Deposition: Rich man’s brick laying
Starter's first deposition.
Construction is something that everyone of you has witnessed time and time again. Most of these buildings are made of brick and concrete. Concrete acts as the adhesive, joining the bricks together. Deposition is the construction of semiconductor manufacturing. Basic understanding of it is constructing thin films by laying atoms on a substrate. Atoms are the bricks and the bond between the atoms is the concrete.
This topic is an absolute mammoth to cover due to the number
of techniques and materials that can be laid on top of one another in such a
minuscule scale. The huge range and properties of materials, whether they be
elements or compounds, calls for multiple processing procedures. One technique
may work for one material and may completely fail for the other one just due to
a difference in melting point.
There are three basic deposition principles: physical deposition, chemical deposition, and hybrid modes. The names tell you half the story, physical implies that the element will remain as is, and will not undergo any reaction. Chemical directs you to the fact that precursors (reactants) will undergo a reaction and then be deposited on the substrate. Hybrid is a mix between the two.
Figure 2: Flowchart Showing the various deposition processes (made using Freeform)We will focus more on the main phenomena such as evaporation
and sputtering in physical, and vapour in chemical. These are the techniques
most used in front-end fabrication [back-end is the assembly, test, marking,
and packaging (ATMP)]. The difference between gas and vapour is the boiling
temperature of material. If a material is solid at room temperature, then its
gaseous form is called vapour, whereas, a material that is in gaseous form at
room temperature is called a gas. Room temperature is generally considered to
be 27℃.
Evaporation is very straightforward in its definition. You must have taken water to its boiling point every day while making tea (or in my case, watched my family do it; I don't really care for tea). We
take a material and we heat it up till we reach its boiling point. The solid
turns into vapour and goes up. The substrate is placed on the roof to capture
the vapour on its surface. It adsorbs (not to be confused with absorb)
the vapour and condenses it back into solid due to the temperature difference. Adsorption is a surface phenomenon which deals with particles on the surface of a base material. Most
techniques use a crucible; it is a small container that is very resistant to
heat and is inert to chemical reactions. It is usually made of alumina,
graphite, or boron nitride.
Resistive and inductive heating are the most straight forward. The crucible is heated up using the electric current passing through the wires. Resistive uses a very high direct current and inductive heating uses alternating current to generate eddy currents on the surface of crucible. The substrate rotates on the holder to ensure uniformity of the film, every point on surface is of equal height. The evaporation of the material is caused due to the increase of crucible temperature. This introduces a problem of contamination by the crucible. In any other field it would not be that big of a deal, but the advances in this field require a method to limit impurities during deposition. This method is great for depositing metals for contacts so some tweaks had to be made in heating the depositing material.
All of these evaporation processes take place in a high vacuum. This is done to increase free path. Free path means the length a particle travels without a collision. A large amount of gas particles would cause scattering of vapour. These particles would not get deposited on the substrate and instead stick to the walls of the chamber. This precaution allows the use of an electron beam for heating. An electron beam is generated by heating up a filament (Tungsten) to a high temperature and providing a positive voltage on the crucible.
Now, a sane person like you or me would probably shoot the electron beam down from somewhere on the side onto the material. This would have created a scattering problem due to the beam coming in the way. Some people saw this and decided the best way to reduce scattering and utilize the heating effect is putting the filament on the bottom of the crucible. Well, what are you going to do now? Use a magnetic field to deflect the beam by 270∘ and focus it onto the top of the crucible, where the opening is to hit the material directly. The crucible gets heated up to quite a low temperature when compared to the other evaporation methods. How do people come up with this stuff?
Figure 4: Types of Evaporation Methods (made using Freeform)
Despite being more expensive than the other techniques, it faces the same sort of problem in terms of deposition, that if a material has different elements then the composition of material will change. For example in an aluminium-copper alloy, the boiling point of Al is lower than Cu, this leads to Al reaching the substrate faster than Cu. The stoichiometry, i.e., the ratio of atoms will change leading to a different allow with different properties to form. To combat this, a new method was brought to life which involves using a very high power laser to completely obliterate a section of a target (deposition material) to generate a plume of material which reaches the substrate and condenses. The laser has a temperature of around 6000K, removing any doubt about the composition of material. This method is named as pulsed laser deposition due to the laser being periodically activated. Another name for it is laser ablation. Science fiction movies were correct all along - lasers really do make everything look cooler.
Sputtering is a technique where atoms are knocked out of the target; the target atoms move towards the surface of the substrate and stick to it. An ionized gas is called a plasma. The plasma is generated by using gaseous ions like Ar+ and energizing it so that it gives up the electron. Energy is given through an electric field that is either direct or alternating. The target becomes a cathode (negative voltage electrode) and the substrate is grounded (acts as anode, it is positive with respect to cathode).
Figure 5: Sputtering of atoms using Argon ions (made using Inkscape)In DC, a constant voltage of -2 to -5 kV is provided on target. A large enough voltage causes some Argon atoms to separate from their electrons to create positively charged ions. Do note that the ratio of ions to atoms is around 0.001 to 0.0001%! In AC, the voltage has a frequency of 13.56 MHz. This frequency is in the radio frequency range, hence the name. Electrons and ions oscillate with AC voltage and end up causing ionizing collisions. This allows AC excitation to happen at a lower voltage (around -1kV).
These are the two main plasma methods, other methods are spin-offs of this principle. Methods like magnetron, bias sputtering, and others utilize one of these two and use different methods to increase ionization efficiency. Yet, there is one technique that is different. You must be thinking that since we can generate an electron beam; why not flip the script and instead throw out ions in the form of a focused beam. Well dear reader, you will be absolutely correct in that assumption as ion beam sputtering is very popular too. It is the best method for smooth and clean films with enhanced adhesion (fancy way of saying it sticks together well). The cost of such a good technique is borne by expensive capital equipment and very low throughput and yield compared to plasma induced sputtering.
Moving onto the final topic for this article (whew, thank god its ending; is what you must be thinking, right?). Chemical vapour deposition includes sending reactants in their gaseous forms to the surface of heated wafers. The heat on the wafer will provide the energy needed to trigger a reaction between the gaseous species and subsequent deposition of the product on the wafer. The names of the techniques correspond with either the precursors used, pressure conditions of the reaction chamber, or any phenomena that assists in reaction formation.
Figure 8: Diagram of various phenomena during CVD (made using Inkscape)Reactions like pyrolysis (thermal decomposition), oxidation (oxygen gas usage), reduction (hydrogen gas usage), and compound formation (hydrides usage). This technique needs raw materials that are in the form of reactants. Meaning - it needs to be volatile and stable at room temperature (seems paradoxical doesn't it); has to have a reaction temperature lower than the melting point of substrate; after reaction the by-products must be easily removable; must have low toxicity. Oh and in case you have forgotten, we are working in nanoscale, a small impurity can send our device into a tailspin. The reactants must also be very highly pure. These are just the bare minimum parameters required. Introducing these many constraints just in the raw material phase has allowed only a few companies to gain a foothold in this domain.
CVD has two in-built limiters on it. One is gas transport limited and the other is reaction rate limited. Gas transport depends on the mechanical properties of gas (viscosity, diffusivity, and density), transport pipe dimensions, and boundary layer thickness. Reaction rate depends on temperature, surface properties, reactant concentration and composition. Only one limiter may dominate at a single time. These are topics that will make us dive deep into the world of growth kinetics in CVD which is beyond the scope of this article.
I hope that this very lengthy and hopefully simplified approach to deposition processes has sparked some form of interest for these deposition processes in you. These processes form the basis of our day-to-day life. In my opinion, everyone should have basic knowledge of these to know why sometimes manufacturing delays occur. Highly specialized manufacturing requires highly specialized raw material. A single hitch in any procurement or fabrication process can tumble the system down.
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