ELP300 Gen2 Excimer Laser Stepper

Direct Patterning Solution for Advanced Packaging, FOWLP, Plastic Electronics and the IoT

SUSS MicroTec‘s ELP300 Gen2 is a mask-based stepper for 200/300mm wafers, with an excimer laser (248nm or 308nm) instead of a UV light source. It enables a one-step, dry-etch patterning process in lieu of traditional photolithography. Through a wider material selection, ultrafine resolution, and less process steps, it addresses many of today’s photolithography challenges. That makes it attractive for semiconductor advanced packaging applications like RDL, fan-out and seed layer removal as well as plastic electronics products such as displays, sensors and the emerging IOT.

Unlike solid-state laser systems that rely on heat for ablation, SUSS MicroTec’s excimer laser system uses high photon energy to break molecular bonds in materials for relatively non-thermal patterning. Also, the ELP300 Gen2 uses a precision laser mask to define the pattern. The mask provides a higher degree of pattern integrity, uniformity and placement accuracy, and enables a much higher throughput compared to single-spot solid-state systems.

The ELP300 Gen2 ablates a wide variety of photo-definable and non-photo-definable polymers, organic dielectrics, epoxies, and EMC’s (Epoxy Mold Compounds). Additionally, the ELP300 Gen2 can pattern or remove thin film metals and silicon nitride layers (< 500nm), off the top of organic layers.

Highlights

Mask-based one-step dry patterning alternative to photolithography

Enabling technology for RDL trench & via, thin metal patterning or removal

Allows patterning of non-photo and photosensitive materials

Resolution down to 2.5 µm L/S vias: 5 µm (top) / 3 µm (bottom)

Overlay accuracy ≤ 1.0 µm (mean + 3 sigma)

High throughput – not depending on pattern or density

SUSS MIcroTec Laser Processing ELP300 Gen2

 

When used in micro structuring applications, excimer lasers offer capabilities beyond traditional solid-state lasers as well as common photolithography steppers. This involves using a pulsed laser beam with high photon energy to remove material from a surface. Since only a minimum amount of heat is produced, the process technology is highly suited for temperature-sensitive materials.

In excimer laser processing, masks are used as patterns, allowing a wide variety of complex structures to be created. Similar to the projection stepper in photolithography, an optical system between the mask and the wafer projects the mask image onto the wafer. The projected image working according to a step and repeat system, directly removes the material from the wafer surface and creates the structure.

Suitable Materials (photo and non-photo):

  • Polymers/dielectrics (PI, PBO, BCB, PET)
  • Epoxies and epoxy mold compounds (filled and unfilled)
  • Thin film metals/seed layers (Ti, TiW, Cu, Pd, TaN, Al, etc.)*
  • TCO’s - transparent conductive oxides (ITO, IZO, parylene, etc.)*
  • Others (consult factory)

*Underlying layer should be organic polymer (PI, PET, etc.)

Excimer ablation allows the one-step dry patterning of vias, nozzles, trenches and complex patterns.

Via/Nozzle Drilling

Excimer ablation is an effective way to drill holes with uniform size, shape and placement, superior to solid-state laser drilling. It is excellent for drilling through polymer layers and stopping on metal pads.  As long as the metal pads (i.e. Cu, Al) are >1 µm thick, they act as a natural stop layer so the pads are not damaged.

Sidewall Angle Control

Excimer ablated features like vias, trenches, etc. have an inherent tapered sidewall angle in the range of 70° - 82°depending on material and fluence (energy). In general, higher fluence creates steeper sidewall angles while lower fluence is used to create shallower angles.

Trenches/Complex Patterning

Mask-based excimer ablation is superior for replicating RDL trench and pads, as well as other complex patterns. Depth of the features can be controlled in the sub-micron level. In addition, multi-depth structures can be generated using multiple patterns on the laser mask.

Excimer Ablation Depth Control

The depth control for excimer ablation is based on the etch rate for the specific material, at a specific fluence. For example, the average etch rate at 800 mJ/cm² fluence for material-A is 0.35 µm per pulse. To ablate a blind trench to a depth of 5 µm it should take 14.29 pulses – so 15 pulses would be used.

Excimer ablation is an excellent way to replicate a precise pattern in thin film metals layers (< 600 nm thick). In fact, in most cases it only requires one pulse for material removal. However, take note that if the metal layer, or combination of multiple metal layers, gets close to a thickness of 1µm, it will not ablate.

Material Stack-up

This type of excimer ablation is best performed with the metal layer on top of an organic layer (i.e. polymer); however, it can still be effective in certain cases with underlying inorganic layers.

Example Applications

  • Sensor electrodes (biosensors, diagnostic sensors)
  • Flexible circuits / interconnects
  • Organic thin film transistor backplanes (OFET)
  • Organic photodiodes (OPD)
  • Biometrics
  • Nanoparticle plasmonics: fabrication of spherical Au spheres

Excimer laser is the dry technique without solvent contamination, for removing thin seed layers in semiconductor applications. This method also eliminates the undercut caused by wet chemistry removal, which is even more crucial as critical dimensions get sub-10 µm. Further, when incorporated as part of the excimer enabled dual-damascene RDL process, it allows for addition of a barrier layer with the seed layer to help prevent copper migration.

Best excimer laser seed layer removal applications

  • Thin seed layers: Total metal thickness (i.e. Ti/Cu) ≤ 400 nm
  • Best throughput/CoO using hybrid approach:
    1. Wet-etch removal of Cu
    2. Excimer removal of Ti, TiW, TaN, etc. (≤ 200 nm)
  • Best when used in embedded trench, planarize process such as excimer enabled dual-damascene process

Benefits

  • Fast one-step dry removal of adhesion layer and copper
  • Faster, lower CoO hybrid process (wet for Cu, excimer laser for adhesion layer)
  • Eliminates undercut caused by wet-etch of adhesion layer
  • Prevents or reduces solvent contamination
  • Allows for removal of barrier layer and seed layer
Details: Alternative to Photolithography

Via and RDL Patterning (Organic Dielectrics)

The combination of Excimer laser ablation, better performing non-photo dielectrics and innovative new plating/de-plating technologies (or CMP) enables the successful front-end dual-damascene process to be performed for next generation RDL applications, at a fraction of the process of record (POR) lithography cost.

Excimer laser ablation creates the needed fine resolution, fine-pitch, embedded RDL structures. The use of better performing dielectrics (i.e. non-photo materials) allows for improved thermal, mechanical and electrical reliability, which is further enhanced by the planarization of dual-damascene layers by plating and de-plating of CMP processing.

Excimer Ablation of Polymers and Unfilled Epoxies

Excimer laser wavelengths of 248nm and 308nm have very high photon energies; much higher than solid-state lasers. The high photon energies allow the excimer laser pulse to break chemical bonds directly within the material – this is called photochemical ablation. Photochemical ablation is considered a relatively cold ablation and only has a small secondary thermal decomposition component. Comparatively, CO₂ and solid-state lasers have a very thermal (hot) ablation mechanism which is why they create considerable heat-affected zone, slag and debris.

Polymer Excimer Ablation Mechanism

  1. Excimer pulse absorbed into material
  2. High photon energies break bonds in polymer chain (fragmentation)
  3. The photon energy excites polymer molecules at the surface and causes them to vibrate and create some heat
  4. The generated heat is thermally transferred through the polymer to a limited extent causing additional polymer bonds to be thermally broken
  5. Gases (CO, CO₂) are generated as by-products of the polymer bonds breaking. That gas builds up pressure under the surface of the material
  6. Smaller polymer chain fragments break the surface over the irradiated area
  7. The trapped gases and kinetic energy (shock wave) create a subsequent “mini-explosion” where the fragmented polymer chains are effectively disintegrated and ejected upward at high velocities (collected by vacuum debris cell)
  8. The result is a well-defined cavity in the polymer at the irradiated location with some submicron carbon powder residue on the surface surrounding the ablation area (easily removed by industry cleaning methods)

Excimer Ablation of Silica-Filled EMC (Epoxy Mold Compounds)

Filled EMC’s for advanced packaging applications are available with various silica-sphere filler sizes and fill densities. Another unique characteristic of excimer ablation is that it does not ablate glass (silica) and therefore, not the silica fillers within the EMC. For fine resolution, patterning applications the filler size needs to be very fine or it will block or partially block the opening being created by excimer ablation.

The ablation mechanism for EMC’s is the same as for polymers, above, except in this case as the epoxy component of the EMC is being ablated and ejected from the cavity – the silica filler spheres are being ejected along with the gases and powder plume. This phenomenon increases the ablation rate because filler volume is removed with epoxy volume. The ablation rate for EMC’s can get up to 0.6 – 1.0 µm per pulse.

The ablation of EMC’s results in a defined cavity with silica sphere faces protruding from the sidewalls and bottom; making the surfaces less smooth. An industry standard descum step usually removes most of the spheres protruding from the surface – making it smoother.

Excimer Thin Film Metal Patterning and Removal

For simplicity, the removal of thin metals and seed layers using excimer lasers can be referred to as ablation. However, it is actually spallation because excimer lasers cannot break the strong molecular bond in metals. Therefore, the ablation mechanism is different from polymer ablation and usually only requires a single pulse for material removal.

Thin Metal Excimer Ablation (Spallation) Mechanism

  1. Excimer pulse absorption (in top skin layer only; the light is blocked by the sea of electrons whirring around in the conductive metal).
  2. Photon energy is immediately transferred to electrons and molecules are excited into vibration creating heat (no bond breaking here).
  3. The thermal conductivity of the metal facilitate the transfer of heat from the surface – down to the bottom at the metal-polymer interface
  4. If enough heat is transferred to interface to thermally decompose the polymer bonds, then they are broken and generate gas by-products (CO, CO₂).
  5. The trapped gases create a separation at the dissimilar material interface and pressure builds up.
  6. The immediate adsorption of the excimer pulse energy generates a downward shockwave that travels through the metal to the interface.
  7. The shockwave energy hits the polymer surface barrier. It is deflected back upward driving the weakened, separated metal layer upward.
  8. The subsequent “mini-explosion” tears the metal into tiny fragments, ripping it from the edges of the non-irradiated boundary areas.
  9. The gases and fragmented metals are ejected upward at high velocity and collected by the tool’s vacuum debris cell.
  10. The result is a well-defined area free of metal with no discernable damage to the underlying polymer layer; except at the beam-stitch boundaries where fine lines with submicron depth are created. These are cosmetic defects which have proven not to cause any negative impact on device functionality.

Seed Layer Removal Applications

For advanced packaging seed layer removal, the ablation mechanism is same as above with the difference that the ablation area on the substrate can contain a combination of seed layer areas and RDL structures. The RDL structures are thicker than 1µm, so the excimer irradiation cannot remove them. However, wherever the thin seed is located in the ablation area it is removed. The result is an area where undamaged RDL structures stand with the seed layer around them removed.

 

Details: ELP300 System Overview

The ELP300 Gen2 excimer ablation system is comprised of the following major assemblies that are integrated into a turnkey solution:

      •  
  • ELP300 Ablation Tool
  • Optional EFEM for Automatic Load/Unload configurations
  • Excimer Laser
  • Class 1 Laser Safety Chamber
  • Environment Control Unit (ECU)

Laser Choices

The ELP300 Gen2 comes with the choice of industrial excimer lasers (248 nm or 308 nm) at various performance and cost ranges. SUSS MicroTec will help to select the proper excimer laser for the intended purpose (i.e. R&D, pilot production, HVM production).

Key Performance Specifications

  •  
  • Wafers: 300 mm (200 mm optional)
  • Resolution down to 2.5 µm L/S
  • Vias: 5 µm (top) / 3 µm (bottom)
  • Optical TSA overlay:  ≤ 1.0 µm  (mean + 3sigma)
  • IR or optical BSA: ≤ 2.5 µm  (mean + 3sigma)
Details: Handling

The standard ELP300 Gen2 is a bridge-tool for 200 mm and 300 mm wafers. The primary tool configuration is for automated handling of 300 mm wafers, optionally for 200 mm wafers. A manual loading and unloading option is available, along with other wafer and substrate sizes.

Automatic Wafer Handling (200 mm and 300 mm)

The system includes a well-appointed equipment front-end module for 300 mm front opening unified pod with dual load ports, a dual arm robot, end-effectors and a pre-aligner (notch finder). For 200 mm wafer handling, either 200 mm front opening unified pod inserts or 200 mm open cassette adapter boxes are available. SEMI standard wafers and bonded wafer pairs.

Wafer Chuck

Aluminum vacuum chuck with dual-zone vacuum for 200 mm and 300 mm wafers standard.

Warped Wafer Handling

Standard and custom warped wafer solutions are available on the ELP300 Gen2 for up to 2 mm bow- standard and optionally up to 5 mm bow.

These solutions primarily consist of the proper choice of wafer chuck and robot end-effector designs.  SUSS MicroTec will test specific warped wafers for customers to confirm the performance its standard solutions; or develop a custom solution, if necessary.

Optional Handling

Please consult factory regarding solutions for Thin Wafers, TAIKO Wafers, Smaller Wafers (6”, 4”, 2”), and Substrates (non-round, up to 300 x 300 mm)

Features & Benefits

  • Fast, safe and reliable handling of  wafers and substrates
  • Flexible for size, type and thickness

The standard ELP300 Gen2 is configured for manual mask loading/unloading. Automated mask loading/unloading system optional.

Laser Masks

Mask Size: 7” x 7”

Materials: Aluminum coating on quartz

Mask Stage

Motorized aluminum vacuum mask stage automatically self-registers mask upon loading. Mask stage travels in one-direction allowing for changing of mask patterns, depending on pattern size.

Automatic Mask Handling System Option

  • Includes automated loading/unloading from mask cassette
  • Mask cassette for up to six masks
  • Cassette manually loaded by operator

Features & Benefits

  • Fast, safe and reliable handling of  laser masks

The ELP300 Gen2 uses a single air gauge sensor to complete 3-point level measurements across the substrate. The sensor is routinely and automatically calibrated to the same reference source to maintain high precision measurements.

Features & Benefits

  • Improved CD uniformity

The ELP300 Gen2 calculates any needed wafer leveling adjustments from the measurement data provided by the air gauge sensor (see height measurement system). If the system determines that a leveling and/or focusing z-axis adjustment is required, the height and tip/tilt of the z-axis stage are changed by three motors. 

Features & Benefits

Improved focus and leveling results in improved imaging and CD uniformity

Details: Optical Alignment

The ELP300 Gen2 comes equipped with a state-of-the-art off-axis optical alignment system capable of 2-Target and 4-Target global alignment. Target locations can be anywhere on the substrate.

System includes one off-axis vision camera and SUSS MicroTec enhanced Cognex VisionPro® Pattern Recognition.

Optical top-side alignment accuracies:

Alignment accuracy: ≤ ± 0.7 µm 3sigma

Overlay accuracy: ≤ 1.0 µm mean + 3sigma

Features & Benefits

  • Precise substrate alignment for high yields
Details: Automation

The ELP300 Gen2 is designed for integration into a fab automation system compatible to the SECS-II/GEM interface standards. Level and details of the communication will be specified on basis of the SUSS MicroTec core software solution.

The ELP300 Gen2 includes a fully interlocked Class 1 laser safety enclosure with a clean-room compatible Environmental Control Unit (ECU), vertical HEPA airflow and temperature control inside the chamber to ± 0.2°C.

Features & Benefits

  • High level of protection for the operator
  • High level of protection for the process - from the operator and environment
Options: Optical Alignment

The ELP300 Gen2 is available with a 1st layer edge alignment option to improve wafer centering beyond the standard pre-alignment wafer centering of ≤ ± 250 µm 3s. The option includes modifications to the wafer chuck to enhance edge contrast when viewed by the standard optical vision camera.

The system will view 4-locations along the edge of the wafer and use the resulting coordinates to calculate the best fit center location and subsequent alignment adjustment (if necessary).

Edge Alignment Accuracy Option

Wafer centering accuracy improved to ≤ ± 25 µm 3s

Features & Benefits

  • Precise substrate centering for higher yields

The ELP300 Gen2 is available with multiple options for IR alignment. View back-side alignment target through silicon wafer from above:

  • Reflection mode: IR illumination from above wafer
  • Transmission mode: IR illumination in below wafer (IR LEDs mounted inside wafer chuck)

IR alignment accuracy: ≤ ± 1.0 µm 3sigma

IR overlay accuracy: ≤ 2.5 µm mean + 3sigma

Features & Benefits

  • Flexible solutions for opaque resists and back-side alignment

View targets on backside of wafer from below using a set of two dedicated optical vision cameras and corresponding reflective optics. Backside target locations subject to fixed positions within 10 mm of wafer’s edge.

IR alignment accuracy: ≤ ± 1.0 µm 3sigma

IR overlay accuracy: ≤ 2.5 µm mean + 3sigma

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Publications

Ultra-Small Via-Technology of Thinfilm Polymers Using Advanced Scanning Laser Ablation

Ultra-Small Via-Technology of Thinfilm Polymers Using Advanced Scanning Laser Ablation

Excimer Laser Ablation – A Novel Patterning Solution for Advanced Packaging

Excimer Laser Ablation – A Novel Patterning Solution for Advanced Packaging

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Excimer Laser Via-Drilling - Options to Further Capabilities of Next Generation Wafer Level Processing Devices

Laser Ablation – Emerging Patterning Technology for Advanced Packaging

Laser Ablation – Emerging Patterning Technology for Advanced Packaging

New Excimer Laser-Based Dual Damascene Processes for High I/O Applications with Ultra-Fine Line Routing

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Excimer Laser Ablation for Microvia and Fine RDL Routings for Advanced Packaging

Excimer Laser Ablation for Microvia and Fine RDL Routings for Advanced Packaging

Creating Planar Embedded RDL Structures Without CMP

Creating Planar Embedded RDL Structures Without CMP

Technical Publications
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