The XRG-800 generator produces radicals, i.e. excited, uncharged molecule parts. The radical generator offers extremely high yield (>95%) of all gas molecules are converted into radicals), extremely high plasma density (>60W/cm³) as well as simple and reliable operation.

The radicals have many applications, some of the most important for Diode wafer are: -

• Exposed Junction Etching
• Wafer Backside Etching
• Wafer Damage Etching
• Etching of Moats in Silicon Wafers

Exposed Junction Etching

Diodes and Thyristors are discrete electronic components which limit the flow of electric current into one or both directions. They are made as whole wafers and then cut into dice (individual elements). The cutting causes contamination and damages to the exposure junction in the silicon crystal structure, this damage must be removed. Until now, this was done with wet chemistry, where the mounted devices were run through several etch baths and then dried and protected with a polymide coating. This could take as long as 90 minutes or longer. 

The disadvantages of this approach are: the high costs of wet chemistry, low reliability of the process and considerable environmental pollution.

Secon has developed a dry etcher, capable of automatically processing several thousand devices per cycle, three cycles per hour. The quality of the dry processed devices is noticeably better than those which are wet processed. The yield is higher and the running costs lower, giving a considerable drop in the costs of the ownership. This process is without competition to date.

The Dry etch process for diodes is more reliable than the usual wet etch process and offers an increase in quality, yield and line capacity. The benefits are: -

• Additional capacity and less space
• More uniform and cleaner die attach process
• Less process variability
• More reliable process

In diode production, one wafer will be doped and coated with conductive layers before is cut into dies. After dicing these dies have to be etched to obtain open junctions free of dicing residues and crystal damage. This conventionally is done with KOH and DI-water, the wet etch lines are difficult to control and the etch rate depends on the doping intensity.

The dry etch technology is based on SF₆- (or CF₄-) and O₂- or N₂O- radicals, which etch the defective parts of the crystal lattice. It is an isotropic etch process with adjustable selectivity against crystal defects. The SF₆/O₂-mixture offers high selectivity compared with the SF₆/N₂O-process with low selectivity but smoother surface. The etch rates are in the range of 2 - 9 µm/min.

Compared to wet (KOH) technologies, this dry process offers even more advantages: -

• KOH 
  • Low etch rates of high p-doped parts
  • Normal etch rates for high n-doped part and barriers with a slight advantage for n-doped parts
• CDE 
  • High etch rates for high n-doped parts and barriers with a slight advantage for n-doped parts
  • High etch rates for high p-doped parts, more than for barriers and almost as high as for p-doped
  • Etch rates are generally higher compared with KOH

Dry Etchers of the XCD - 7XX series are already used successfully in the Diode production lines of leading manufacturers, such as Bosch, Motorola, Temic, Tesla, Vishay Zetex, where they are used in the production of highly cost sensitive devices.

Besides the open junction, more and more planar power devices are being manufactured in which whole wafers are being processed from diffusion to passivation. For this type of device, Secon etching technology is offered in either the XPL - 300 cluster for single wafer processing, or the XCD - 7XX etcher for batch processing of several wafers per run.

The next step after etching is the passivation, here the Secon equipment can deposit a protection layer of amorphous silicon or silicon nitride on the open junction of the devices. This can offer tremendous advantages for some device manufacturers. Improved performance and the elimination of a polymide coating stage, a time and cost-intensive post treatment step.

The Secon passivation technology offers a short additional process in the etch chamber. The elements will be passivated immediately after the etch step without breaking vacuum. The additional time required is < 10% of the whole process.

Two options are available: -

• Passivation by am amorphous silicon layer for selective (at the open junction only) passivation for low temperature applications 
• Passivation by silicon nitride for high temperature applications

The benefit for the customers: -

• Elimination of environmental problems (no pollution)
• Higher quality passivation layers
• Increased throughput by polymide process elimination
• Reduced production cost

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Wafer Backside Etching

During the fabrication of semiconductor devices, the wafers are repeatedly subjected to deposition processes, where they are coated on both sides. This lead to the undesirable build up on the wafer backside which can cause problems in later processing. Nowadays, it is becoming standard practice to remove these layers and some silicon stock as final stage in fabrication, to improve the thermal performance of the devices. This is usually done by protecting  the front side with a tape coating and removing the respective layer from the backside, either wet or dry with the CDE-process or by a combination of grinding and wet processing. Then the coating must be removed from the front side. These two additional steps require process time and equipment capacity, as well as a considerable consumption of materials.

An alternative approach which is becoming increasingly used involved a wet etch but with no need for front side coating. In this process, the wafers, are rotated fast, face down on an air cushion, while from above wet chemical etch solution is sprayed onto the wafer backside. By fast rotation of the wafer, the chemicals are thrown outwards over the edge of the wafer backside and thus can not attack the front side.

This process has established itself on the world market, but is also has serious disadvantages:

• The consumption of wet chemicals including DI-water for rinsing is considerable, this is despite the possibility of some re-use of the media (costs, media disposition, disposal)
• The etch rate for LPCVD-nitride is rather low compared to the otherwise reached poly-Si and oxide rates, and the throughput of wafers is accordingly low.

Secon has in conjunction with Siemens developed a new process, whereby the layers to be removed from the wafer backside are etched by use of a radical generator in a dry process. In this case, the wafer front side does not need to be coated with resist, but is protected by a neutral gas without being contacted physically. The wafers are loaded into the process chamber fully automatically for the single wafer etchers, in their natural position (face up). During backside etching, etch gases reach the wafer from below and etch the backside, the wafer edge and a very thin ring of adjustable width on the wafer front side. This is the ideal method chip manufacturers have sought for a long time.

The advantages of the backside etcher XCD - 2XX developed by Secon, compared to all other methods, are: 

• Dry chemical process - clean and much cheaper than wet chemical processes
• No additional process step necessary for protection of wafer front side
• Standard wafer handling with front side up
• High nitride etch rate
• High wafer throughput
• Possibility of etching wafer edges
• Possibility of simultaneous etching of wafer front- and backside
• Considerably reduced cost of ownership

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Wafer Damage Etch

There are increasing demands for thin chips for a variety of reasons, weight, flexibility, thermal performance etc. This means that an increasing proportion of all process wafers are having to be thinned in their final stages of production to target thickness in the region of 150 - 200 microns.

The wafers in the production line therefore are processed to completion and then thinned by mechanical back grinding. This process damages the crystal lattice of the wafer, which leads to wafer bow, and microcrack, these reduce the tensile strength. Damage the lattice and microcrack are usually removed by wet etching.

Secon has therefore developed a system with the damage etcher XCD - 2XX, in which thinly ground wafers are treated, to removed lattice damage and microcrack.

The advantages of this process are: 

• Dry chemical process - cleaner than wet chemical processes
• No additional process step necessary for frontside protection 
• Standard wafer handling with front side up
• High wafer throughput
• Possibility of etching wafer edges
• High cost saving per wafer

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Etching of moats in silicon wafers

Some applications in diode manufacturing require deep moats in silicon wafers, This is usually achieved using photoresist masks and wet etching. For a dry etch solution for this application, Secon has developed the series model XCD-7XX etcher, which together with the XRG-900 high rate source and an HF bias voltage (100 - 400 Hz), produces ions, radicals and UV radiation, for optimum etch rate. In the large process chamber batches with several wafers can be processed, etch rate up to 50 µm/min can be achieved with hard aluminum masks replacing photoresist.

The benefit for customers is one process step for two requirements: -

• Damage etch at the open junction
• Moat etch for separation of elements

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Copyright © 2002 Secon Semiconductor Equipment (M) Sdn. Bhd.
Last modified: Juli 03, 2002