Plasma Etch and the MATEC Plasma Etcher Simulation Top Down Method Plasma Etch and the MATEC Plasma Etcher Simulation

Plasma (dry) Etch Advantages No need for immersion More precise control of etch feature sizes More rapid than chemical etch without plasma assist Plasma can create reactive species in gases Plasma can be used to perform physical etch as the chemical etch proceeds Dry etch methods have the advantage of being able to work on smaller feature sizes. A simple chemical vapor etch typically proceeds at a given rate, but may be isotropic just as wet etch methods are. When a plasma is used, either remotely or within the chamber, the gasses can dissociate into more reactive radicals that combine with the surface to be etched to create volatile species that can be drawn away. The plasma can also be used to perform a sputter etch process at the same time if it is provided in the chamber and help to form the anistropic profiles that are necessary for some features.

Examples of species to be etched Aluminum Silicon Silicon Dioxide Silicon Nitride Tungsten Polysilicon There are several different materials used in top-down manufacturing. The materials above are all common to semiconductor manufacturing, but are found elsewhere in MEMS, filters, and other devices. Aluminum and tungsten are common metals in fabrication. Pure silicon is what the wafer is originally made of. Silicon dioxide is an insulator and an important part of field effect transistor construction. Silicon nitride is used as an insulating or protective layer and is very hard. Polysilicon is doped silicon that is conductive to a degree dependent upon the concentration of impurities added.

Etch chemistry/method depends on Surface deposition to be etched What is immediately below the deposition High selectivity will minimize damage to the layer below Low selectivity can result in damage to the layer below Etch Profile Required Isotropic Etch (equal etch in both planes) Anisotropic Etch (for tall, vertical sidewalls) The etch chemistry and the amount of plasma used in the process must be matched to the layer to be removed. If it is not, the chemical process may leave residues behind that are difficult to remove or the process may take longer and possibly subject the layers below to damage. High selectivity in the chemistry is desirable so that the etching will have minimal impact on the layer immediately below the species to be etched. The composition of the etch gases and the plasma used can define the etch profile. Tall sidewalls require anisotropic etch, and the addition of other species besides the main gas can help to provide this.

Dry Etch Materials and Etchants Material to be Etched Common Etchant(s) Aluminum BCl3 & Cl2(N2) Oxide Ar(N2) Silicon CF4, SF6/Cl Silicon Dioxide CF4 Silicon Nitride CF4 & SF6 (N2) This table shows some typical dry etch chemistries for different materials. It is a matter of not only the layer to be etched, but the layer below as well that determines the species used. Source: MATEC Module 47 (M047NR01)

Etch Mechanisms Damaging Mechanism Ion bombardment breaks chemical bonds between the atoms on the surface Increased RF energy and plasma etch Dangling bonds are more attacked by free radicals Surface atoms bond with radicals to form volatiles that are swept away Anisotropic profile due to plasma etch Normally used with oxides and nitride One mechanism used commonly with hard dielectric layers such as silicon nitrides and oxides is the so-called damaging mechanism. The etch process is more physical as higher plasma energy is used to break up the surface so that it can bond with the gaseous species.

Etch Mechanisms Damaging Mechanism (2) Plasma is also used to dissociate gases used plasma CF4 → CF3 + F F + Si3N 4 → SiF4 + N (SF6 may also be used to provide fluorine) Silicon tetrafluoride is volatile (from Xaio (2001) Introduction to Semiconductor Manufacturing Technology, Prentice-Hall, NJ) The plasma in this case is not only providing ion bombardment to break the surface bonds, but is also forcing the dissociation of CF4 which frees up fluorine radicals that combine to form silicon tetrafluoride with the dangling surface bonds, which is volatile and carried away from the surface.

Etch Mechanisms Blocking Mechanism Additional gaseous species added to process provides a passivation layer on the sidewall Creates a boundary on sidewall to minimize etch to provide anisotropic etch profile Commonly used in metal etch Chlorine (Cl2) is commonly used for metal etch Plasma breaks up Cl2 into Cl radicals that react with metals such as aluminum to form volatile by-products. The blocking mechanism uses a second gaseous species that forms a layer on the sidewalls. Since plasma etch tends to be straight line, the vertical etch continues.

Etch Mechanisms Blocking Mechanism (2) Additional gaseous species added to process provides a passivation layer on the sidewall BCl3 is commonly used for sidewall passivation N2 and CF4 can also be used to improve passivation More chemical etch and less plasma (physical) etch The “recipe” includes several species The blocking mechanism uses lower plasma energy and more chemical action, with the additional species aiding in keeping the process anisotropic.

Plasma Etch Processes Generally include several steps Preparation of chamber Temperature/Low Pressure Cleaning (Nitrogen or other gas) Concentration of vapors Main Etch Process Removal of volatile residues Secondary Etch Process Clearing Chamber/Return to initial conditions Rather than one single operation, the plasma etch “recipe” as it is called usually has several steps. In some cases, the chamber and wafer may be conditioned by introduction of dry nitrogen or other gases. The main etch step, where most of the etch occurs, is midway in the entire process. Some processes use secondary etch processes and may include cleaning or passivation steps in between.

Example Etch Recipe Note that the layer to be etched is silicon nitride, an insulator and very hard layer. The gas to be used is CF4. Argon has been included to support the ion bombardment needed for the damaging mechanism. In step 3, a very high amount of RF power is used for the plasma etch. Steps 4 and 5 show the introduction of CF4, with the plasma activation occuring in step 5. Step 6 is used to return the chamber to pre-etch conditions so that the wafer can be removed.

Click once for each question. Practice Questions Click once for each question. 1. What are the advantages of plasma aided etch? Anisotropic profiles, higher etch rates, and sidewall passivation 2. What must be considered in choosing dry etch chemical agents? Plasma aided etch uses the physical action of ion bombardment with the chemical etch of the gaseous species to improve etch rates. The polymerization of the sidewalls in some processes can prevent isotropic etch profiles. When selecting the etch chemistry, consideration of the effectiveness of the etch process on the surface layer is the first concern, but it is important not to damage the layer below, so the chemistry must be selective enough to prevent this from occurring. The damaging mechanism uses plasma to create dangling bonds on the surface that will be susceptible to the free radicals generated in the gaseous species from the plasma. The blocking mechanism uses other chemicals to perform depositions on the sidewalls. The depositions do not form on the bottom of the etch due to the straight line nature of the ion bombardment that comes from plasma etch. Selectivity, the surface layer and the layer below 3. What are the two anisotropic mechanisms? Damaging and Blocking

MATEC Etch Simulator Guide (2) Process the lot Enter the concentration, power level, and other parameters for each processing step The process is real time, so start and stop as indicated in the recipe When complete, the simulator will verify if the process was successful If unsuccessful, the simulator will show what needs to be changed Print a copy and re-process until successful! Start the etch process, entering the parameters for each step. The simulator will process the lot one step at a time, and you can control the duration of each step with the start and stop buttons on-screen. At the conclusion of the 6th step, the simulator will indicate what needs to be changed, if anything, on the screen. Recipes are just that and sometimes require modifications for a process. Metrology using profilometers and other devices can indicate the depth and shape of the etched feature. Modifications to obtain successful results are made as needed.

MATEC Dry Etch Simulator Guide Select a “Lot Traveler” A process document that indicates what processes a wafer undergoes and who Start the MATEC Dry Etch training program Enter your name and the information on the substrate to be etched, underlying layer, and thickness. Refer to MATEC tech manual Select a recipe based on the species Copy down etch parameters A lot traveler identifies the in-process wafers. In the notes provided on the traveler, the substrate to be etched is identified as is its thickness and underlying layer. Start the etch simulator program and enter this information. Refer also to the MATEC technical manual that explains the steps in the process. Select a recipe from the recipe screen and copy the parameters on the Etch Recipe Notes page. Use the information in this presentation to guide you to the selection of a recipe.