 it is clearly shown that “for smaller area of sheath there is larger voltage drop occurring from the plasma to the smaller electrode” . Now one can think that where is the wafer to be etched away? The answer is that wafer is between plasma and electrode and it is surrounded in plasma region.
Plasma is the fourth state of matter and it generates tremendous amount of energy just for few tenths of seconds. It has different colors. But it is said that real plasma is as equivalent as solar energy and it serves purpose of Sun on the earth. So, now-a-days countries are running after it and try to invite a revolution on earth. Now let's go back to our etching.
To etch other than photoresist material some gases are added in the plasma such as CF4, CL2 and HBR. Sometimes H2, O2 and Ar are also added . Reaction of plasma to wafer in plasma etching is as shown in figure 9. There are mainly two components present in plasma etching process. One is neutral species which are known as free radicals and other one is ions. These components can work independently as well as together . “When the reactive neutral species act by themselves, the process or mechanism is called chemical etching and “when ions acting by themselves can result in physical etching. ”
Chemical etching mechanism in Dry etching:
In this process neutral species are used because they have free electrons in their outer orbit. So, they can combine with any one nay make a pair. So, they can increase a rate of etching through chemical reactions. Let's take an example of fluorine and CF3 both have fewer electrons in their outer orbit. So, they will be combined and they will quickly react with the other species. The main purpose for this kind of process is to etch the material by reactive neutral reactants . So, the materials to be etched can easily be etched away. In this process same as we discussed in wet etching, etching is done isotropically, so it etches some parts of material that are not to be etched. So, to reduce this problem we are doing this mechanism at particular angle.
Physical etching is also known as ‘sputtering'. In this process main part is ions that are present in plasma. As we discussed above electric field across sheath, wafers are placed on the electrode and voltage drop occurs between plasma and each electrode . Due to this process, ionic species (sputter) will be accelerated to the surface of wafer and etching will start. Here, ionic species penetrate only in one direction so it is called as anisotropic etching. This is called ‘sputtering' . Sputtering is accelerated by electric field. It is directly proportional to electric field. Due to this property of sputtering etching of a wafer is done perfectly and all ions arrive to wafer surface. But this has low etching rate compare to others. So, to increase sputter efficiency, ionic species are bombarded at particular angle. Sputter yield is also a function of incident angle. So, to increase sputtering yield angle of incident should be increased. But the effect of etching in sputtering is less compare to Ion beam etching . Because in Ion beam etching both neutral and ionized species are involved. Now, one new technology of etching is High density plasma etching. In this method bias supply is used to etch the material .
Etching in High Density Plasma (HDP) system:
This method has a special feature in it and a use of RF supply to improve etching. This system separates the plasma density and ion energy by using bias voltage through wafer electrode. Instead of using capacitive load for plasma non capacitive load is used . It produces a very high density of plasma. There are mainly two sources used for it: ‘Electron Cyclotron Resonance (ECR)' and ‘Inductive Coupled Plasma (ICP)' . Among these ICP is very popular because of much simpler design and less use of equipments. It is shown in figure 10 . In this system there are not different power supplies for sputtering ions and for wafer. This one is preferable because in ICP high density plasma is generated compare to ECR . Due to this density we can get more etching in very low pressure of supply. So, there is less gas collision in the sheath. In ECR we have to increase pressure of supply to get more etching rate. So, it is possible that wafer can be damaged. Due to this reason ICP is preferable. In ICR due to low pressure and low energy, etching will become more isotropic and we can get more perfection in etching process of material . In ICP system Inductive supply is used in order get more etch rate in low power only. Wafer is kept in the plasma surroundings and also within dielectric window. So, we can have both directional (isotropic) as well as low power etching. So, this method of etching is reasonable .
If we want to improve etching rate more then it is possible by higher ion flux. By keeping lower power only and higher ion flux we can get more isotropic etching than HDP etching. To get higher flux, sheath has to be more supplied and power supply and gas should also higher .There is one same kind of method which uses ion milling. Here, some advantages of sputtering method are also used. So, this method is called as Ion milling and sputtering. Because in sputtering process, separate power source is used. So, there is more flexibility in using the Sputter etching and Ion milling method.
Sputter Etching and Ion Milling
This system is different from chemical method and only sputtering method. So, this one is totally physical method of etching. The sputtering has its own purposes. It is used to produce species and deposition of material. As we have discussed sputtering method is used to planarization the film which is to be etched and filling some holes or defects in the material. One more thing about sputtering is that it needs RF supply . Because in this system ions are bombarded to wafer and plasma is used to etch away some material. Plasma does reaction with Argon gas in this system. Here, there is one inlet of Gas is provided. So, it is very easy to start a reaction with plasma through this gas. In this method, cathode is small compare to anode. So, energy of bombarding ions is very high and it is greater than 500ev. Due to this, etching is done very fast and effective. There are some advantages in this system .
The main advantage of this system is by using this method of etching all materials can be etched away very easily. Moreover, ions and sputtering both are used to etch away some material in this system. So, this is very convenient method . In this method etching is very directional. So, it is anisotropic. Here, one disadvantage is that this method doesn't use any chemical reaction so; it is totally depending on the sputtering. To increase a yield of etching sputtering yield should be increased. But sputter yield is better is improved by argon gas. So, after all we can get good etching in this process. This etching method used when other methods can not work properly. Sputtering yield is a function incident angle . There are some problems related to sputtering and ion milling etching. This problem is due to ion bombarding and will become strong due to only sputtering because this is totally physical.
One of these problems is called trenching, or “microtrenching” . In this problem ions are glance off the sides and will strike the bottom corner. So, corners have higher ion flux at corners than other regions of the chamber which is used for sputter etching and ion milling process. Another problem is of redeposition. In this case, other ions are also sputtered and decrease the efficiency of yield. These unwanted ions can redeposit anywhere and create another layer. One more problem is of radiation damage. “During sputtering, surface is subjected to bombardment of energetic ions, electrons and photons. Ion bombardment can lead to electron traps being produced in the in gate oxides. ” Another problem is surface charging. This is related to bombardment of ions. Because of nonuniformities in plasma dielectric materials can be charged by the local differences between ions and electron flux. This flux is induced by ion bombarding on the surface of wafer or plasma. Tunneling Current damage is also produced due to this so, uneven etching is occurring .
By using ion milling technique problems associated with charged species are eliminated. In this technique, plasma is used to generate a beam of argon ions. This is done by accelerating the ions towards target of wafer fro plasma chamber. “A separate electron is used to supply electrons to neutralize the ion beam .” This allows for independent control of ion energy and ion density. “The amount of reactive species may also be added to, or replace, the argon in ion milling systems. These species may include CF4, CCl4, or O2. This will add some amount of selectivity to the process. This method is called ‘Reactive Ion Beam Etching (RIBE)' ”. Ion beam is used to sputter ions and etch a wafer which has small area. This is called Focused Ion Beam (FIB) technique. But this is used for lithography process .
We have discussed a lot about etching. We also discussed about all etching techniques and methods. There are lots of benefits and lots of problems for different methods. Now question arises that how a process or material knows that it has to stop etching or not? The answer of this question is there is one layer beneath the material to be etched away. So, there is a method to stop etching. There is a layer for etch stop. There are different methods for etch stop. It is discussed as follows:
It is defined as material featuring drastically different etch characteristics than material to be etched, layer of “etch stop” material is placed underneath etched material to stop etching process .
An etch-stop is where some form of barriers is obtained within the material which holds up the etching process. This can be obtained by the process below:
- High level of boron
- Electrochemical bias
- Embedded layer
In the first method, by making silicon highly doped by boron atoms it becomes almost impossible to dissolve into alkaline etchants.
In the second method, the potential is created within the wafer by which etching can make inhibited at one portion and not to other. The drawback of this method is that it will create porous silicon of a few micrometers, which not gives the polished layer.  
In the last method, a thin layer of different type of material is embedded within the wafer which stops the further etching process. Other way to obtain this layer is ion implantation. The drawback of ion implantation is it needs to heated, which can done under room temperature. 
I am going to follow all these procedures to get the etching of silicon wafer done. And my goal is to take cross-section of a silicon wafer to get the view of a thin silicon wafer with device.