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1 - Introduction
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The dicing process arises at the end of the wafer manufacturing to separate the chips or after deposition of semiconductor based materials to isolate the device to characterize. The substrates to dice can be of various types and natures. The most diced are composed of semiconductor materials (Silicon, GaAs, GaN, SiC, compound III-V…), but there are also some ceramics or glass supports that hold some microelectronic devices or layers and they may need to be cut too.
Dicing small & thin semiconductors and related substrates is a delicate process. The semiconductor wafers & substrates are thin and fragile and sometimes now the dicing streets are extremely narrow.
It is important to precise here that the below explanation and related products concern the dicing of materials used in the frame of microelectronic production and research which are typically thin materials, 0.1 mm to 1 mm thick (3 to 4 mm in some specific cases). Thicker materials are cut with different processes.
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The thin substrates are usually diced with 2 processes.
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- The scribing/breaking dry process. The initial scribing is made with hard diamond tools, it gives a partial cut then the pieces are separated by breaking. The process is typically used in glass dicing for windows… Applied to semiconductors, it is mostly concerning the very thin substrates (100µm) and particularly some materials (GaAs) that can be cut along the crystal angle with absolutely no chipping. This process is not very common and we will not talk too much about. Amongst the advantages, there is the perfect cut, no coolant, no kerf loss. Some disadvantages: the full process is not easy and can't be much automated, the substrates thickness is limited. The scribing/breaking is being alternate nowadays with the laser dicing that can respond to more applications.
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- The typical semiconductor dicing process use dicing machines (Disco, ADT, TSK, Loadpoint, Bernet, Meyer-Burger…) specifically made for the application. In those equipments, a spindle rotates at high speed (30kRPM). The dicing wheel (or dicing blade) is being mounted onto the spindle and turns at the same speed. The X-Y and Z movement is controlled by the machine software. The sample being diced is maintained either by a hard mount cement (thermally reacting wax type) in the case of thick and hard materials (ceramics, package dicing…) or by an adhesive tape (wafers and thin materials). Holding by adhesive tapes is obviously and easier process, especially with the newest UV tapes which offer a very high adhesion.
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2 - Adhesive plastic tapes for dicing (include UV tapes)
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The adhesive tapes are widely used to maintain the substrate or wafer before dicing. It is indeed like a standard adhesive tape used in all offices except that the characteristics both technical and dimensional are specific to the semiconductor dicing process. As it concerns microelectronic products which can be expensive and fragile, the tapes used are special in their technical features, cleanliness, initial adhesion and evolution by the time, contamination, expandability…
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The tapes are usually presented in roll of 100m long with the width adapted to the size of the substrate (165 to 350mm). There are two main families
: Standards tapes (which means with standard adhesive) and
UV tapes
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The objective if those UV films is to offer a very high adhesion during the dicing operation (5 to 20 times compared to standard tapes) and a possibility to lower that adhesion to a very low level in order to pick up the dies very smoothly and without damage. This can be extremely important in the case of dies and substrates very thin and fragile (smartcard applications, mobile…). Lowering the adhesion force after dicing is obtained by curing the film adhesive thru X-rays at UV frequency. The value then obtained is around 5% of the initial value and much lower than conventional tapes. As a matter of example, a standard tape could present an adhesion of 100g/25mm during dicing and after when dies pick up occurs. A UV tape would present an initial adhesion of 500g/25mm (5 times more) and a final adhesion after curing by UV of 20g/25mm (5 times less) thus responding perfectly to the dilemma, very adhesion first, very low adhesion after.
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The plastic tapes are tensed around a metal carrier called the "frame". They are sticked onto the frame which is maintained by magnetism during the dicing process. Certain types of films can then be "expanded" after the dicing so as to separate clearly the diced chips and facilitate the pick-up.
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3 - Dicing wheels with metal and resin bond
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Microelectronic materials, wafers, ceramic or other substrates, thin glasses coated with electronic layers…of thicknesses ranging from 0.1mm to 2 or 3 mm can be cut with specific dicing machine and appropriated dicing blades.
The dicing blades are made for the active part (the cutting part) of diamonds grains embedded in a bonding structure. They present physical and technical characteristics specifically adapted for the nature and dimensions of the material to cut, and optionally to the dicing process as well.
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There are two main families of dicing blades, the "metal bonded blades" and the "resin bonded blades". The difference between them resides in the bonding structure in which the diamonds grains are embedded
The bonding structure can be made with a metal based materials (typically Nickel) and allows then the dicing of soft to medium-hard materials (which are most of the Silicon-based materials used in semiconductor processes) or it is made with a resin based materials and is more adapter to dicing of hard and brittle materials such as glass and ceramics.
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The "metal blades" are available practically in a unique OD/ID size (OD = outside diameter = 55.55mm or 2.187", ID = Inside diameter = 19.05mm or 0.75") and the thickness can vary from 10µm to >150µm.
They are "Hubtype" blades, which means that they are complete and ready for mounting on the machine spindle for dicing. They are replaced after use (after having cut on a distance varying with the materials and blades characteristics). The diamonds grit sizes range from 0.5µm to 40/50 µm but most silicon-based materials use a grit size comprised between 2 and 8 µm.
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The "resin blades" are "hubless" type blades, their shape is annular, a ring where the OD/ID can be identical to metal blades which allows to use the same dicing machine or comprised in a range of 50-116mm (OD) which then needs the use of a specific dicing equipment accepting these diameters.
The resin blades need to be mounted in mechanical flanges which will hold the blade while cutting. The diameter of the flange determines the necessary exposure function of the material thickness. The diamonds grit size range is wider than the metal blades (0.5 to 150µm) and the bonding structure of the resin blades reacts differently than the metal bonding.
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The goal is different when using resin blades, the objective is to cut with diamonds particles always "fresh" i.e very sharp. The bonding structure is calculated for a particular P/N so that the "worn out" diamonds can be ejected from the bonding matrix and newest diamonds particles embedded in the structure be presented for an efficient and clean cut. The drawback is obviously that the blade wears out. It is for that reason that it is mounted within flanges & can be replaced easily after use or when it is worn out and exposure or thickness & square shape have become out of specs.
Additionally, there is also a range of "Hubless" metal blades, for mounting within flanges. They are used for particular applications (big exposure or biggest diamonds grit than standard metal blades).
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4 - Manual equipment for film frame mounting, UV curing, expansion
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Usually, the wafers or substrates to dice are sticked onto a plastic adhesive tape, being itself tensed over a metal frame. The assembly is then put on the dicing machine and a vacuum system holds the wafer while the metal frame is held by magnets.
The operation of laminating the film over the wafer must be perfectly done, the tape must be tensed and there should not be bubbles between the film and the wafer which would prevent from a good adhesion of every part while dicing. The operation requests the use of automated equipment for high volume production or manual or semi-automatic equipment for small production or labs.
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In the case of using Ultra-Violet sensitive tapes, lowering the adhesion value in order to pick up the chips is obtained by curing the tape under X-ray at specific wavelength and power.
Equipments are available to make the curing operation and easy and repeatable process. Upon size of wafers, power, automation needed, several UV curing systems are responding to all applications.
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After the dicing, the frame is removed and all dies keep sticked onto the tape, separated and awaiting to be gently removed. It may be necessary to increase the space between the dies more than the current spacing given by the blade thickness.
The expansion process consists in expanding the plastic film which will separate the dies with larger spacing so that picking one die will not touch and possibly damage the neighbor one. The expansion process is automatic on production equipment, it can be made manually with manual expander. The plastic tapes can typically be greatly expanded up to 300 or 400% (except the PET tapes).
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5 - Accessories used in the dicing processes
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This section concerns the product used as "consumable", such as dicing blades, adhesive tapes, wafer film frame and plastic grip-rings (used in the expansion process). Other products last more but need care and regular maintenance and/or cleaning.
The flanges particularly can last several years if properly used and maintained with periodical cleaning and manual polishing of the inside surfaces holding the annular blade. |
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A new blade needs usually to be "dressed" in order to not produce too much chipping at the beginning of its use and this is particularly true in the case of hubless blades to perfectly round the concentricity and test the intimate contact with the inside holding surface. A big problem with cutting hard materials is the risk that the blade turns inside the flange, either because the contact is not perfect due possibly to some particles inside or a damaged flange or an insufficient tightening of the flange.
The dressing will assure a very good quality cut (depending also on the machine itself) in the production material and optimize the lifetime of the blade. Special dressing blocks are used to reach these objectives. They are used to receive the first cuts and are made of a specific material to dress the blade.
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Another important aspect of the dicing process is the antistatic properties of materials used. Adhesive films are made of plastic and can generate static electricity during the unwinding pr lamination onto the wafer. These static charges are potentially dangerous for microelectronic components.
The laminating equipments can be equipped of antistatic systems. A ionised air can be blown onto the surface to protect in order to avoid development of static charges. Also some versions of the adhesive tapes include a special layer to become antistatic films and minimize the risk of damage to the semiconductor devices.
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Electrical characterization of semiconductor layers
