Month: November 2020

What is a light-activated adhesive?

30th November 2020

A simple introduction to and understanding of how a light-activated adhesive works (IKB-077).

A light-activated adhesive is used to bond non-transparent materials together without using an additional mechanism to cure the adhesive in the shadow zone between the two substrates.

Whether the adhesive is an epoxy or an acrylate, the principle is the same: apply the adhesive to the first substrate, illuminate the adhesive with high-intensity light, and join the second substrate. The adhesive between the substrates will cure completely over time. Optionally, the cure time can be accelerated, either by adding heat (for every 10 degree rise in temperature above RT, the cure time halves), or, if a fillet of adhesive is visible around the edge of the bond area, further illumination for just a few seconds can significantly increase the handling strength.

Light-activated epoxies have been around for over twenty years, but recent innovations have now produced light-activated acrylates. While the end result is the same, i.e. full cure of an adhesive between non-transparent parts, the different acrylate chemistry results in significantly more flexible bonds, making them ideal for stress-sensitive applications.

Light-activated adhesives are generally shipped and stored at room temperature, therefore avoiding the necessity of dry ice during shipping (which can result in significantly reduced transport costs and which also expands the number of couriers who will carry these adhesives). This also means that expensive industrial freezers are not required, reducing energy bills and freeing up valuable storage space within manufacturing facilities.

A much more comprehensive explanation concerning all aspects of light-activated adhesives is also available on Inseto’s website (HERE), but this is a pictorial explanation of the process of using light-activated adhesives:

For further information on our range of “Light Activated Adhesives”, please click HERE.

Author

Date

Version

Author

Eamonn Redmond

Date

19 November 2020

Version

IKB077 Rev. 1

Download

Author Biography

Eamonn Redmond is a Senior Technical Sales Engineer at Inseto (UK) Ltd, leading the Materials and Adhesives divisions. With nearly four decades of experience in microelectronics, he combines strong technical knowledge with practical commercial insight.

After completing a B.Eng. in Electronics at the University of Limerick, Eamonn began his career at Analog Devices, working as an In‑Line Test Engineer and Process Engineer. During this time, he developed hands‑on expertise in die attach, wire bonding and environmental testing, including burn‑in, temperature cycling and thermal shock. He later progressed to IC Production Manager, gaining experience in wafer‑level and package‑level testing and modern semiconductor production operations.

In the early 1990s, Eamonn moved into technical sales, specialising in die attach adhesives, eutectic solders and materials for wire and ribbon bonding. This role established a strong foundation in adhesive technologies for high‑reliability microelectronic applications.

Since joining Inseto in 1996, Eamonn has supported customers with a wide range of microelectronic materials, including die attach adhesives and encapsulation compounds, alongside substrate materials such as thick‑film inks and thin‑film substrates. He is widely regarded as a key technical resource for customers seeking reliable bonding and encapsulation solutions.

Resist Coating Methods

10th November 2020

This document provides an overview of techniques for the uniform coating of a substrate with photoresist (IKB-067).

Resist Coating Methods:

A crucial requirement to ensure repeatable, reliable and acceptable results with photolithography, is to have a uniform coating of a photoresist over the surface of the substrate. Photoresist is typically dispersed in a solvent or aqueous solution and is a high viscosity material. There are a number of options available to coat a photoresist depending on the process requirements:

Spin-coating
Spray-coating
Dip-coating
Inkjet printing
Slot-die coating

Spin- coating:

Spin-coating is the most common method used when coating a substrate with photoresist. It is a method that presents a high potential for throughput and homogeneity. The principle of spin-coating is that typically a few millilitres of photoresist are dispensed on a substrate which is spinning at several 1000 rpm (typically 4000 rpm). The resist can either be dispensed when the substrate is stationary and then accelerated up to speed (static spin-coating), or it can be dispensed once the wafer is already rotating (dynamic spin-coating). Any excess resist is spun off the edge of the substrate during the spinning process.

The centrifugal force experienced by the resist on the surface of the wafer causes the viscous resist to spread out into a uniform thin film. The height of this film is directly controlled by the rotational speed of the substrate, allowing the operator to achieve the desired film thickness.

Alongside the spin speed the spin time can also be used to control the film thickness. This is due to the evaporation of some of the solvent or aqueous liquid used to disperse the resist, which causes further thinning of the resist. The loss of the solvent also results in the stabilisation of the film, so that it will not collapse during later handling of the substrate.

The main advantages of spin-coating are that the coating step is quite short, typically 10-20 seconds, which when combined with the dispensing and handling time, can lead to process times less than 1 minute. The other advantage is that the films obtained are very smooth and the thickness can be reproducibly controlled very accurately.

The disadvantages and limitations of spin-coating arise when using non-circular substrates or thick (very viscous) resists. In these cases the air turbulences at the edges and especially the corners cause the resist to dry in an accelerated manner. This excess drying then supresses the spin-off of the resist from these regions, causing a bead of resist to build up at the perimeter of the substrate; this built up sidewall of resist is referred to as an edge bead. In more advanced spin-coat systems, techniques for removing this edge bead by precise application of solvent, or limiting its growth by controlling the air turbulences, have been developed.

The other limitation that can impact spin-coating is if the surface of the substrate has a large number of features or a varied topography, then the homogeneity of the film thickness can be affected. With build-up of resist in holes or spaces leading to thicker films and thinner films on the edges of the features. This can be overcome by a two-stage spin profile, or by using one of the alternate coating techniques.

Spin Coat Nozzles for Automatic Dispensing of Photo Resist

Spray-coating:

Spray-coating is an alternative to spin-coating, particularly when the substrate surface or morphology means that the photoresist cannot be coated with the required uniformity. The basic principle of spray-coating is that the resist film is formed from the deposition of photoresist that is atomised into droplets in the µm range.

The formation of the droplets is possible through a variety of techniques; the simplest is to produce an atomised spray from a nozzle similar to what is used in a conventional airbrush gun and a nitrogen nozzle. Nitrogen is preferred, as it helps to reduce contamination of the resist with humidity or particles and produces a dryer mist of droplets.

The second standard method to generate an atomised spray is through the use of an ultrasonic atomiser. The ultrasonic atomiser creates the droplets of resist via the high frequency mechanical vibration of the resist media, which is then transported to the substrate by a carrier gas.

The droplets of resist are then deposited on the surface of the substrate where they form a continuous thin film of the photoresist. Spray-coating is thus able to cover the entire surface of the substrate even in arbitrary shapes and to provide a conformal coating regardless of the topology. Additionally, there is less photoresist wasted allowing for a higher yield in comparison to spin-coating.

To be able to spray-coat a photoresist, the resist must have a suitably low viscosity. Typically this is a few cSt and may require the dilution of the resist with a solvent. Dilution of the resist can lead to the resist ageing process accelerating and to particle formation within the media. The other limitation of spray-coating, is that formation of films <1 µm is difficult given the stochastic distribution of droplets landing on the surface. To form a continuous film requires a minimum critical resist droplet density to be reached, which increases the minimum film thickness as well as increasing the processing time.

Schematic of a typical Spray Coat process.

Dip-coating:

Dip-coating is used as a solution for resist coating if the size and type of substrate is not suitable for spin-coating and the photoresist represents a significant cost to the overall process, that even when compared with spray-coating, the consumption of resist must be further reduced.

The process of dip coating is that a wafer is submerged vertically in a cuvette of resist and lifted out slowly. The resist film then forms on the surface and thins out as the substrate is removed from the bath of photoresist. The thickness of the resist is controlled by the dwell time in an atmosphere saturated with solvent, which controls the rate of solvent evaporation. The higher the withdrawal speed from the resist bath, the thicker the photoresist film.

As a result of this process, the yield of resist can be 100% if both sides of the substrate need coating, representing a huge increase in resist yield compared to spin and spray-coating. The tank may have to be replaced if the resist expires before it is consumed, which will decrease the yield but still offers the most cost effective of the three coating methods covered.

The photoresist often must be significantly diluted, which can significantly increase both the ageing of the resist film and the frequency with which the tank must be replaced. Additionally, substrates with large changes in surface topography are not suitable as the resist can flow over the substrate and significantly reduce the surface homogeneity.

Inkjet printing:

An alternative method utilised to dispense resist is ink-jet printing. This works in a similar fashion to spray-coating, producing droplets of photoresist. Unlike spray-coating however, these drops are produced in a stream rather than a mist. This stream of droplets can then be precisely controlled and patterned onto the substrate.

Ink-jet printing differs from the other coating methods, in that whilst it can be used to produce a thin homogeneous film of photoresist, the real advantages in its use come from using it to directly pattern the substrate. This can dramatically save the amount of resist used by only depositing where required, reducing both the cost and environmental impact of the photolithographic process. Additionally, it is possible to directly deposit some materials such as organic semiconductors and conductive inks onto the surface, without needing to run a photolithographic process.

The disadvantages of the increased flexibility and versatility of ink-jet printing, are an increased coating time leading to a reduced throughput of coated wafers and substrates. This can be offset however through the use of multiple print-heads and path optimisation. Additionally, ink-jet printing is not the ideal solution for the coating of thicker resists, as the layer thickness is typically only a few microns.

Slot-die coating:

The final method of coating photoresist discussed here is slot-die coating. Slot-die coating is a scalable manufacturing technique used in a range of industrial processes to produce uniform films and coatings. The principle of slot-die coating is shown in the image above. The print-head continuously dispenses the photoresist onto the moving surface of the substrate, producing a uniform film of photoresist. As the solvent within the wet photoresist evaporates, the photoresist film dries leaving a uniform thin film that can then be processed further.

Slot-die coating is a pre-metered coating technique; this means that all the material that is dispensed from the print-head of the coater is used to coat the surface of the substrates. This enables very low (to no) levels of photoresist wastage, which is a great advantage if the photoresist represents a large material cost. The other advantages of slot-die coating are that it is a readily scalable technique, allowing the number of substrates to be greatly increased. Slot-die coating is also perfectly suited to coating flexible substrates and to being used in a roll-to-roll manufacturing process. Beyond the coating of photoresists, slot-die coating (like ink-jet printing) can be utilised to coat any functional material that can be dispersed into a printable ink.

Schematic of a Slot-die Coating process.

For further information on our range of Spin Coaters, Spray Coaters & Inkjet Printers please click HERE

Author

Date

Version

Author

Chris Valentine

Date

10 November 2020

Version

IKB067 Rev. 1

Download

Author Biography

Chris is a Technical Sales Engineer at Inseto, responsible for both front-end equipment and wafers projects within the company.

After completing his undergraduate studies at the University of Glasgow achieving a first class M.Sci Physics degree, Chris moved to Cambridge to pursue further study achieving an M.Res in Sensor Technologies in 2016. He then spent a further 3 years developing scalable carbon nanotube based sensors and electrochemical devices, publishing a papers on this topic and on energy storage applications. As a researcher Chris focussed on wafer level fabrication, characterisation and test, focussing on the use of contact lithography to pattern the catalysts with which he grew carbon nanotubes.

Chris joined Inseto in 2019 as a sales engineer, initially with a focus on front end equipment. Within Inseto his responsibilities have expanded to include wafer materials, as well as a wider range of equipment within the broader semiconductor and microelectronics industry.

Fine Wire Bonding Explained

10th November 2020

What is fine wire bonding and how does it work? (IKB-064)

Fine Wire Bonding Process:

Wire bonding is the process of providing electrical interconnects between an Integrated circuit or component and the external leads of its packaging such as a lead frame or PCB with very fine bonding wire (<75 micron diameter wire).

The materials of the wire used in wire bonding are usually made of gold (Au), aluminum (Al) or increasingly copper (Cu) & Silver Palladium (AgPd). In fine wire bonding there are two main process variations: Ball bonding and Wedge bonding.

Ball Bonding:

During the ball bonding process, the wire is thread through and held by a bonding tool known as a capillary. The end of this wire is then melted by means of an EFO (Electronic-Flame-Off) to achieve a free-air-ball roughly 1.5-2.5 times the size of the wire diameter used. The free-air-ball size is controlled by the energy provided by the EFO and also the length of the wire provided (known as the tail).

The free-air-ball is brought into contact with the bond pad, coupled securely by the capillary. Pressure, heat and ultrasonic energy are then applied to the ball for an amount of time, forming an intermetallic weld between the ball and the bondpad. The ball is deformed during this process to a shape defined by the dimensions and geometry of the capillary tip (note: bond parameters will also have an effect on the final bond geometry).

The wire is then guided to the next bond position, where the wire is brought into contact with the package interconnect pad underneath the capillary, the wire is guided through the capillary during this movement making a loop of wire between the two bond locations. Controlling this movement can help determine the shape, height and length of this loop.

Pressure, heat and ultrasonic energy are then applied to the wire to create the second bond (Sometimes referred to a ‘stitch’ or ‘fish tail’ bond). The process is completed by means of breaking the wire in preparation for the next wire bonding cycle, by clamping the wire and raising the capillary.

Ball bonding can be used to bond both Au and Cu wire, as Cu wire is considerably harder than Au, increased parameters and harder materials are needed for the capillary along with a localised inert atmosphere for the EFO, in order to prevent oxidization during the creation of the free-air-ball.

Gold Ball Bond: Complete Wire, 1st Ball Bond & 2nd Bond

Wedge Bonding:

During wedge bonding, a clamped piece of wire is coupled under a bonding tool (referred to as a wedge) and a bond pad. Pressure and ultrasonic energy are applied for a given period of time forming a first wedge bond. The shape and dimensions of this bond are determined by the dimensions of geometry of wedge (note bond parameters will also have an effect on the final bond geometry).

The wire is guided through the wedge during this movement making a loop of wire between the two bond locations. Controlling this movement can help determine the shape, height and length of this loop. The wire is then guided through the wedge tool to the second bond location and the pressure and ultrasonic energy are applied again to form the second bond location. The process is completed by means of breaking the wire in preparation for the next wire bonding cycle, by clamping the wire and movement of the wire.

Wedge bonding can be performed using Al, and Au wire with the addition of heating the bonding surface and modifications to the wedge tools material construct and tip shape. Al wire will use a concave tip shape made from tungsten carbide while Au wire will use a cross-groove tip shape made from Titanium Carbide.

Comparison:

Ball bonding is non-directional unlike wedge bonding which is unidirectional which makes ball bonding a much faster process than wedge, as the part being bonded does not to need to be aligned to the path of the wire and tool.

Wedge bonding has the advantage of been able to bond to pads with a smaller pitch compared to ball bonding. Wedge bond footprints are considerably narrower as there is no need for a free-air-ball to be created.

Unlike gold ball and gold wedge bonding, which require an elevated temperature (typically 125-150 degrees Celsius), Aluminium wedge bonding is performed at ambient temperature and can therefore be used where devices / materials are temperature sensitive.

The elevated temperature required for bonding wire gold wire helps ‘soften’ the bond pad surface and introduce more energy into the bonding junction, as well as help remove organic contaminates off of the bond pad surface; widening the process window.

Fine Wire Wedge Bond versus Ball Bond Pitch Comparison — Wedge Bond versus Ball Bond Pitch Comparison

View the range of “MPP Equipment for Wire Bonding“.

View the range of “K&S Automatic Wire Bonders“.

View the range of “Coining Wire Bonding Materials“.

View the range of “MPP Wire Bonding Wedges“.

Author

Date

Version

Author

Adam Marshall

Date

29 July 2020

Version

IKB064 Rev. 1

Download

Author Biography

Adam Marshall is a Senior Technical Support Engineer at Inseto Ltd with over 14 years of experience in the semiconductor industry, including 10 years specialising in assembly processes and related equipment. He supports customers across the microelectronics and semiconductor sectors who rely on precision equipment to maintain reliable and repeatable results in production and research environments.

Known for a calm and structured approach, Adam works closely with customers, suppliers and internal teams to deliver clear technical advice and dependable support when it matters.

Probe Station – Optoelectronic Device Test

10th November 2020

How to configure a wafer probe station for probe testing optoelectronic devices (IKB-070).

Wafer probe stations can be utilised to test and characterise devices for a wide range of applications. One such application of interest is optoelectronic devices. Optoelectronics (OE) is the study and application of electronic devices that interact with light. This interaction could be the emission of light (LEDs, light bulbs, LASER diodes), channelling of light (fibre-optic cables & waveguides), the detection of light (photodiodes, sensors and photoresistors) or be controlled by the light (optoisolators and phototransistors).

Broadly speaking the interactions listed above can be broken into two groups when thinking about configuring a probe station. In the first group an electronic signal is produced in response to an external optical stimulus, i.e. the device is exposed to light which causes a current to flow or a potential difference to occur. In the second light is emitted from a device due to a current flowing or other process occurring within the device. In both of these cases to fully characterise and test the device, it is important to be able to both probe a device electronically and to be able to measure the optical signal.

Customised Optoelectronic Probing Solutions

As no two devices are exactly the same any optoelectronic probe station will be highly tailored and customised to the specific application. The probe system for life (PS4L) from SemiProbe allows the integration of standard optical test components with traditional probing accessories to produce a custom test platform specific to any application. The following will outline common features and concepts that must be considered to configure a probe station to work in an optoelectronic setting.

An important question to ask when configuring an optoelectronic probe station is how will the device interact with the light? Will there be a direct optical path from the light source to the device to the optical detector? If so a glass chuck or double sided probing chuck may be required. These will need to be able to hold the device under test (DUT) but also allow the unimpeded transmission of the light.

Optical stimulation, light collection & measurement:

Another consideration for the optical path is how will the light be delivered to the system or if it is produced by the device how will it be collected and measured. One common method is to use an optical fibre to illuminate the device or collect the optical signal. This optical fibre is often mounted on an X, Y, Z and theta (θ) stage allowing the precise control of the fibre. Additionally a goniometer, a device used to precisely measure angles between surfaces, or a non-contact height sensor may be required to allow the full characterisation of the optical path. An alternate solution to collecting emitted light is to use an integrating sphere. An integrating sphere is a hollow cavity which scatters light evenly over all angles and enables the total power (flux) to be measured without directional inaccuracies. This allows the calculation of the total power from a light source to be captured in a single measurement.

Wafer probe station configured for the characterisation of VSCELs with both probe arms and an optical fibre to measure the output of each device. — This probe station is configured for the characterisation of VSCELs with both probe arms and an optical fibre to measure the output of each device.

Other considerations:

Other factors to consider for the optical setup of a probe station are what parameters of the light need to be measured; typically this could be, wavelength, phase, intensity, flux or power. Additional filters for polarising, selecting specific wavelengths and modulating intensities may also be required.

Beyond the optical setup required for probe testing optoelectronic devices, the other aspects to consider are common to most probing applications. These include what configuration of manipulators will be required to probe the device? Will these need to be optical, DC, high frequency (HF), high voltage (HV) or even magnetic and how many manipulators will be required. In some cases the wafer chuck will need to provide a bias to the device under test (DUT). The feature sizes and pitch of the devices will also have an impact on the manipulators and probe arms chosen, as will the tolerances required by the application.

Additionally any thermal or environmental requirements must be considered when configuring the probe station. Will the probing be carried out at increased temperatures, if so how will the application of heat be applied without interfering with the optical setup? One common solution is to use a forced hot air system to increase the temperature of the device and to integrate in thermocouples to provide closed-loop temperature feedback. Additionally a localised environmental chamber (LEC) may be required to further control the surrounding environment.

The LEC can be of further use if the probe station is required to probe at negative temperatures. In these circumstances it is important to ensure a stable frost-free environment that is shielded from the external surroundings. Typical thermal ranges incorporated to an optoelectronic probe station are -40–200°C or 25–200°C. This can be extended to include -65–300°C if the application requires.

The final consideration for an optoelectronic probe station is what industry standards such as SEMI or UL that the tool must meet. Commonly incorporated to optoelectronic probe stations are laser safety interlocks to avoid exposing the operator to any intense light whilst the tool is in operation.

In summary an optoelectronic probe station must be highly customised to be able to meet the exact specifications of the optical device. The probe system for life (PS4L) from SemiProbe is a tool which will allow the user to integrate in application specific optical components whilst utilising standard electronic semiconductor probing techniques. These platforms have been deployed in a number of applications such as vertical surface cavity emitting lasers (VSCELs), edge emitting laser diodes (EELDs), optoelectronic MEMs devices, LEDs and optical sensing platforms.

Close up of wafer probe system configured to characterise LED devices. — Close up of probe system configured to characterise LED devices

Semi-automatic equipment for 150mm wafer probe testing optoelectronic devices

For more information on SemiProbe Optoelectronic Probe Stations, please click HERE.

Author

Date

Version

Author

Chris Valentine

Date

10 November 2020

Version

IKB070 Rev. 1

Download

Author Biography

Chris is a Technical Sales Engineer at Inseto, responsible for both front-end equipment and wafers projects within the company.

Lamps for Curing UV Adhesives

9th November 2020

An overview of the different lamp technologies involved in curing UV adhesives (IKB-074).

Historically, wide-spectrum lamps have been used for curing uv adhesives, but with advances in LED technology, using lamps containing Light Emitting Diodes has become more popular over the last few years due to their lower cost-of-ownership.

The term “UV adhesive” is used to denote any adhesive that needs light to cure, and is factually misleading because it refers to adhesives that cure in the Ultra-Violet part of the spectrum AND adhesives that cure in the visible light (VIS) part. The generally-accepted definition of a pure UV adhesive is one that cures between 100 and 400nm, while VIS adhesives cure between 400 and 780nm.

The UV spectrum can be broken down further into Vacuum UV (100 – 200nm), UV-C (200 – 280nm), UV-B (280 – 315nm) and UV-A (315 – 400nm).

Wide-spectrum UV lamps should contain a filter at the lamp output to remove all of these except UV-A, operating in a similar manner to the ozone layer above the Earth that filters out these same UV wavelengths.

Wide-Spectrum Lamps

These will usually emit light between 200nm & 550nm and so are suitable for curing almost all light-cured adhesives on the market. Mercury bulbs, or derivatives thereof, are used in these lamps because they are relatively straightforward to manufacture. As stated above, the bulb works over a wide wavelength spectrum, relying on a filter in the glass casing to remove the harmful aspects of UV light.

When setting up a manufacturing process with a wide-spectrum lamp, factors that need to be considered are: a) the time it takes for the bulb to reach operating strength after switching it on (~ 5 minutes); b) frequent monitoring of the intensity of the light, as new bulbs go through a burn-in process when first installed, meaning that after around 50 hours, the intensity will drop by ~ 20% to its regular operating strength; c) because the bulbs take 15 – 20 minutes to cool down after switching the lamp off, it cannot be switched on again during this period because the mercury in the bulb needs to condense, so switching a lamp back on before this can happen will cause the bulb to crack; and d) the operating lifetime of the bulb is in the region of 1,000 hours (depending on the lamp), after which the intensity drops off considerably and the bulb will need to be replaced.

Wide-spectrum lamps emit ~ 30% UVA light, 10% VIS light, and 40% infra-red light. A consequence of this is that for some adhesives such as light-activated epoxies, it can be an added advantage because these adhesives cure exothermally, and the additional heat generated by the bulb can reduce the cure time. But because these lamps do produce heat, care needs to be taken to ensure that the bulb is properly ventilated. Also, some temperature-sensitive substrates cannot be used with these lamps – they could distort and maybe even give off smoke. Mercury bulb lamps also emit the light in a wide arc from the exit area of the lamp, so some sort of shielding needs to be placed around the workspace to protect operators and any casual passers-by. Coloured Polycarbonate is usually used for this purpose, as most grades of Polycarbonate act as UV blockers.

Mercury Flood Lamp for Curing UV Adhesives

LED Lamps

Unlike wide-spectrum lamps, LED lamps operate at very specific wavelengths, namely 365nm, 400nm or 460nm. 365nm LED lamps will cure adhesives that only contain a UV photoinitiator; 460nm LED lamps cure adhesives that only have a VIS photoinitiator, while 400nm LED lamps straddle the border between the two and will cure most adhesives that contain either photoinitiator. For this reason, 400nm LED lamps are the most popular of the three wavelengths, because they will cure most light-cured adhesives on the market. In addition, 365nm LEDs tend to be more expensive than the others, making the lamps more expensive, sometimes by a substantial amount. It can also make it easier to switch between adhesives, without investing in a new lamp, given that most adhesives can be cured with 400nm lamps.

The other significant difference from wide-spectrum lamps is that, because the heat is removed from the LEDs by either active or passive cooling, these lamps are cold lamps. So there is no cooling-down period when LED lamps are switched off, which results in curing-on-demand for the system – the lamp is only “on” when light is required. Therefore, while a wide-spectrum lamp is switched on for the duration of the production period, say 8 hours a day, it is only actually curing for a fraction of that time. During the same 8-hour shift, an LED lamp will be “on” for significantly less time, resulting in a considerable reduction in energy consumption. Combined with the much longer life time of LED lamps (> 20,000 “on” hours), it makes LED lamps significantly cheaper to operate and results in a quicker return-on-investment. Another consequence of this is that there is no waiting period when an LED lamp is switched on at the beginning of the working day – the typical start time for LED lamps is 0.1ms.

The intensity of light produced by an LED lamp is typically one or two orders of magnitude greater than that produced by a wide-spectrum lamp. LED lamps are capable of generating up to 25,000 W/cm² (measured directly at the adhesive using a DELOLUX 30 400nm LED lamp), while for a wide spectrum lamp the light intensity will be in the region of low hundreds of W/cm², resulting in faster curing of the adhesive, shorter cycle times, and greater UPH when using LED lamps.

Please click HERE for more details on our area, spot and high intensity curing lamps: https://www.inseto.com/adhesives/delo-adhesive-equipment/#adhesive-equipment+category:curing-lamps

Process Control

Whether a mercury bulb lamp or LED lamp is used for curing uv adhesives, it is critical to control the curing process as tightly as possible to ensure consistent results. That means that the intensity of the light that reaches the adhesive is the most important parameter to measure, not the power or the energy of the lamp. Light intensity is defined as radiation performance per area unit, and is typically measured in Watts per Centimetre Squared (W/cm²). This can be controlled by changing the intensity of the light that the lamp produces, assuming that the lamp has this capability, or by altering the distance of the lamp from the adhesive – moving the lamp closer to the adhesive increases the intensity of the light reaching the adhesive, while moving it further away decreases the light intensity. It’s difficult to define this relationship due to a number of factors, but in general, doubling the distance of the lamp from the adhesive reduces the intensity of the light by 30 – 40%, assuming no change in other variables.

The other factor to consider is the frequency of measurements. For a new production line, light intensity should be measure multiple times during each shift, whereas for more mature processes, daily or even weekly measurements can be taken. The type of lamp used will affect this: mercury bulb lamps, because of the wide variation seen in light intensity during the lifetime of a bulb, should have much more frequent measurements taken than for LED lamps, due to their significantly longer lifetime and minimum intensity variation during this lifetime.

Please click on the link for more information on Inseto’s light measurement device, the DELOLUXcontrol.

Light Intensity Meter for measuring UV and Visible Light used to Cure UV Adhesives.

Author

Date

Version

Author

Eamonn Redmond

Date

06 August 2020

Version

IKB074 Rev. 1

Download

Author Biography

What is Wire Bonding?

5th November 2020

This document overviews the different Wire Bonding techniques and processes used for in Semiconductor, Microelectronics and other Electronics and Battery assembly applications (IKB-015).

What is Wire Bonding?

A method of making interconnects between an integrated circuit (IC) or similar semiconductor device and its package or leadframe during manufacturing. It is also commonly used now to provide electrical connections in Lithium-ion battery pack assemblies

Wire bonding is generally considered the most cost-effective and flexible of the available microelectronic interconnect technologies, and is used in the majority of semiconductor packages produced today.

There are several wire bonding techniques, comprising:

Thermo-compression Wire Bonding:
Thermo-compression (combining two similar surfaces (usually Au) together under a clamping force with high interface temperatures, typically greater than 300°C, to produce a weld), was initially developed in the 1950’s for microelectronics interconnects, however this was quickly replaced by Ultrasonic & Thermosonic bonding in the 1960’s as the dominant interconnect technology. Thermo-compression bonding is still in use for niche applications today, but generally avoided by manufacturers due to the high (often damaging) interface temperatures needed in order to make a successful bond.

Ultrasonic Wedge Bonding:
In the 1960’s Ultrasonic wedge bonding became the dominant interconnect methodology. Through the application of a high-frequency vibration (via a resonating transducer) to a bonding tool with a simultaneous clamping force, allowed Aluminium and Gold wires to be welded at room temperature. This Ultrasonic vibration assists in removing contaminants (oxides, impurities, etc.) from the bonding surfaces at the start of the bonding cycle, and in promoting intermetallic growth to further develop and strengthen the bond. Typical frequencies for ultrasonic wire bonding are 60 – 120 KHz.

The ultrasonic wedge technique has two main process technologies:

Large (heavy) wire, for >100µm diameter wires

Fine (small) wire, for <75µm diameter wires

Examples of typical Ultrasonic bonding cycles can be found on our knowledge base documents “Fine Wire Wedge Bond Sequence” for fine wire and “Heavy Wire Wedge Bonding Cycles” for large wire.

Ultrasonic wedge bonding uses a specific bonding tool or “wedge,” usually constructed from Tungsten Carbide (for Aluminium wire) or Titanium Carbide (for Gold wire) depending on the process requirements and wire diameters; ceramic tipped wedges for distinct applications are also available.

Thermosonic Wire Bonding:
Where supplementary heating is required (typically for Gold wire, with bonding interfaces in the range of 100 – 250°C), the process is called Thermosonic wire bonding. This has great advantages over the traditional thermo-compression system, as much lower interface temperatures are required (Au bonding at room temperature has been mentioned but in practice it is unreliable without additional heat).

Thermosonic Ball Bonding:
Another form of Thermosonic wire bonding is Ball Bonding (see the “ball bond cycle“). This methodology uses a ceramic capillary bonding tool over the traditional wedge designs to combine the best qualities in both thermo-compression and ultrasonic bonding without the drawbacks. Thermosonic vibration ensures the interface temperature remains low, while the first interconnect, the thermally-compressed ball bond allows the wire and secondary bond to be placed in any direction, not in-line with the first bond, which is a constraint in Ultrasonic wire bonding. For automatic, high volume manufacture, ball bonders are considerably faster than Ultrasonic / Thermosonic (Wedge) bonders, making Thermosonic ball bonding the dominant interconnect technology in microelectronics for the last 50+ years.

Ribbon Bonding:
Ribbon bonding, utilising flat metallic tapes, has been dominant in RF and Microwave electronics for decades (ribbon providing a significant improvement in signal loss [skin effect] versus traditional round wire). Small Gold ribbons, typically up to 75µm wide and 25µm thick, are bonded via a Thermosonic process with a large flat-faced wedge bonding tool.

Aluminium ribbons up to 2,000µm wide and 250µm thick can also be bonded with an Ultrasonic wedge process, as the requirement for lower loop, high density interconnects has increased.

View further information on this website:

View the range of “MPP Equipment for Wire Bonding“.

View the range of “K&S Automatic Wire Bonders“.

View further information on our suppliers website’s:

MPP Manual Wire Bonders website

Kulicke and Soffa website

Author

Date

Version

Author

Jim Rhodes

Date

05 November 2020

Version

IKB015 Rev. 1

Download

What Is Spin Coating?

4th November 2020

This document overviews the semiconductor fabrication process known as “spin-coating” and explains how it works (IKB-075).

What is spin coating?

Spin-coating is the most widely deployed method for dispensing photoresists and other materials uniformly onto substrates. Spin-coating is used to produce thin films of the desired material with high levels of process control and repeatability.

Photoresist as discussed in the lithography basics knowledge base document is crucial to the photolithography process. Typically, photoresist is a highly viscous material and the uniformity of its coating plays an important role in the reliability of any photolithography process, as well as the resolution achievable. Spin-coating is a technique widely used in research, development and industrial processes, in order to produce specific uniform film coatings.

The principle of spin-coating is that a few millilitres of photoresist are dispended onto the substrate. The substrate is then spun at high speeds in the range of 500 – 4000 rpm. The viscosity of the photoresist is then selected to keep the spin speed in the optimal range whilst producing a coating of the required film thickness. These parameters are usually specified by the resist manufacturer and are specific to the resist used. The other source of information for these parameters is to research the wide body of literature about photolithography and to adapt a published process to your needs.

The photoresist is dispensed at the centre of the substrate prior to the spinning, this is called static dispense. An alternative to this is dynamic dispense, where the wafer is already spinning at the desired speed and to then dispense the photoresist. This is the more commonly used technique when spin speeds are in excess of 1000 rpm.

At these high spin speeds the centrifugal force causes the viscous solution to spread outwards and flow towards the edge of the wafer. At the edge material builds up until the surface tension of the photoresist solution is overcome, at which point the resist is ejected from the spinning wafer. The thickness of this thin film is defined by a number of parameters: spin speed, concentration, viscosity and spin time.

The requirements for uniformity are demanding on the spinning process, as the quality of the film is critical to the number and size of defects in the pattern transferred. Photolithographic processes can require high uniformity, both across a single wafer and from one wafer in a cassette to another. With typical photoresist film thicknesses of 2 µm, this is a uniformity requirement of ±1.0%.

To achieve results with uniformity to this high tolerance the spin speed and time must be precisely controlled, as well as the acceleration of the wafer up to the specified spin speed. In some cases a multi-speed spin protocol is recommended by the resist manufacturer, this is to ensure that the final resist thickness is finely controlled and within the tolerances required for further processing.

Pictorial Representation of Photo Resist Spin Coating

For further information on our range of Spin Coaters, please click HERE

Author

Date

Version

Author

Chris Valentine

Date

04 November 2020

Version

IKB075 Rev. 1

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Author Biography

Chris is a Technical Sales Engineer at Inseto, responsible for both front-end equipment and wafers projects within the company.