Etch Processes for Microsystems Fabrication - Part II

0:00 welcome to etge processes for Microsystems fabrication part two presented by scme the Southwest Center for Microsystems education for information about scme please visit our website at sc- nm.org in part one of edge processes for Microsystems fabrication we talked about the different types of edge processes surface and bulk wet and dry we went into detail on the wet Edge processes in this presentation we will discuss the dry Edge processes as well as the differences between chemical and physical etching in dry etge process a single wafer or a batch of Wafers is

0:46 placed in a sealed chamber and then exposed to a gasas etchant which is suspended in an RF a radio frequency energized plasma collisions between the gas molecules and energized electrons create a soup made up of electrons ions and radicals each of these particles have a definitive purpose as we'll discuss in this presentation by Design dry Edge methods provide more control over the factors which influence etched results dry etge processes provide directional control of the etch and isotropic profiles and greater control over the process parameters such as pressure temperature gas flow and Power this control allows very detailed and

1:33 specific etching to be performed because gases are the primary etent and are housed in a sailed chamber human exposure to dangerous chemicals is limited however the equipment is very expensive to purchase and just as expensive sometimes to maintain dry etch is normally used to remove selected areas from the surface layer rather than bulk material of a sub Strate or a sacrificial layer dry Edge can be a chemical Edge a physical Edge or it can be a combination of both physical etching is similar to sand blasting where High Velocity sand is used to chip away as at an exposed surface in sand blasting decorative

2:18 glass a picture such as the one shown here is formed on a sheet of glass using a strong protective material this material stands up to being blasted with high velocity sand particles the unprotected areas of the glass are etched by the sand which removes particles of glass from the surface when the desired depth is reach the blasting stops the mask is removed and the design is exposed physical Edge is very similar to this instead of sand positive ions created by collisions in a plasma bombard the surface of a wafer causing molecules to sputter off the surface the hard mask on the wafer surface protects the areas that we don't want etched in this image the hard mask is

3:05 the photo resist notice how the photo resist protects areas of the Silicon nitride while exposing other areas the Silicon nitrite is etched wherever there is an opening in the photo resist in this image the photo resist has been removed to show the patterned silicon nitride layer if you watched part one of edge processes you'll know that the Silicon nitride pattern identifies the area silicon that are to be etched later on in a wet chemical process like sand blasting physical dry Edge is an entirely physical process with no chemical reaction occurring so let's see how this works Wafers are placed on a negatively grounded holder in a vacuum chamber a

3:51 gas is introduced into the low pressure chamber which is usually less than 50 milor or down into the high vacuum region RF power is applied to the chamber and a plasma is struck in the chamber the gas molecules pass through the plasma and collide with high energy electrons the energy is transferred from the electrons to the gas edent molecules these collisions result in high energy State positive ions as well as free radicals these positive ions are attracted to the negatively charged cathode where the wafer set the ions accelerate as they move toward the cathode when the ions hit the

4:37 wafer surface layer molecules are sputtered off the surface this process continues until the pattern is etched through the surface layer exposing the underlying layer as I mentioned before the plasma consists of electrons ions and free radicals the ions are used for physical etching while the free radicals are used for dry chemical etching free radicals are atoms or molecules that have at least one unpaired electron and are therefore unstable and highly reactive it is the free radicals that react with the surface creating volatile particles that are released from the surface and then removed from the chamber this process is called a dry chemical etch

5:22 chemical etching requires a gas edent that will chemically react with the wafer surface layer free radicals of this ENT are generated in the plasma and once produced they travel towards the wafer where they are absorbed by the material on the wafer surface a chemical reaction occurs between the material to be etched and the radicals byproducts of this reaction in part one of this series the dry plasma etch process uses two primary parameters to control the

6:08 type of etch that takes place whether the chch is chemical or physical the parameters are process pressure and RF power level this chart shows how these two parameters affect the type of edch chemical etching at the W for surface can be increased by increasing the process pressure or decreasing the r F power level are both of course for any chemical reaction to occur one of the plasma gases or etchant must be reactive with the material to be edged with physical Edge the amount of physical Edge increases with the decrease in chamber pressure and or an increase in RF Power by carefully controlling these two parameters you could have all

6:54 physical Edge All chemical Edge are a combination of physical and chemical Edge this control allows for specific shapes and sizes to be fabricated regardless of the technique used wet Edge or dry edch physical or chemical The Edge quality is influenced by several factors a few important factors for Microsystems include chch rate directional control and seel activity H rate is the rate at which the material is removed from the wafer in other words 50 angstroms per minut minute or five angstroms per minute the faster the etch rate the faster the material is removed for instance 50 angstroms per minute etches faster than

7:39 five angstroms per minute directional control results in achieving the desired shape through the type of edge profile isotropic and isotropic are a combination of both as you've seen different types of ents results in different profiles selectivity is the property of the ENT which permits it to selectively etch specific materials at a faster etch rate than other materials on the wafer selectivity is the ratio of the etch rate of the material to be etched to the etch rate of the material not to be edged for example if we refer back to the backside Edge for the mim's pressure sensor which we talked about in part one

8:26 of the series we need to use a mask that can withstand the wet pottassium hydroxide Edge which can last up to 4 hours this process has to etge all the way through the Silicon therefore we want the masking layer to be basically non-reactive with k or potassium hydroxide or to have a very very low hch rate relative to the hch rate of silicon so to make sure that you understand selectivity I have a question for you you saw this picture previous previously and I mentioned that it was an etched silicon nitride mask that would be used in a wet chemical etch to etch exposed silicon the wet etge process uses potassium hydroxide or Koh

9:13 to etch the Silicon this particular etch process can take up to 4 hours therefore we do not want the Silicon nitride layer to be removed or etched away before the Silicon is completely etched through so when thinking about the term selectivity and comparing the etch rate of silicon to Silicon nitrate what would our selectivity ratio ideally be very low or very high you can pause this video and think about the question for a moment if you said that we want a high selectivity you are correct a high selectivity means that with a correct timed edch the material to be etched which in this case is silicon can be

9:58 completely removed before the masking layer of silicon nitrite is even affected now let's take a look at Ed rate in the scme H learning module we have an activity that is supported by a kit called the rainbow wafer one part of this activity is to estimate at least six oxide thicknesses on a wafer graph the relationship of the amount of oxide removed or the oxide thickness versus time and calculate the etch rate this chart is an example of the relationship between the amount of oxide removed and time the x-axis is time and the y- AIS is oxide thickness removed as you can see it shows that the longer the edge the more oxide is removed using the

10:45 information on this chart we can estimate the etch rate in angstroms per minute by determining the slope or rise over run of the line the line you see is the line of best fit for the data points that were gathered take a moment and determine the edge rate of this process again the etge rate is basically the slope of the line if you said 300 angstroms per minute you are correct as you can see from the graph at 6 minutes 2,000 angstroms have been removed and at 4 minutes approximately 1,400 angstroms have been removed so therefore that is a difference of 600 angstroms the time has

11:31 a difference of 2 minutes so 600 angstroms removed over a 2-minute period leaves a etch rate of 300 angstroms per minute another type of dry Edge is reactive ion etching r r e Rie uses midlevel RF power and mid-range pressure to combine both physical and chemical etching in one process the positive I ions from the plasma bombard the waer surface at the same time that the radicals are absorbed to the surface you remember that we can control the Edge by adjusting the process pressure and RF power so that we can get both type of etching physical and

12:16 chemical occurring at the same time this process provides High selectivity ratios it also produces anisotropic profiles on features less than three microns its ability to capitalize on the advantages of both physical and chemical etching makes Rie an invaluable tool in the manufacturing of microsystem components a special subclass of R is deep Rie or D deep reactive ion etching is used to etch deep cavities or trenches in substrates with relatively High aspect ratios these cavities can be hundreds of micrometers deep while only a few

13:02 micrometers wide aspect ratios up to 50 to1 can be achieved with diie as new Dr methods are developed it is very likely that these aspect ratios will get even higher this scanning electron microscope image or Sim shows a series of cavities edged using a Dr process notice that the deeper cavities have the wider openings this Sim shows that one of the ways to control cavity depth is by controlling line width in addition to creating cavities Dr E can be used to create tall thin objects or components for Microsystems devices these objects can later be

13:49 released through other etch methods such as wet etching the scme shows a leaf spring created by Dr notice how the spring is attached to a block at one end the front and to a slider at the opposite end the entire spring has been released from the substrate allowing it to expand and contract so as you can see from this presentation dry Edge processes have enabled the construction of very unique components with low to high aspect ratios that can be used throughout microsystem devices for more detailed information on these ET processes be sure to download the etch overview learning module from

14:35 the scme website under educational materials both part one and part two of etge processes are available through YouTube and are online at the scme website thank you for viewing this presentation produced by the Southwest Center for Microsystems education