Suraiya Nafis, head of sales for semiconductor equipment, PVA TePla America
During the manufacture of integrated circuits (ICs), it is necessary to perform many deposition and etching steps to build the necessary internal structures. Among the most common are the ashing, or stripping, process to remove photoresist from etched wafers, and the descum process to remove any residue.
Although solvents can be used to remove photoresist in a wet process, alternatives that allow for dry processing are often sought to reduce chemical handling and acquisition costs. Among the alternatives for dry processing are plasma tools that apply ionised energy for wafer ashing and descum.
Plasma etchers have been available since the earliest days in the semiconductor industry. Since then, plasma processing has been expanded to include the removal of polymers, SU-8 (a commonly used epoxy-based negative photoresist) and sacrificial layers: for wafer cleaning, chip carrier cleaning, encapsulation and flip-chip underfill; and for wafer stress relief, chip side healing (CSH) and passivation (for ultra-thin wafer technologies).
Given the maturity of the market and the refinement of processing techniques, plasma etchers are relatively standardised, with fixed format, off-the-shelf options. However, with the addition of more back-end applications, along with the huge demand for microelectromechanical systems (MEMS), microfluidic devices, power devices, high-brightness LEDs (HB-LEDs), polymer LEDs (PLEDs) and photovoltaics, which utilise similar manufacturing techniques and tools, more semi-customised solutions are now required to adapt to the varying sizes, substrates and other manufacturing challenges.
Ashing and descum
The IoN 100Q RF plasma treatment system for ashing.
Plasma ashing is the process of removing the photoresist by burning off the organics. Using a processing tool, monatomic plasma is created by exposing oxygen or fluorine gas at low pressure to high-power radio waves, which ionise it. This process is performed under vacuum to create a plasma that turns the photoresist to ash.
Two forms of plasma ashing are typically performed on wafers. Ashing, or stripping, is to remove as much photoresist as possible, while descum is to remove residual photoresist. The main difference between the two processes is the high temperature the wafer is exposed to while in an ashing chamber.
As previously mentioned, high-powered radio waves ionise the oxygen or fluorine gas. For photoresist removal, this means a critical decision needs to be made between radio frequency- (RF-) and microwave-based units. If underlying layers are sensitive, they can be damaged when photoresist is removed, and some devices are sensitive to plasma damage. In cases where the device is not sensitive, an RF-based unit can be used, but in cases where they are, a microwave-based unit is the better option.
RF-based units are generally superior for ashing. They etch the surface via a physical process, namely bombarding the surface with plasma in a specific direction.
Microwave-based units are gentler because application of the plasma is not so much a physical process. They mainly adopt an isotropic chemical approach, meaning the plasma permeates everywhere. This makes them more suitable for removing material from the underside of items such as flip chips that cannot be reached through direct physical bombardment.
Microwave-based units are also especially useful for removing SU-8 photoresist, since parts exposed to ultraviolet (UV) become polymerised but the remainder of the film remains soluble and can be washed away.
was developed to provide the high-resolution masks required for microfluidics, MEMS and other microelectronics applications. Among the many merits of is its chemical stability, but this can also make it difficult to remove. As a result, PVA TePla has developed a dry plasma ashing technology that removes SU-8. Microwave-based units are used due to their isotropic chemical etching approach, as this affords the advantage of being able to remove sacrificial layers and undercutting the top layer. Another critical factor is their precise temperature control, since in the past, plasma etchers have often measured temperature but provided little to no control over it.
The microwave plasma stripper must have very exact temperature controls because if the temperature is too high, SU-8 becomes very hard and even more difficult to remove.
In addition to the choice between RF- and microwave-based units, ashing and descum require batch and single wafer processing, respectively.
Unlike ashing, descum is used to remove minute, very exacting amounts of material. It is important that the critical dimensions of the wafer remain unaffected. This means that descum must be performed one wafer at a time, thus achieving the uniformity required.
Single wafer processing is more precisely controlled but can still be performed at relatively high speeds.
Cleaning
When it comes to wafer processing, ashing and descum are essentially similar processes to cleaning, both involve the removal of organic material. With this in mind, plasma is also used to remove particulates and other contaminates from the glass and sapphire carrier wafers used to provide flat support to ultra-thin silicon wafers during manufacturing.
Carrier and silicon wafers are temporarily bonded. Once the manufacturing process is complete, a UV laser is used to de-bond the substrates, leaving behind the silicon wafer, which is around 50 µm thick. As glass and sapphire substrates can be expensive, cleaning and re-using them is ideal.
However, the cleaning of carrier wafers is currently mostly carried out via a wet process using solvents, so it is much more expensive and the chemicals must be disposed of after. It is much easier to clean via dry processing with plasma and it will remove all of the adhesive.
Customisation
The ultimate lesson is that plasma tools are not one-size-fits-all. Each application will often have a very different set of requirements. However, because semiconductor manufacturing has so many fixed formats, some providers stick only to off-the-shelf options and provide little to no customisation.
For providers that are willing to customise, the equipment manufacturer must invest in research and development engineers, facilities and equipment to propose appropriate solutions. These include RF- or microwave- and single wafer or batch processing systems for semi-automated to fully automated, high-speed production. Then there are details such as chamber size, type of loading/unloading mechanisms and degree of temperature control, which also play a critical role.
PVA TePla America
www.pvateplaamerica.com