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Automated dispense systems for small substrate applications

  
  
  

Many research laboratories and institutes use spin-coating technology to cost-effectively create thin-film coatings with precise thickness and uniformity control. In many cases, the most significant initial costs of development work are for the semiconductor-grade substrates. Consequently, many spin-on application projects may use irregularly shaped wafer pieces, microscope slides, and/or wafer die (1 cm × 1 cm) in early development work. Material deposition is typically performed with handheld syringes, manual pipettes, or more sophisticated digital repeater pipettes.

As process optimization begins to accelerate, the need to eliminate human variables becomes increasingly critical. Transferring the application method from a handheld dispense technique to a fixed mechanical nozzle with microprocessor control is a primary area of focus. Implementing automatic dispense options provides a feasible pathway; however, many variables must be considered when pursuing this option. An ideal solution combines the programmable control provided by an intelligent spin-coating (host) system with a disposable syringe barrel. This configuration is inherently capable of minimizing material consumption, mitigating cross-contamination, and providing precise programmable control.

auto dispensed used for spin coating

One such system is the disposable syringe dispense system (DSD-1), offered by Brewer Science. This system consists of a stainless steel/anodized aluminum housing with a proprietary design that features a mechanical roller suckback that ensures a positive clamp for drips and completely seals the nozzle tip from exposure to open air. This system provides extremely accurate, repeatable dispense rates and volumes. Each syringe accommodates various sizes of barrels and typically holds 30 cc or 55 cc of material. Various barrels are available, including transparently clear for general-purpose applications, light-sensitive amber, and opaque black (lightproof). Barrels also feature precision mated pistons that mitigate trapped air and enhance shot size uniformity. The DSD-1 system utilizes a proprietary flexible 16-gauge ID dispense luer tip and is compatible with viscosities up to 400 cP with a standard N2 or CDA pressure of 30 psi. With higher N2 pressures (to a maximum of 60 psi), the DSD-1 can support viscosities up to 13.5 kcP and dispense rates of 0.3 cc/s. The spin coater host software provides discrete control of the dispense timing to the nearest 0.1 second and combines with syringe pressure to control the dispense volume.

Auto Dispense used with spin coater

Syringe barrels are self contained, allowing users to interchange various process materials in a matter of seconds. The syringes are easily removable for filling and weighing and can be capped for storage over several days or weeks. The entire wetted assembly can be cleaned and reused and/or disposed of and replaced for the next chemical application. Inline Whatman™ Puradisc syringe filters are compatible with the syringe barrels and are available in a wide array of media types, pore sizes, and flow rates. They are simply inserted between the pressurized syringe barrel and the dispense tip. Common filter media types include Teflon®, polypropylene, nylon, and Kynar, and pore sizes include 0.05, 0.1, 0.2, 0.45, 1, 5, 10, and 25 µm. Optimal setup procedures include prewetting all PTFE Teflon® filters with solvent before plumbing on the process material. Prewetting will optimize the flow characteristics of the material through the filter and mitigate the risk of trapped air. The solvent purging process should be used in combination with opening the cartridge valve and purging any residual trapped air. Always invert the position of the filter to allow the output to flow from the top of the barrel.

Auto Dispense for Spin Coater

The DSD-1 dispense nozzle housing mounted in the lid of a Brewer Science® Cee® spin coater will accommodate multiple automated and/or manual syringe dispense options. A maximum of three other automated cartridges, pressure cans, syringes, or positive displacement or diaphragm pumps can be mounted simultaneously. With the Brewer Science® Cee® equipment user-friendly design, dispense recipes are easily entered, monitored, and stored through our stand-alone Windows®-based graphical user interface. Please refer to the Brewer Science® Cee® spin coating equipment landing page for additional information and detailed tool specifications.

Whatman™ is a trademark of GE Healthcare Companies. Teflon® is a registered trademark of E. I. du Pont de Nemours. Windows® is a registered trademark of Microsoft.

Process methodologies for temporary thin wafer handling solutions

  
  
  

Use of temporary bonding/debonding as part of thin wafer handling processes is rapidly increasing. Users must determine which temporary bonding/debonding method is appropriate for a specific application. Some of the technologies that have been developed are chemical release, thermal slide separation, and ZoneBOND® debonding.

types of debonding

The chemical release process requires very little force to release the thinned device wafers from carriers and is recommended for low-volume, small-format, and compound semiconductor (CS) III-V materials.

The thermal slide debonding process is ideal for small-format applications with throughput requirements of 500-600 wafers (50-150 mm in diameter) per week and with thermal budgets up to 220°C.

The ZoneBOND® process is capable of separating large-format wafer pairs (with diameters of 200 mm or larger) containing large topography and/or perforations. It is also compatible with all wafer sizes, thicknesses, materials, and surface topographies.

Download the white paper to learn more about these three thin wafer handling process methodologies.

 

Thermal slide debonding for temporary bonding processes (Part 3 of 3)

  
  
  

Thermal slide debonding represents the next significant advancement in obtaining high-yield thin wafer results. Initial detection of anomalies and cracks usually occurs during debonding; however, many causes for this damage originate during upstream bonding material coating, curing, bonding, and thinning processes. Moreover, only thermal separation tools that are highly precise and highly accurate will consistently render desirable process yields. The bonded pair is subjected to many thermal and compressive forces during processing and debonding. The thinned device layers are often very sensitive to outside factors including temperature, vacuum, and mechanical compression and release.

For debonding, relatively speaking, silicon materials are inherently more flexible than III-V compounds and allow greater control tolerances. Consequently compound semiconductor (CS) materials require significantly tighter control of platen temperature uniformity, pull force, vacuum fluctuations, platen separation, and platen co-planarity. The Cee® 1300CSX thermal slide debonder (see image below) has been specifically designed to meet and exceed these specifications.

 1300 CSX debonder

 

In this tool, wafer stacks are processed with the thinned device wafer orientated to contact the upper chuck and the carrier substrate is held in place by a vacuum emanating from the lower chuck. A manual insertion tool accurately aligns the bonded pair onto the lower platen lift pins. The lift pins are programmed electronically and provide precisely controlled acceleration for bringing the substrate to the desired set point temperature for heating the thermoplastic bonding material to the appropriate temperature to achieve a viscosity of <300 Pa·s. The following steps outline the standard process flow for thermal separation (see diagram below).

thermal slide

 

Operating the debonder involves the following processing steps:

  1. Load the debonding recipe (platen temperatures, pull force, thermal dwell times, electronic lift pin (ELP) positions, platen positional search windows, vacuum sensing threshold, etc.).
  2. Confirm the stack configuration (diameter and height/thickness). Recipes must match these physical characteristics.
  3. Using insertion tool, load the wafer onto the lift pins.
  4. Initiate the debonder recipe.
  5. The first recipe step is adjusting the lift pin positional height to allow a gradual increase in temperature for thermally sensitive materials.
  6. After achieving the appropriate thermal stabilization, the pre-bake step concludes, and the lower platen moves from the load to the press position.
  7. Upon reaching the press position, the platen is raised to the appropriate height to a predetermined vacuum search position.
  8. Once the minimum vacuum threshold is achieved, the system will enter the thermal stabilization step to ensure minimal temperature gradients.
  9. After the minimum temperature gradient is reached, the lower platen will pull in the x direction within preprogrammed force and speed limitations. During this pull, the following variables are monitored, controlled, and logged: z position, x position, force, speeds, upper and lower vacuum, upper and lower temperature, and process sampling time. The consistent profile of these variables is evidence of appropriate debonding parameters and may be used as a statistical process control.
  10. Upon debonding completion, the lower platen will drop and move to the load/unload position.
  11. The vacuum will shut off, and the lift pins will rise to the unload position for removal of the carrier.
  12. A specialized Gel-Pak® end effector is then inserted into tool under the upper platen and raised to a minimal distance (<2 mm) from the device wafer. Vacuum is released on the upper platen, and the wafer is gently transferred to the top surface of the Gel-Pak® end effector.

 

 thinned device

Transferring the intact device to a post-debonding spin-cleaning tool is the next critical step. The most common transfer method is outlined below:

 

  1. The device wafer and Gel-Pak® end effector are removed using the extraction tool assembly, and the entire Gel-Pak®/thinned device wafer structure is flipped onto a full sized porous ceramic spin chuck. Vacuum is applied through the spin chuck, and the device is released from the Gel-Pak® layer for cleaning.
  2. The remaining bonding material is removed from the device in a spin-cleaning process similar to a stream-puddle develop process. Megasonic cleaning is also a popular, safe method for cleaning residual bonding material to decrease cycle times and reduce solvent consumption (see photo below).
  3. After being cleaned, the device is lifted from the spin chuck with another clean Gel-Pak® carrier and mounted to a film frame on dicing tape.

 gel pak

megasonic cleaning

device wafer

 

Brewer Science is uniquely positioned to seamlessly integrate the materials, processes, and machines for adopting a precision temporary thermal bonding/debonding application. Our product portfolio includes high-temperature temporary bonding materials (WaferBOND® HT-10.10 material), precision spin coaters (Cee® 200X and 300X tools), bake plates (Cee®1300X and 200CBX tools), temporary bonders, debonders (Cee®1300DB and 1300CSX tools), thin-wafer transfer tooling, and cleaning tools (Cee®200XD, 300XD, and 300MXD tools). Please contact us today to learn more about these advanced technologies and products for overcoming daunting process challenges. We are standing by to assist you.

Gel-Pak® is a registered trademark of Gel-Pak LLC.

Thermal slide debonding for temporary bonding processes (Part 2 of 3)

  
  
  

In addition to precisely controlling application of the materials that enable wafer bonding, a solvent-enriched sealed spin chamber contributes to process integrity. One of the most critical variables in achieving optimal uniformities at the desired target thickness is airflow dynamics. Ideal conditions are created in a sealed chamber with a prewet solvent nozzle, a backside rinse, a lid gasket, a splash ring (air-flow baffle), a programmable exhaust, and center-stream bonding material delivery. Radial and reverse-radial scanning dispense arms are not recommended because they require open bowl environments. Additionally, the scanning technique has not demonstrated significant advantages related to uniformity or material conservation.

A sealed bowl chamber (see bowl image below) improves material consumption, casting characteristics, and coating uniformity. A closed bowl environment combined with a programmable exhaust module allows the solvent vapor concentration to be precisely controlled at various stages in the spin-coating recipe.

programmable exhaust

Closed lid with programmable exhaust

Prewet dispense techniques deliver nominal amounts of base-solvent at the onset of depositing the bonding material. This dispense step serves the dual purpose of preparing the surface and increasing the initial bowl vapor concentration. Generally, exhaust is set at 0% (see solvent-enriched environment figure below) during the dynamic dispense and casting portions of the spin recipe. Exhaust flow is typically throttled to 50-100% (see open exhaust environment figure below) during the subsequent drying steps for adequate vapor removal.

solvent enriched environment

Solvent-enriched environment

open exhaust

Open exhaust environment

The lid gasket and splash rings seal the chamber and shape the exhaust flow for ideal edge coating. The gasket and rings also mitigate the risk of materials contacting the straight sidewall and redepositing on the upper device area of the substrate.

The backside rinse also helps prevent potential downstream bake plate, bonder, and debonder platen contamination by removing any residual bonding materials that collect on the rear side of the coated substrate.

Proper baking control is essential to achieve void-free media layers and overall high process yields for thick films. After the spin-coating/drying procedure, residual solvent will remain in the bonding material layer. Multistage bake cycles regulate the solvent evaporation rate and mitigate the risk of a “skinning effect.” Skinning results when the outer, exposed layers dry at a faster rate than lower, interior layers. Once skin forms, evaporating solvent below that skin can form blisters in the coating. This phenomenon can be observed if the film is not baked at multiple setpoints (120º-180ºC) and/or is too aggressively ramped to the respective baking temperatures.

During the backside processes (previously depicted in Part 1 figures) and in the thermal debonding process, failure to properly drive off residual vapors can also lead to vapor flash, greatly increasing the risk of device cracking. The use of two high-uniformity bake plates, featuring pneumatically controlled lift pins to permit proximity bake, soft contact bake, and hard contact bake methods (depicted in the figure below), are recommended for optimal curing results. Utilizing an initial proximity bake method with a nitrogen (N2) pillow lowers the risk of the substrate physically contacting the bake plate and receiving a thermal shock. The slower heating ramp also reduces the risk of blistering and cracking the outer, exposed film surface.

 

methods

N2 proximity, soft contact, and vacuum hard bake methods

A higher level of precision can be attained by using electronically controlled programmable lift pins. The electronic lift pins provide the user with specific proximity heights above the surface in any sequence or combination. In Brewer Science® Cee® bake plates, the heights are programmed in 0.001-inch increments, with an overall operating window from 0.001 inch through 0.750 inch (± 0.002 inch). This feature allows for more controlled temperature ramping and can emulate several bake plate temperatures with a single bake plate. This feature is also extremely valuable for safely handling thermomechanically sensitive materials, such as GaAs, InP, GaN, SiC, and sapphire substrates, because it reduces thermal shock.

lift pins

Thermocompressive bonding is often either overlooked or underestimated as a trivial step in the process flow. Precisely controlling process temperatures, achieving platen co-planarity, and maintaining the physical proximity gap between carrier and device during full evacuation (<5 mTorr) prior to physically joining the two substrates (see image below) are all critically important for successful thermocompressive bonding. Failure to control these parameters can lead to trapped pockets of air and solvent, which create voids. Scanning acoustic microscopes can reveal the existence and magnitude of such voids. These voids represent areas of non-uniform support during post-bonding backgrinding and could potentially cause adherence failure during backside processing and debonding processes. Close attention to the recipe parameters and carrier/device orientation is required to achieve a viable material bond line between wafers.

 

bonded wafers

 

Loading the bonding material–coated device in the lower stack position with the uncoated carrier immediately above is highly recommended. This loading position will reduce the risk of the bonding material making physical contact with the proximity separation pins during evacuation. Such contact could cause subsequent contamination of the next stack and also require excessive cleaning of the equipment.

Post-bonding thinning to thicknesses less than 100 µm requires the utmost in precision controls and programmability. Qualified vendors such as DISCO Corporation and Okamoto Corporation should perform this portion of the rough backside grinding process. Following the mechanical rough grinding procedure, post-thinning polishing is recommended to eliminate process-induced subsurface microcracks. SEM images clearly illustrate the presence of countless cleave planes resulting in non-uniform surface stress and warpage. A subsequent mechanical ultrafine grinding or chemical etching process can be used to remove this stress prior to additional thermomechanical processes.

backgrinding

Thermal slide debonding for temporary bonding processes (Part 1 of 3)

  
  
  

The microelectronics industry is rapidly migrating to fabricating 3-D wafer stacking interconnects using through-silicon via (TSV) technology. Major market segments seeking to benefit from TSV technology include advanced packaging for memory/logic, light-emitting diodes (LEDs), and compound semiconductor (III-V) high-power radio-frequency (RF) devices. In this cutting-edge technology, fragile device substrates are bonded to carrier substrates with polymeric bonding materials for uniform support during backgrinding (thinning) processes. Device wafers containing various topographies, including etched topographies, high-aspect-ratio structures, trenches, and bored holes throughout the active component area, are first coated and planarized using spin-on bonding materials. Each coated device wafer is then mounted to a rigid carrier and thinned, usually to a thickness of less than100 µm, for further processing. The bonded pair, including the bonding material layer, will be subjected to a wide variety of thermo-mechanical stresses generated during backside processing to thin the device wafer and create electrical input/output redistribution layers (Figures 1 through 5). An effective temporary wafer bonding solution is expected to provide complete support, retain bond strength, and remain soluble for relatively low-temperature (>150°C) separation and cleaning. Temporary bonding process steps include the following:

  1. Coating the bonding material on device surfaces
  2. Baking the bonding material layer (by baking at temperatures of ≤180°C)
  3. Thermo-compressive bonding (at temperatures of ≤180°C)
  4. Mechanical and chemical backgrinding
  5. Post-thinning polishing to mitigate micro-crack propagation (also called stress relief etching)
  6. Photolithography (including baking at temperatures of ≤180°C)
  7. Plasma etching/reactive ion etching (RIE)
  8. Resist stripping
  9. Dielectric deposition (150°-220°C SiO2process)
  10. Seed layer deposition (by sputtering 220°C copper)
  11. Electroplating (nickel)
  12. Thermal debonding (at temperatures of 150°-190°C)
Tempory Bonding

 

During these process steps, the bonding material must remain adhered and provide sufficient support to the entire thinned device. To obtain consistently high process yields and throughputs, all upstream and downstream processes must be precisely controlled, often by very specialized equipment and tooling to achieve accurate and repeatable results. Coefficient of thermal expansion (CTE) mismatch poses a considerable threat to process integrity by inducing thermal stress. Using device and carrier materials with matching CTEs should be strongly considered. When the carrier and device CTEs cannot be matched, selecting materials with the closest possible CTEs will reduce risks associated with thermal stresses. Additionally, the carrier wafer retains greater structural integrity and will dominate the wafer bow effect. The thermoplastic bonding material layer provides a degree of compliance to relax this internal stress, and thicker bond lines (50-100 µm) work to amplify this effect.

Edge-trimming device wafers and/or using oversized carriers provides loading alignment tolerance and lessens the potential for knife-edge creation along the wafer’s edge, which creates a non-uniform radius and is the main source of chipping and subsequent crack propagation. Many issues related to this type of yield loss can be averted by using edge-prep techniques to replace the rounded edge with a straight wall. Removing 0.5-1.0 mm of the total diameter [of the device wafer?], depending on the overall wafer substrate size, is recommended.

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Brewer Science bonding materials are spin applied and require a precision spin coater with environmental chamber controls. Because of the relatively high viscosity of the bonding materials, standard (dip tube) pneumatic dispenses are not recommended for medium-volume, prototyping environments. Gravity-fed pneumatic reservoirs are viable for R&D and low-volume laboratory settings; however, manually pouring and/or transferring the material can result in the infusion of air, which creates microbubbles, and will detrimentally affect suck-back control. Thick-film bonding materials require significantly higher pressures to achieve nominal flow rates, and the overall pressure is directly proportional to the amount of gas dissolution. Recommended flow rates are >1 ml/s. Although bonding material volume will depend on the desired coating thickness, a good rule of thumb is to start with a minimum of 1.5 ml of bonding material per every 100 cm2 of coated surface area.

The ideal equipment for material delivery is a positive displacement pump system. These pumps have the significant advantage of not exposing the material to pressurized gas during fill, dispense, suck-back, and refill sequences. The pumps utilize stepper-motor-controlled diaphragms and provide optimal shot size, dispense rate, suck-back and refill accuracy. One top-of-the-line option, the Cybor 610 pump, provides a specialized recirculation feature to assist in outgassing during the initial purging operation. A reservoir system especially suited for high-viscosity materials is another beneficial accessory to mitigate the introduction of air bubbles during bottle-changing procedures. The system is installed upstream from a high-viscosity pump and features a capacitive sensor that monitors the liquid level and will auto-fill before air can reach the pump source line.

cybor

Cybor 610 and 5000 positive displacement pumps

Programmable bake plates and electronic proximity lift-pins

  
  
  

 Since the dawn of the microelectronics industry, application engineers have required the ability to uniformly cure photosensitive materials. Particularly, curing wet-developable spin-applied films evenly across the substrate surface has presented a technical challenge. Such curing has become increasingly difficult with the introduction of large format substrates (200- to 300-mm) and the continual decrease in critical dimensions. Uniform baking across the substrate surface is critical to reduce over-curing, which causes "scumming," and under-baking, which causes undercut erosion and pattern collapse.

Cee Spincoater

High-uniformity precision bake plates have successfully achieved the level of performance needed to overcome these hurdles. These bake plates include features such as programmable bake times and multiple bake methods (contact, proximity, and vacuum) and are capable of maintaining temperature uniformity of ± 0.3% within the active working surface.

Multiple bake methods are performed with the use of a chuck having a series of concentric hole patterns that deliver either vacuum or nitrogen proximity gas. The concentric patterns determine the active size-specific (2 inches to > 200 mm) areas for N2 gas or vacuum distribution. These ports serve a dual function by supplying N2 for proximity baking and by floating the substrate into the active process position during the loading and unloading bake recipe sequences.

Graph1

N2 Proximity Contact: In this method, the substrate is floated on a self-leveling “pillow” of nitrogen at about 0.006-0.020 inch from the bake plate surface during baking. The gradual heating of the substrate reduces the blistering and cracking of films that sometimes results from the use of fast-drying solvents. This bake method can eliminate the need for two separate bake plates with similar temperature set points (within 10°C). Proximity baking with an N2 pillow mitigates the risks associated with physical contact between the substrate and the bake plate and has been adopted for many photomask and display processing applications. The N2 gas pillow accurately aligns the substrate against the vertical centering stop pins during the loading process.

Gravity Soft Contact: In this method, the substrate is held against the surface of the chuck by gravity. This style provides an intermediate method between hard vacuum contact and proximity bakes where a controlled multiple-step set point acceleration ramp is required. This style is very flexible and is compatible with any substrate size within the working surface area of the bake plate (1 inch from the surface plate perimeter).

Hard Vacuum Contact: This method is the most accurate baking method for bake plates and uses vacuum ports to ensure intimate contact between the substrate and the bake plate. This style ensures baking uniformity and minimizes warping and/or bowing of the substrate.

Additionally, a bake plate should maintain minimal variation from the temperature set point during loading and unloading of a substrate. This objective can be achieved by utilizing a large thermal mass in the bake plate surface. Although critically important to minimizing temperature variation, this large mass typically limits the speed of the set point acceleration. Multiple bake plates are generally utilized for spanning large ranges of set point temperatures.

Brewer Science® Cee® bake plates equipped with programmable ramping temperature controllers and compatible software can give a single bake plate the ability to ramp from low to high temperatures with precise soak times and up to ten incremental temperature step changes. This feature is limited to temperature increases up to about 50°C, and requires significant cooling times between bake recipes.

Brewer Science® Cee® bake plates incorporate a sophisticated technology that can address this handicap by utilizing program-controlled electronic lift-pins. This option utilizes accurate stepper motor control (±0.002 inch) that will drive the lift-pins to 100 specific proximity process heights above the baking surface in any sequence or combination. The heights are programmed in 0.001-inch increments, with an overall operating window from 0.001 inch through 0.750 inch. Traditionally, this technology was only available on million-dollar track equipment. Recently, this capability has been adapted to a compact benchtop version that is intended specifically for prototype-scale production. This flexible feature enables faster ramping acceleration/deceleration and emulates several bake plate temperatures simultaneously, while maintaining a high degree of bake uniformity. This feature is extremely valuable for the safe handling of thermomechanical shock-sensitive materials such as gallium arsenide, lithium niobate, indium phosphide, gallium nitride, silicon carbide, and sapphire substrates.

screen wafer bake plate

Brewer Science® Cee® equipment engineers have utilized the KLA-Tencor® SensArray measurement probe to measure, track, and record temperature conditions produced by Cee® bake plates. The engineers have meticulously performed these process trials and developed user-friendly temperature matrixes for a variety of common temperatures used for a soft bake (100°C), post-exposure bake (PEB), and post-develop hard bake (205°C) for final curing.

ProgrammableBakePlates4

The following chart represents a temperature matrix reflecting a surface temperature of 200°C, and incremental descent from 0.600 inch to hard vacuum contact in downward steps in increments of 0.100-0.020 inch. Each positional height is allowed to stabilize for a period of 300 seconds (5 minutes), and uniformity is recorded at these heights.

graph

By eliminating the need for multiple bake plates, this programmable system is a cost-effective option in a space-saving design. This configuration is ideal for performing multiple set points (dual stage), baking "from the inside out" for thick-film materials such as MicroChem SU-8 materials, MicroChem KMPR® materials, Shipley BPR™-100 photoresist, and Brewer Science® WaferBOND® HT-10.10 materials, and mitigating concerns associated with the "skinning" effect.

The Brewer Science® Cee® high-uniformity bake plate product family brings together all these capabilities in a compact footprint designed for a lab-scale environment. Click here to learn more.

Developer options for spin-on photosensitive materials

  
  
  

Developing photosensitive film layers to produce features of targeted sizes is a critical process step within any photolithography application. Application engineers have created several processes for performing this step with tank immersion (that is, a bath) and/or several adaptations of spin developing a single wafer to make patterns of features based on film areas of differing solubility. The use of immersion tank processes has steadily declined in MEMS fabrication and advanced lithography over the past decade due to excessive material consumption, non-uniform resolution, and poor clearing from high-aspect-ratio features due to insufficient agitation. Additionally, increased throughput requirements and smaller critical dimensions (CDs) have further shifted mainstream applications to single-wafer (track) spray/puddle process flows. 

 

The standard developer materials consist of low-concentration (< 3%) bases and acids (TMAH, KOH, lactic acid in aqueous solutions). Other popular solvent-based photoresists, dry films, and polyamides are typically resolved and rinsed with polar solvents including propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monomethyl ether (PGME), acetone, and isopropanol (IPA).

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The most common single-wafer process is designed to deposit the developer solution onto the center of the wafer surface through a stream puddle technique. The wafer is then spun at low speed (0-50 rpm). The rotation is then stopped and held at 0 rpm to enable the material to “puddle” and allow the chemical reaction to actively dissolve the soluble areas of the patterned film. This process is very popular for thin films ≤ 1 μm thick and/or for small wafer applications. For thick films and/or high-aspect-ratio features, the stream puddle step may be repeated over several iterations to reapply fresh developer. However, because this method of development is inherently non-uniform, it can be problematic for many thick-film and large-surface-area processes. Often features along the wafer perimeter are underdeveloped, and center features are often over-exposed.

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The use of side-angle spray nozzles significantly enhances fluid deposition uniformity. The standard configuration utilizes two side-spray, V-line spray nozzles to evenly apply developer solution and deionized (DI) water across the substrate simultaneously. Some configurations use an open UHMW polyethylene lid with spray nozzles mounted either outside the wafer plane, spraying inward from the center of the wafer out (puddle spray), or directly over the substrate for continuous (direct) spray applications. The side spray nozzles are factory positioned outside the wafer plane (side orientation) and ensure uniform deposition for all substrate sizes (2-8 inches). The spray nozzles can also be used in a puddle developer application by quickly applying material to the substrate at a slow speed and then reducing the substrate speed to 0 rpm. Programming and setup options will also enable continuous-spray applications with the omission of all static (0 rpm) steps during the recipe and spray continuously throughout processing steps.

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Many of the latest specialized lift-off resists and dry films (MicroChem LOR and SU-8 resists and Shipley BPR™-100 resist) are not compatible with spray/stream puddle development and require immersion, continuous direct-angle spray, and/or megasonic-assisted processes. Immersion methods are less than ideal and are not preferable for the reasons mentioned above. Direct-angle nozzles are often used for continuous-spray applications and allow accelerated development of thick films (5-100 μm) with features having high aspect ratios. The direct angle provides sufficient agitation to penetrate the film and remove soluble material. Another distinct advantage of this method is the continuous replenishment of fresh solution to accelerate development.

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Introduction of megasonic development was a dramatic technology leap in this area. This development/cleaning technology combines a ProSys MegPie megasonic transducer with spray/stream developer systems. The radial megasonic array is specifically designed to apply uniform acoustic energy across the entire surface of the spinning substrate and gently lift away soluble materials without damaging fragile structures. This technology is uniquely suited to resolve extremely small CDs with high-aspect-ratio (> 5:1) structures for MEMS fabrication and advanced lithography (193- and 248-nm).

 

The Brewer Science® Cee® equipment line has been specifically engineered to accommodate all spin-developer process techniques including stream, spray, direct-angle, and megasonic-enhanced. As the predominant lab-scale equipment supplier for advanced R&D and prototyping, the Brewer Science® Cee® product team is eager to meet your specific application needs.

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Automated dispense systems for applying high-viscosity materials

  
  
  

Process Engineers have explored several techniques for consistently dispensing high-viscosity resins for spin-on material applications. Manually pouring and/or using pre-filled syringes requires significant material consumption and excessive time and introduces microbubbles into the viscous material. Achieving consistently accurate dispense rates and volumes and controlled suck-back is a daunting challenge. Automatic dispense options provide a feasible solution, however, many variables must be considered when pursuing this option.

 

One cost-effective solution involves using a combination of gravity-fed pressurized cartridges (12-ounce capacity) and stainless steel reservoir cans (1-gallon capacity). This medium-volume option features disposable polypropylene liners within the transparent cartridge housing, which allow users to switch materials in a matter of seconds without risk of cross-contamination. Each entire chemically exposed liner can be cleaned and reused or disposed of and replaced for the next chemical application. Precision pneumatic dispense valves are triggered by host spin coaters and provide accurate shot size and suck-back operation. The host software provides discrete control of the valve timing to the nearest 0.1 second. Other flow-rate variables include the N2 or clean dry air (CDA) pressure to the reservoir and the volume control adjustment on the dispense valve near the point of use. 

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   This option provides significant improvements over manual techniques in delivering viscous materials to the substrate surface. However, all pneumatically pressurized reservoirs are prone to allowing dissolution of gases into the process material. This phenomenon varies depending on the material solubility to the pressurized gas (N2) and the flow characteristics and/or viscosity. Thick-film materials require significantly higher pressures to achieve nominal flow rates, and the overall pressure is directly proportional to the amount of dissolution. This trapped gas can result in anomalies in the coated film and inhibit control of the suck-back function, resulting in problematic drips. While this option is feasible for initial small-scale proof-of-concept and development laboratories, it is not ideal for medium-volume prototyping and pilot-line environments.

In direct contrast to pressurized reservoirs, positive displacement pumps (such as the Cybor 5000-610) do not directly pressurize the material during dispense operations or subsequent recharging. Also, many positive displacement pumps feature a recirculating system to facilitate a specific outgassing procedure when the material is being initially purged or not in continuous use. The Cybor 5000-610 intelligent dispense pump is considered the industry-standard positive displacement pump for dispensing high-viscosity thick-film resists and polyimides (≤ 50,000 cP). The Cybor 5000-610 pump delivers extremely accurate shot size, volume, and suck-back control as well as microbubble-free operation. This pump is controlled through a Windows®-based PC. The software is fully compatible with today’s automated wafer-processing cluster and track systems.

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Unfortunately, many R&D laboratories simply cannot justify the purchase of million-dollar track systems that would be compatible with the positive displacement pump solution. Furthermore, the vast majority of laboratory-scale, entry-level spin coaters are not PC-based and will not support the software necessary to interface with this solution.

All Brewer Science® Cee® spin coaters include PC-based controllers capable of hosting the IDI/Cybor ChemNet® software. Visit the Cee® spin coaters page to learn more.

 

Bake plate enhancements for optimal thick-film curing results

  
  
  

High-uniformity bake plates have been displacing convection ovens for well over two decades in the microelectronics industry. The disadvantages of variable temperature zones, lengthened cure times, and considerable particle contamination have been thoroughly analyzed and confirmed. Although temperature uniformity remains the primary advantage for precision hot plates, virtually eliminating skinning effects for thin-film (< 1-5 µm) applications has been equally beneficial. The skin effect is well understood when baking “from the outside in” and is typical of baking in a conventional oven. This phenomenon is more pronounced with thick-film materials such as SU-8 photoresist, KMPR photoresist, BPR-100 photoresist, and WaferBOND® HT-10.10 material, which are very prone to the skin effect. This effect occurs when the outer exposed layer of the film dries and forms a skin before all of the solvents in the lower layers have evaporated. These process anomalies are often observed with today’s bake plates if the heating accelerations are too aggressive and/or the bake temperature or time is below recommended levels. Often the result is post-bake blistering and/or cracking, which renders the film unusable for downstream processing.

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To combat this issue, customers have deployed the use of multiple high-uniformity bake plates with multiple set points (dual stage) for performing a soft bake, post-exposure bake, and post-develop hard bake for final curing. However, the multi-plate option fails to provide a repeatable method for ramping up from ambient temperature to the set point and over-utilizes precious clean room or laboratory working surfaces. One potential solution to this problem is instituting programmable plates with multiple bake method options (proximity, gravity, and hard contact).

Proximity baking essentially floats the substrates on a “pillow” of inert gas that is blown through orifices in the chuck surface. A combination of heated gas, radiant heat from the chuck, and reflective radiant heat from the hood baffle uniformly heats the substrate. This slower heating of the substrate reduces blistering and cracking of films made from materials containing fast-drying solvents. Proximity baking can eliminate the need for two independent bake plates if the set points are relatively close (± 10°C). Proximity baking with an N2 pillow mitigates the risk of physical contact between the substrate and the hot plate and has been adopted for many photomask and display processing applications.

In a “soft-contact” bake, gravity alone holds the substrate to the surface of the chuck. This method is typically utilized as an intermediate option between a “hard contact” bake and a proximity bake, for multiple step warm-up.

In a “hard contact” bake, the substrate is held to the surface by applying a vacuum to the underside of the substrate. This method ensures baking uniformity and minimizes bowing and warping of the substrate.  


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The latest process enhancement combines the existing bake methods (proximity, soft contact, or hard contact) with programmable lift pins. The electronic lift pins are used for automated loading and unloading of the process wafers and provide enhanced repeatability. Furthermore, they have been designed for maximum flexibility and will accommodate all standard and semistandard sizes from 2 inches (with proper placement of the flat) through 200 mm. The electronic lift pins provide the user with 100 specific proximity process steps above the surface in any sequence or combination. The heights are programmed in 0.001-inch increments with an overall operating window from 0.001 inch through 0.750 inch (± 0.002 inch). In many scenarios, this feature allows for faster ramping acceleration/deceleration and emulates several bake plates temperatures simultaneously. This feature is also extremely valuable for safe handling of thermomechanically sensitive materials such as GaAs, LiNbO3, InP, GaN, SiC, and sapphire substrates. The Brewer Science® Cee® line of high-uniformity bake plates are the only ones to bring together all referenced capabilities in a compact footprint designed for a lab-scale environment. See the Cee® bake plate web page for additional information and detailed tool specifications.

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Processing wafers with high-topography 3-D structures

  
  
  

43-µm deep pillars with oxide hard mask remainingExtreme trench fillingVia filling

Over the past decade, Brewer Science has developed and published various methods of creating a planar surface over three-dimensional (3-D) structures. Sharing these results invariably leads to a potential customer asking, “Can you fill the holes in my substrate that are x in size?” The short answer to the question is “Yes, we can.” Filling holes, however, is rarely the only thing customers want to do. Typically they want to do something else, for instance, coat a photoresist, maintain chuck vacuum, support a structure, etc. That “something else” usually presents a challenge that is very different than just filling holes. For Brewer Science to provide the best solution for your particular structure and need, our highly experienced engineers and scientists must ask many more questions about what processes are acceptable and what downstream conditions the substrate must withstand.

To minimize cost and complexity, we offer process and material flexibility and compatibility with a standard photolithography track. We provide materials to level 3-D topography using the following methods:

Dry etch back

  • Requires an etching tool and bay transfer
  • Includes a variety of processes for which Brewer Science coatings are highly effective

Expose and develop

  • Requires an exposure tool and a developer spin bowl
  • Removes with dry etching
  • Takes advantage of the very good chemical resistance offered by Brewer Science materials

Wet etch back

  • Uses TMAH or solvent developer
  • Requires a developer spin bowl
  • Offers processing with no bay
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