Published on September 16th, 2015 | by ryankapsar0
Working For A Chipmaker: Part III
In this series we take a look at what it’s like to work in the semiconductor industry. These articles reflect the opinion and experiences of the author and do not reflect the opinions of KBMOD. See Part I and Part II
In the last two parts of this series I’ve talked about Photo-lithography, which lays out each layer and the etching groups which make that layer permanent on the wafer. Unless the wafer is brand new, where these steps happen on the bare silicon, some other material has been deposited onto the wafer creating features like Gates, Capacitors, and Resistors. There are three groups that deposit material onto the wafer and each use different methods for doing so – CVD, PVD/Sputter/Metals, and Diffusion. We’ll start with CVD and move on from there.
Chemical Vapor Deposition, CVD, is a process not unlike Etch – depending on the specifics of the tool, where a mixtures of gases are flowed over the wafer and a plasma is struck; however, that’s where the similarities end. Rather than etching into the material to create trenches, this plasma and gas combination quickly deposits a thin layer of material onto the wafer. These materials are typically variations of “glass”, such as silicon nitride, and act as insulators between the various layers of the chip.
So that’s technically what CVD does; however, that’s not very enlightening is it? If we look at the image to the left we can see that a (c) gas flows into the CVD chamber (it’s a single wafer tool – meaning it processes one wafer at a time). This is a mixture of various gases that then react in the plasma. This reaction is what gives us materials like the silicon nitride. Once the reaction is complete the new material is pushed down onto the wafer by the incoming gases where it binds to the wafer. This produces a “thin” layer, which is the second thickest material on the wafer and can range from a few nanometers thick to a micron thick depending on the use of that material. For example, when making the capacitors on a wafer of memory, CVD will deposit material that is the full height of the capacitor so the shape of the capacitor can be etched out. In this case all of that material will eventually be removed by other etching processes as it was intended only to be etched away. A full micron of material deposited only to be removed!
Physical Vapor Deposition (PVD) Sputtering is very closely related to CVD; however, instead of using gases to form the new layer of material through a reaction, material is etched from a platter and then falls onto the wafer. As you can see in the picture to the right, positive Argon gas flows and strikes a target, which is a massive platter about 3 feet in diameter. These materials range from Aluminum to Copper to Gold. While many of the CVD processes result in an insulation material, the purpose of Sputtering is to lay down a layer of conductive material. The type of material changes depending on the specific purpose of that layer. For example, a Tungsten layer might be used at the bit line, while a word line or a metal layer might be Copper. Bit lines contain data only for one single capacitor, so only a single one or zero, while a word line is multiple bits together and metal layers are buses between the internal processing of the chip and the exterior world. Each material has different resistivities which changes the amount of electrons into heat energy.
While I was working at Samsung we transitioned from Aluminum to Copper because Copper had the conductive properties required for sub 40 nm flash drives; however, this required massive changes to the structure of the Fab because of copper properties. While in an acidic solution, Copper is extremely mobile and while in a Wethood (see my last article where I discuss Wethoods) copper particles will move from the acid bath onto other wafers. This can be problematic whenever the wafer moves into a diffusion furnace (See below) and turns into gigantic particles, which are chip killers.
The final group that deposits material onto a wafer is called Diffusion. This is because, unlike CVD or Sputter, gas flows into a large furnace and diffuses over all the wafers. This material reacts with the surface and “grows” a new material. These processes are slow and high temperature, roughly 600 Celsius. Because of this, anywhere between 100-150 wafers are processed at the same time (see image to the left). Diffusion, unlike CVD, grows very, very thin films. In fact, some Diffusion layers are called Atomic Layer Depositions and have a thickness measured in Angstroms (1 A = 10^-10 meters, a cm is 10^-3 m), where variation in the thickness of layer could be the result of one additional molecule of material.
Diffusion processes are used to create the transistors, capacitors, and resistors. They need to be thin so that electrons can eventually move through the layer. but still thick enough to prevent electron leakage, which could result in a 0 bit being read as a 1 bit in a memory chip or processor.
All of these groups are vitally important to the quality of any chip. If Photo and Etch are important for transferring the design of a layer to the semiconductor, these groups are important to make sure that the layer functions properly in terms of semiconductor physics and chemistry. Poorly deposited CVD means that the layers aren’t properly insulated and there can be internal arcing between bit lines. Issues with Sputtering can prevent high desired speeds from being achieve, while Diffusion issues means capacitors or transistors don’t perform as expected. This can be more devastating than issues with Photo because the issue might not because until the material is tested at the end of line. In some cases that means a bad tool could have impacted 30 days or more worth of material.
For the most part, all three groups have very similar needs; Chemists and Chemical Engineers to develop processes while using Mechanical Engineers and Electrical Engineers for Equipment Engineering roles. One key difference for these groups compared to Photo and Etch is that a deep understanding of Semiconductor physics will be extremely helpful in analyzing root causes of yield problems. This means that having some Electrical Engineers that focused on Semiconductor theory will be important to work with the Chemical Engineers.
In my next article, I’ll discuss the two odd ball processing groups: Ion Implant and Chemical Mechanical Planar.