19 Etching Techniques

Dr. Ayushi Paliwal and Dr. Monika Tomar

1. Introduction

Etching is a process used extensively in microfabrication process. Etching process includes all the techniques which aim at uniform removal of material from the surface of a wafer. It is mostly used in parallel with the processing process and is used to cut open the protective film on the surface. These opening are further explored to develop semiconductor region by means of diffusion and implantation. Etching can also be used to delineate metallised layers. Chemical matching of a semiconductor is incorporated into fabrication process by the process of etching.

Microelectromechanical MEMS devices have very small size, mass and volume with low cost of production and low power consumption. They can be easily integrated into systems and can be batch fabricated into large arrays. They find applications in various aspects of day to day life and also at industrial scale, such as in shock and tilt sensing, gas shutoff, STM’s and many more. Bulk micromachining as well as surface micromachining is used to develop microstructures on the surface of these MEMS devices. Bulk micromachining mostly utilises wet etching techniques for the purpose of fabricating microstructures, where as surface micromachining uses dry etching.

2. Bulk and Surface Micromachining

As the name signifies bulk etching focuses on removal of bulk material from a substrate. In bulk micromachining silicon substrate is selectively etched, creating cavities, hence forming microstructures. A silicon dioxide (SiO2) layer is deposited first over silicon to protect certain areas from etching. This is done mostly by thermal evaporation technique. Then windows are opened on this SiO2 layer to pave the path for etching. As the name suggests wet etching involves the use of aqueous chemicals which then etch the wafer surface away. Chemicals like KOH, TMAH, hydrazine-water are used for the purpose of wet etching. These etchants are isotropic in nature, that is, silicon is etched equally in all directions. Whereas when this etching takes place uniformly in only one direction (vertical direction) it is termed as anisotropic etching. This etching takes place along a certain plain of the substrate and etch rate is different along different planes.

On the other hand surface micromachining involves the deposition of a pattern layer and then sacrificing the same layer eventually. Micromechanical systems are created by deposition and etching of these thin structural films. Using surface micromachining simple as well as complex structures can be fabricated on the substrate. When silicon is used as a substrate then mostly silicon oxide (SiO2) is used as the sacrificial layer while polysilicon is used as the material for microstructures. Very small microstructures which can be fabricated using surface micromachining, this is a big advantage of surface micromachining. Also it allows integrating of microstructures with microelectronics on the same chip along with low cost and mass production advantages.

3. Some Points about Wet Etching

Wet etching can be done by various methods and is dependent on parameters of thin film, porosity of the film, process of film formation or the various processes it has been subjected to before etching. While etching a few things should be kept in mind, like – Chemicals which are used to dissolve materials in bulk form are commonly used for thin film etching also, but the film material will suffer a faster rate of etching as compared to bulk materials. In order to avoid over etching the chemicals must be used in dilute form. Diluted chemicals reduce the etch rate to a desirable state.

Thin films which are grown by e-beam evaporation or have been subjected to e-beam in any of the previous stages or the ion-implanted thin films are characterised as irradiated films. We should remember that irradiated films will etch rapidly in comparison to non-irradiated film. Though using a negative resist can help as these resists get toughen due to polymerisation induced by irradiation.

Substrate temperature, change in temperature of surroundings and rate and manner of film deposition influences the amount of stress that develops on the thin film. This stress alters the rate of etching and it increases for high stress.

Rapid etching can also be seen in films that have poor microstructures which can be because of loose structures or high porosity of the film.

If the preparation techniques of a compound lead to departure from its stoichiometry then films of such compounds will have a high etch rate.

Films which have a single phase will show a higher etch rate in comparison to multi phase films as preferential etching of one component rapidly increases the porosity of film and hence lead to higher etch rate.

4. Plasma Etching:

Physical bombardment of ions or atoms on a substrate leads to removal of material. This etching is assisted by a gas discharge. It imparts energy to ions. Projectiles of these ions impinge on the substrate with high velocity and transfer their momentum to substrate atoms via elastic interactions. Plasma assisted etching has been rapidly used in recent years. Despite high cost of initial instalment this technique has flourished due to the following reasons:

• Etch directionality can be achieved without dependence on crystal orientation.
• The patterns formed by the process of lithography can be transferred faithfully.
• It is a comparatively cleaned method and compatible with vacuum processing techniques.

The plasma based dry etching techniques are able to achieve etch directionality due to the occurrence of a film forming process on the sidewalls of the features which are undergoing etching. These films are given the name of passivation layers. The passivation layers shields the sidewalls and stops the etchant species from attacking the sidewalls. This prevents mask undercutting. It also comprehensively prevents any damage to sidewalls from off-normal ion bombardment. This reduces the bowing effect.

But how are these layers generated? Energy and chemistry of glow discharge, the type of the substrate used, temperature of substrate and the material used for the purpose of masking all play an important role in formation of these layers but a mechanistic details for their formation are still up for discussion.

A “right” combination of these factors leads to deposition of this passivation layer on the features being shaped. The thickness of these layers can vary from 0.1 µm to a few microns.

4. Reactive Ion Etching

Reactive Ion Etching belongs to the class of dry etching techniques and uses chemically reactive plasma for the purpose etching. It is accomplished by replacing the neutral species, used for the purpose of etching in processes like sputtering technique, by chemical species. Ionized and neutral species are created by the interaction of these chemicals with plasma. These highly energetic species are used for the purpose of etching.

Figure (): A planar reactor.

A planar reactor (shown in the figure ()) configuration is used for this technique. The substrate is immersed in plasma and the gas flow is kept normal to substrate position. This set up allows the energetic species with small lifetime to be used effectively. This set up also helps in achieving high anisotropic etching as energetic species are highly directional. This directionality is due to the fact that the substrate is kept normal to the R.F. field.

This anisotropic nature of reactive ion etching can be enhanced further and is controlled by various factors. These factors can be altered or varied to yield better results, like

• We can use chemicals which will give products with very large number of ionised components.
• An increase in the voltage drop across the cathode sheath. This significantly increases the velocity of species responsible for etching.
• The system can be operated at a reduced value of pressure. By doing so we can minimise the collision in the sheath and improve the directionality.

Etch rate is also dependent on the area of the wafer or rather the area of the surface that is to be etched. As this area decreases the rate increases and hence the etch time has to be reduced. This loading effect can be calculated by making some simple assumptions. Let us assume that plasma produces a single reactive species with G as the generation rate per unit volume per unit time and recombination rate is given by . Also for a given wafer temperature, k is the constant for the linear reaction rate where a single product is formed. Now, if N is the concentration of reactant species, A is the area and j is the flux density, then the flux density of species, then flux will be

A change in etch rate may lead to complications in patterning fine lines. For      ⁄   > 1 as the film clears the etch rate increases and leads to undercutting at accelerated rates. This can lead to loss of anisotropy. Hence, it is advised to load the reactor very lightly. This can be done by increasing the inter electrode spacing. By doing so we can increase the plasma volume and reduce the surface area, but the throughput decreases. So it is rather better to perform single wafer processing.

Two processes, that is physical and chemical process, happen simultaneously in reactive ion etching. Both of them are responsible for the completion of etching process. The physical process is based on sputter removal technique. This ion bombardment process the rate of etching is dependent on the sputtering yield. It shows poor selectivity. The chemical process is highly isotropic in nature. The energetic chemical species that participate in etching are selective and specific towards the material it can etch. Keeping a balance between these two processes is essential for successful etching. This balance can be achieved by controlling the parameters such as r.f. voltage, substrate self bias, chemical species etc.

As mentioned earlier, the formation of a “passivation” layer during RIE is very advantageous. The ion beam which is highly directional removes these layers of native oxides and polymeric products normal to the substrate surface. Also due to directionality of this beam the sidewalls are not attacked and hence remain unetched. Due to this undercutting of grooves is prevented. The ion beam also increases desorption of the etching products. These etching products are desorbed and removed by using a pumping system.

End point detection schemes are a major part of reactive ion etching. Such schemes are needed because it is difficult to predict the extent of etching.

5. Dry Reactive Ion Etching

Etching refers to the chemical removal of layers from the surface of a wafer (substrate) to create cavities which can be filled with desired materials hence developing the micro devices. One of the methods to achieve etching is dry reactive ion etching. Dry RIE makes use of the chemically reactive plasma to remove materials deposited on the substrate. This gaseous form of etching is mainly used for obtaining highly anistropic etching, a disadvantage of the wet etching process. A typical (parallel plate) RIE system is made of a cylindrical vacuum chamber, with an electrically isolated wafer platter that is situated in the bottom of the chamber. Gas is made to enter through small inlets present at the top of the chamber, and exit to the vacuum pump system through the bottom. The types and amount of gas used depends upon the etching process for e.g. Sulfur hexafluoride is commonly used for etching silicon. Gas pressure ranges between a few mili torr to a few hundred mili torr by adjusting gas flow rates.

Plasma is formed in the chamber by applying a strong RF (radio frequency) electromagnetic field to the wafer platter. The oscillating electric field ionizes the gas molecules by stripping them of electrons, creating plasma. The electrons so formed are electrically accelerated up and down in the chamber, striking both the upper wall of the chamber and the wafer platter. The ions, however, being heavy move relatively little in response to the RF applied. The electrons, striking the walls are simply grounded but those deposited on the wafer platter cause charge build up on the electrically isolated wafer. This negative voltage attracts the positively charged ions which accelerate towards the surface. The ions collide with the surface to be etched, both chemically reacting as well as removing the materials from the surface by shear momentum transfer. As most of the ions reach the surface vertically, very anisotropic etch profiles can be produced using this method unlike the typically isotropic profiles from wet chemical etching. This high directionality can also be achieved by varying parameters, such as pressure, gas flows, and RF power according to our requirements.

6. Bosch Etching

Even deeper trenches / penetrations with higher aspect ratios on the surface than those obtained from the above method can be achieved by using deep RIE also known as Bosch etching. This process can fabricate almost vertical (90~ ) but slightly tapered walls. The process is similar to dry RIE except alternates between a polymer like C4F8 and a normal plasma gas etching as achieved through dry RIE. The polymer forms a passivated layer like that of teflon on the surface preventing it from etching. However during the collision of directional ions with surface, the ions scrape off the polymer from the surface but not from the sides exposing the surface to etching. Apart from manufacturing MEMS devices, this process is also extravagantly used to excavate trenches for high-density capacitors for DRAM and also creating through silicon via’s in advanced 3D wafer level packaging technology.

7. Summary

1. Bulk and surface micromachining techniques of etching were discussed
2. Two types of etching i.e. wet and plasma etching
3. Detailed description of reactive ion etching and dry reactive ion etching