Processes in front-end and back-end technology / Glossary

Photoresist Processes/Removing Photoresist

Photoresist can be removed very easily via a plasma process, usually with excitation by microwaves. This configuration provides the best removal rate along with minimal damage to the components in the plasma.

Processing of a large number of wafers in a batch is the most cost-effective variant with regard to costs per wafer, and enables the paint to be removed from both sides simultaneously. However, the results are limited in terms of homogeneity of removal due to the geometry of wafer loading. Even so, this effect is barely relevant in a process with pure oxygen with total removal of the paint, as this process stops automatically at the inorganic layer underneath.

The removal rate in the oxygen process gets higher as the temperature of the wafers rises. To limit this procedure, we have developed a process that takes place at a consistent temperature. The advantage is attainment of the highest possible removal rate without exceeding the preset wafer temperature. Temperature monitoring is carried out by an infrared thermometer with corresponding control of the microwave output.

Our Batch Systems

With larger wafers, a single-wafer process is usually preferred, as it can be easily automated and provides better results in terms of uniformity.

The process for paint removal is usually performed with pure oxygen but occasionally also in combination with special gases so that special requirements for removal, removal rate, and wafer temperature can be met. This is particularly important when removing paint after high-dose implantation. In the case of layers with a strong tendency to oxidation, hydrogen can alternatively be used as a process gas.

Our Single-Wafer Systems

  • GIGAfabM
  • GIGAfab A300
  • GIGAfab LED for 150 mm and 200 mm wafers with automated loading GIGAfab LED for flat-panel displays

Descumming after Lithography

In descumming, a very small layer of the photoresist is removed in a very uniform way in order to prepare the exposed substrate surface for the subsequent step of galvanization or a lift-off process. In both cases, a clearly defined resist profile and a substrate surface with no organic impurities is required so that the subsequent metallization results in good adhesion and has the desired profiles.

Smaller substrates can be processed well and cost-effectively in a batch process, e.g. when manufacturing optoelectronic components or SAW filters. To this end, in line with the substrate size and the requirements of the components, we can structure the process chamber and the loading options and develop special processes with multiple steps using special gases.

Larger wafers can only be processed in the single-wafer process due to the strict requirements, for example in the bumping process. Uniformity of 5% can be attained on a 300 mm silicon wafer here with a corresponding design of the chamber and process.

To the Single - Wafer Systems

To the Batch Packaging Systems

Removing Sacrificial Layers

In the manufacture of three-dimensional structures, sacrificial layers are used, acting as a supporting structure during creation of the elements. Once the three-dimensional structures are complete, these layers are removed so that the structures can fulfill their mechanical function as inertial mass, a sensor or a drive.

When these sacrificial layers are removed, use is made of the outstanding groove-penetration properties of the microwave plasma that enable the radicals from the plasma to go through very small openings. In combination with the direct microwave excitation in the process chamber, PVA TePla AG’s plasma systems are ideal for removing organic sacrificial layers that are only accessible through very small openings. Furthermore, our processes are highly suited to removing SU-8 as a sacrificial layer.

To the Plasma System GIGA 690 (Batch Packaging System)

To the Strip Type System 80 Plus

Removing Polymers

The etching process for deep trenches in monocrystalline silicon consists of multiple repetition of etching and depositing steps in line with the Bosch process. The side wall of the already etched trench is passivated with a polymer here in order to protect it from the further etching in the subsequent step. Once the preferred etching depth has been reached, this side-wall polymer must be removed before the next process steps. This can be done by means of a relatively simple plasma process in which the radicals penetrate deep into the etched trenches and break down the polymer layer into volatile components.

One key criterion for this process is the wafer temperature: the higher the temperature, the better the polymer removal. If the process chamber is structured appropriately, this process can be performed in a batch as well as in a single-wafer reactor.

The Bosch process was developed by Robert Bosch GmbH and is protected by various patents.

Surface Activation

The surface of an object can assume various energy states. One way of describing them relates to how resistant they are to water. Highly resistant surfaces are called hydrophobic. This state can be determined by a simple test in which a small drop of liquid is applied to the surface. The extent to which the drop spreads or forms a sphere is observed here. The related measurement is known as the contact angle. In the case of hydrophobic surfaces, this angle is very large, and wet etching proceeds very uniformly.

The surface energy can be transferred from the hydrophobic to the hydrophilic state in a very simple activation process. A short process with oxygen, possibly mixed with argon, changes the surface to a very low contact angle, with the result that a subsequent wet-chemical process takes place much more quickly and uniformly than without this pretreatment.

Another application of plasma processes is conversion with various contact angles of different surfaces of the same substrate. This is used, for example, as pretreatment of glasses for OLED flat screens that are manufactured with the inkjet method.

Solar-Cell Edge Etching

Edge isolation is an important step in the manufacture of solar cells. Here, the doping on the edges is removed, as this layer acts as a short for the P-N junction. For this process step, we have devised a special plasma-etching process in the cell stack that works with a process mixture of a gas containing fluorine, and with an oxidant.

The cell stack is only exposed to the etching process at the edges; the adjacent cells protect the surfaces. Rotation of the cell stack during the process ensures uniformity of the process. The throughput for this process is around 800 cells per hour.

The cell stack is held together in a cell clamp, which is available as an optional extra. With the loading aid, it is very easy to load a stack into the cell clamp or remove it. The cell clamp and loading aid are designed for cells of 4″, 5″ or 6″ and for different stack heights.

The process requires a relatively high proportion of gas containing fluorine in combination with an oxidant in order to attain the desired removal rate.

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