Wet etch is the process by which patterns are formed in the substrate material.

It can be defined as "the process of solid material removal from a substrate by means of a physical or chemical reaction".

Wet etching involves the immersion of the product wafers in a chemical bath.

There is a danger of over etch if the material is left in the chemical for to long – to much material is etched away. All the film could be etched away if left in the bath for to long. Variables used to describe the critical features of an etch process are isotropy, etch rate, selectivity, uniformity and critical dimensions.

The more uniform the process, the more consistent the output and the better the process. The goals of any etch process are:

Reproduce the mask features with fidelity, control the etch feature side-walls, control the degree of undercut of the mask.

In addition any useful process must also have the following characteristics; minimal mask etching, minimal substrate etching, minimal damage, high throughput, good uniformity, little contamination, easily removable masking layer post etch.

As stated previously etching can be categorised as either wet etching or plasma etching. The major application areas for etch processes are as follows.

Etching to remove thin films from a substrate: Rework, cleaning and decontamination, thin oxide removal, backside film stripping.

Etching for analysis

Inspection (destructive)

Defect analysis for yield and reliability problem diagnosis.

Etching for pattern transfer.

Wet etching is the simpler of the two processes. The simplest wet etch process involves immersing the substrate in a bath of chemical substance containing etchants which react with the film and remove it. The process sequence is as follows:

1] Etchants approach film substrate.

2] Etchants absorb onto film surface.

3] Etchants and film react to form etch product.

4] Etch product moves away into solution, etc.

The complete process sequence is wet etch, rinse and then dry. Wet etching is carried out at a wet bench. At the etch endpoint (end of etch) the wafer substrate or batch is transferred from the immersion bath into a fast dump rinser which removes the remaining chemicals by continually flushing the batch with de-ionised water. The dump rinser is basically a tank with a trap door, which sprays de-ionised water over the batch until the tank fills to a pre-set level. The trap door then opens and the water is dumped. Spraying, filling and dumping continues for a set number of cycles until the chemicals have been completely removed.

The batch must then be thoroughly dried in a controlled manner in order to avoid streaks or other defects remaining on the surface. This is achieved using a spin dryer which heats the batch; blows dry nitrogen over the surface and simultaneously spins the batch at high speed.

At the end of this stage the batch is ready to proceed to the next stage which would be mask removal.

The technology of wet etch equipment has not changed dramatically since its initial implementation, with the majority of equipment presently used falling into the middle of the following three types:

Immersion, dump rinse, spin dry

Much of the drive towards automation has been to improve safety and process control as well as reducing particle contamination. Recent developments have seen the use of spray systems, which ensure fresh reactant always gets to the wafer surface. This aids process control by ensuring that the wafer is always exposed to the same conditions. Immersion tanks become depleted with time as the reactant is used up in the reaction.

Is the material removal rate with time. Wet etch processes generally only remove the material film without damaging other exposed layers such as the substrate and therefore the only important etch rate is the film material etch rate.

Etch rates are generally used for routine process inspection and verification in the industry. Etch rates measured at different points on the wafer can give an estimate of process uniformity.

It is also necessary to consider the isotropy or shapes of the final etch profile. A completely isotropic process has an etch rate which is the same in all directions and at the etch endpoint results in rounded profiles (note perfect quarter circle at mask edges on the wet etch example. Isotropic profiles therefore do not exactly transfer the masking pattern to the underlying layer, as there is some undercut present. The undercut is the masking pattern when the film material has been removed from directly underneath leaving some overhang. Isotropic processes are very chemical in nature and are typically of all wet etch processes and a few plasma processes. For better pattern transfers, it is necessary to have an etch rate which is greater in the vertical direction than in the lateral direction. This is termed anisotropy and directional or vertical profiles are dependent on the particular process involved. Anisotropy profiles are any profiles, which are not completely isotropic.

As all the different materials which are exposed to the etch medium can potentially be etched it is important to know how well tuned the processes to the removal of the one particular layer which it is desired to be removed.

Wet etch processes have very high selectivity’s (they only remove the material that they are supposed to and can have near infinite selectivity and plasma etch processes have much lower selectivity’s as low as 1 or as high as 100)

In order to characterise the output of the equipment it is also normal to measure the critical dimensions CD’s of the patterned features on the wafer surface. Indeed, this is the measure that is relied upon for the characterising the day to day output of plasma etchers in the industry. This generally involves identification of a particular feature on a die and measuring its width using a specialised optical microscope or an automatic CD measurement machine. The same feature is measured on a different die to give an estimate of the process uniformity. The feature to be measured depends on the particular layer to be patterned but is commonly a line, which gives rise to the term linewidth.

Uniformity can be measured and assessed for the following; cross wafer, cross batch, batch to batch (run to run), machine to machine.

Uniformity’s would most commonly be assessed for etch rates and CD’s definitions for uniformity may vary.

In order to have repeatability and control of wet etching, it is necessary to control the following major variables:

Time exposed to the etchants (more time means more etching, which can lead to excessive undercutting and CD problems due to too much material removal).

Temperature of the etchants (must be constant as this directly affects the reaction rate and therefore the etch rate.).

Agitation (affects the etch rate in some processes; agitation is also sometimes necessary to remove gas bubble reaction products from the surface so that etching can continue.).

Concentration or chemical strength (this directly affects the reaction rate and therefore offers more control as the process is less likely to quickly become over-etched. Glacial acetic acid is used in recipes as a thickener.).

6]. Equipment used: -

Commercial systems feature temperature control and chemical filtering to maintain process control and reduce particle problems.

Types, diagram, basic operation.

Diagram1 – acid etching workstation

Diagram 2 – a quick dump rinser

Diagram 3 – a recirculating, filtered, temperature-controlled bath

Process performance is generally assessed by examining CD’s. Film step heights or feature profiles in extreme cases.

Measurement Equipment includes:

Optical microscopes- useful in assessing overetch and underetch. Also useful for other problems such as particulates or resist erosion problems.

Optical shearing microscope –manual CD measurement technique.

Automatic linewidth measurement – this has superseded image shearing as the industry standard for routine CD measurement.

Profilometer – a needle or stylus is moved slowly across an etched step. Changes in the stylus pressure are caused by the step and interpolated as a height.

Scanning electron microscope – this can be used for CD measurement, profile assessment and analysis of process problems such as contamination. It is a very complex and expensive technique and is not often currently used for routine CD measurement. It is the only technique which can be used for profile assessment although this is a destructive analysis.

Particle scanners – necessary to routinely check the equipment cleanliness. A bare silicon wafer or "clean wafer" is processed through the etch equipment and then the number of particles counted.

Plasma etching was originally only used for photoresist stripping but is now widely used for pattern transfer after it was discovered that oxygen plasmas have a very powerful oxidising action and the resists created by ion implantation could be removed more easily than with wet chemicals.

Dry Etching relies on plasmas. A useful plasma definition for the type of plasma used for etching is that it is a hot, ionised, low-pressure gas. If a low-pressure gas has a large enough voltage applied across it, a breakdown point is reached. The voltage across the low-pressure gas accelerates free electrons (there are always some free electrons) and these then collide with gas atoms or molecules. Some collisions cause more electrons to become "free" and they then accelerate to become involved in more collisions and create more electrons. This is the breakdown point, After the break down the gas reaches equilibrium and is characterised by its "glow". A plasma glows because some electrons do not have enough energy to free an electron collision. Their impacts cause the gas molecule to become exited. When that gas molecule relaxes, it gives off light, which is the "glow". The plasma splits the process feed gas into very reactive components called radicals. These are very chemically active and are the supply of etchants to perform material removal.

Depending on the equipment configuration, the plasma also provides a supply of positively charged ions, which hit the substrate surface at an angle of 90 with high energy. These ions are essential for anisotropic etch profiles.

1]. Plasma splits "inert" feed gas into reactive radicals.

2]. Radicals absorb onto wafer surface.

3]. Radicals react with wafer surface to form products.

4]. Products desorb.

5]. Gaseous products are pumped away by the vacuum system.

In order that etching takes place, it is essential to have process gases present which when split by the plasma produce radicals which are capable of forming a gaseous compound with the substrate. Otherwise the surface of the film may react, but cannot be removed by the pumping system.

Radio frequency voltages are used to break down the gas into a plasma. These plasmas are often called RF plasmas or RF glow discharges.

Similar to wet etch but plasma etch processes can remove all layers which are exposed to the plasma, it is important to consider all the following:

Etch rates

Film vertical etch rate

Film laterals etch rate

Mask verticals etch rate

Mask lateral etch rate

Substrate etch rate.

Plasma etching can have both isotropic and anisotropic profiles wet etching has isotropic profiles.

Much lower selectivities than wet etch processes typically between 1 and 100.

see wet uniformity previous.

The variables which can effect the plasma etch process as a result of all the machine set-up parameters are:

Gas mix i.e. the ratio of different process gases (process chemistry)

Reactor Geometry – in some systems there can be small variations in the reactor geometry due to spacing between components being changed after maintenance re-assembly. All reactor chamber spacing should always be the same.

Contamination in the Chamber – contamination on any surface of the process chamber can spread to the product wafer surface and cause defects.

6]. Equipment Used: -

The simplest RF plasma etch system is know as a barrel Asher or barrel etcher and is most commonly used to strip photoresist masks.

The principle is fairly simple. Molecular oxygen flows into the chamber. The RF voltage applied across the two electrodes creates a stable plasma. The plasma provides a steady stream of oxygen radicals which are long lived enough to drift to the wafer surface and react. Photoresist is mainly carbon and hydrogen and so the major reaction products are water and carbon dioxide. These are gases, which are then removed by the pumping system.

This system features an etch tunnel although some systems do not. This perforated metal shield is used to isolate the plasma from the wafers. Because the wafers are isolated from the plasma, radiation damage in the form of energetic ions is reduced. Radiation can cause damage to the sensitive gate oxide layer in the MOS processes and it is therefore important to minimise the effect whenever ion bombardment is not required as in this case. Barrel ashing is a relatively high-pressure process, which typically operates in the 0.3 to 0.4 Torr range. The chamber can be made of quartz, aluminium or stainless steel. Because the wafer are isolated from the plasma, no ion bombardment takes place and therefore only isotropic profiles with severe resist undercutting result if used for pattern transfer. These systems are the dry etch equivalent of wet etching. With suitable chemistry, barrel Ashers have been used for etching photoresist and for etching patterns in polimide, silicon, silicon dioxide and silicon nitride.

With this configuration, the product wafers are not isolated from the plasma, as in the barrel Asher, and are therefore subject to ion bombardment. This means that anisotropic profiles are possible. The major disadvantage of RIE is that it has much lower selectivities due to the ion bombardment component of the etch process. This means that the photoresist-masking layer and eventually the substrate material are also being eroded during processing. Careful process control is necessary to achieve the desired result.

The basic process is similar to that of the barrel etcher. Process gases enter the chamber and are dissociated to from radicals by the plasma. The choice of process gas chemistry is dependent on the film to be etched and would be set up by the process engineer and is generally a chlorine-based process or fluorine based process

The combination at the film surface of radicals and ion bombardment from the plasma leads to the anisotropic etching, which is essential for high fidelity pattern transfer.

RIE is a much lower pressure process than barrel ashing with common processing pressure of 10 to 350 mtorr. Chambers can be again constructed from stainless steel or quartz but are most commonly made from aluminium. The key difference is that the wafers are directly mounted on the RF powered electrode.. This electrode is "driven" from a RF generator through a matching network. The matching network makes user that no power is reflected from the complex chamber impedance back to the 50 ohms generator. This configuration means that there is a "blocking capacitor" between the generator and the chamber which can develop a large DC voltage drop across it called the DC bias (approximately 200 to 400 V).

This DC bias is dropped across the dark space and is the voltage, which forces the positive ions to impact the wafer surface.

The key system components of an RIE are:-

Vacuum chamber – stainless steel, anodised aluminium or quartz. Vacuum seals are either elastomer or metal.

Pumping system – older systems use oil sealed rotary vane pumps with boosters. Newer systems use dry pumps or dry pumps and turbomolecular pumps.

Gas Flow control standard thermal mass flow controllers MFC’s are used. The sensor has two elements with the first transferring heat to the gas and the second measuring the gas heat energy. The difference in thermal energy is proportional to the number and type of molecules in the sensor tube, which gives the mass flow rate. Gas flow is measured in sccm and is the rate at which molecules enter the chamber.

Gas Jungle – multiple MFC’s are required to feed the chamber because their may be up to five different feed gases in the process recipe. These gases are then introduced into the chamber usually by some sort of " showerhead" arrangement, which distributes the mixed process gas to all points on the wafer surface.

Pressure controller – this consists of a pressure gauge; a variable conductance valve and an automatic pressure controller to provide feedback. The pressure controller is set to a pressure and it then varies the valve angle to maintain that pressure.

RF system – this generally composed of two major units: the generator and the matching network. The generator operates at 13.56 MHz

Electrode cover – because electrode is receiving a similar chemical attack and ion bombardment component to the wafer surface, the electrode cover material are chosen carefully based on :

Chemical resistances i.e. very low etch rate

Low out gassing rate

Ease of cleaning

Temperature control – carried out either using heater/chiller bath controls the temperature of thermally conductive liquid. This liquid is then circulated through machined channels in the electrode so that heating or cooling takes place dependent on the process recipe. With the helium technique, wafers are firmly clamped to the electrode and He gas (a good thermal conductor) flows over the back of the wafer.

Load locks – most modern etch systems use load locks because they exclude atmospheric water vapour from the processing chamber. Water vapour can cause process problems as it interferes with the process chemistry and may prevent etching in extreme cases.

Automated controller – this is the system controller and is responsible for control of all the individual process modules which make up the etch system. It can also act as a data monitor and recorder. These systems are very complex but necessary in order to allow the multi-stage process recipes and complex automated wafer transfer systems which are now common.

Endpoint detector – these systems automatically detect when the etch process has finished. They usually monitor the light emitted by the plasma and changes in particular wavelengths of light indicate that different layers have been reached in the etch process.

Diagram 1 Barrel asher

Diagram 2 Reactive ion etcher.

See above as in wet etch process.

The only area in which dry etching has replaced wet etching is in the area of pattern transfer as they are unable to resolve small features because of their isotropic nature, which leads to undercutting and a difference in CD between mask and film. Features sizes of approximately 6m m and above still employ wet etch processes for pattern transfer. However the majority of leading edge processes are operating at minimum CD’s of 1 m m and below and these processes rely on plasma etching for pattern transfer. Plasmas are very sensitive to very small changes in equipment set-up.

Plasma etch processes have much lower selectivity’s than wet etch processes as low as 1 or as high as 100). The major application areas for etch processes are as follows:

Etching to remove thin films from a substance.

Rework, cleaning and decontamination, thin oxide removal, backside film stripping, etching and analysis, inspection (destructive), defect analysis for yield and reliability problem diagnosis, etching for pattern transfer.