04-08-2012, 01:04 PM
Laser micromachining of optical biochips
[attachment=30215]
ABSTRACT
Optical biochips may incorporate both optical and microfluidic components as well as integrated light emitting semiconductor devices. They make use of a wide range of materials including polymers, glasses and thin metal films which are particularly suitable if low cost devices are envisaged. Precision laser micromachining is an ideal flexible manufacturing technique for such materials with the ability to fabricate structures to sub-micron resolutions and a proven track record in manufacturing scale up.
Described here is the manufacture of a range of optical biochip devices and components using laser micromachining techniques. The devices employ both microfluidics and electrokinetic processes for biological cell manipulation and characterization. Excimer laser micromachining has been used to create complex microelectrode arrays and microfluidic channels. Excimer lasers have also been employed to create on-chip optical components such as microlenses and waveguides to allow integrated vertical and edge emitting LEDs and lasers to deliver light to analysis sites within the biochips.
Ultra short pulse lasers have been used to structure wafer level semiconductor light emitting devices. Both surface patterning and bulk machining of these active wafers while maintaining functionality has been demonstrated. Described here is the use of combinations of ultra short pulse and excimer lasers for the fabrication of structures to provide ring illumination of in-wafer reaction chambers.
The laser micromachining processes employed in this work require minimal post-processing and so make them ideally suited to all stages of optical biochip production from development through to small and large volume production.
Keywords: Laser Micromachining, Optical Biochip, Excimer, Electrokinetics, Femtosecond, Ultra Short Pulse.
INTRODUCTION
Biochips bring together the accuracy and flexibility of microfabrication processes and biological or chemical assays to create an integrated environment for biotechnological processing. The potential of biochip technology extends over a wide range of market sectors dominated by the benefits in healthcare and drug discovery but also including sectors such as food and environmental monitoring and analysis. Optical biochips extend the fundamental biochip concept to include optical principles either as part of a monitoring process such as absorbance or fluorescence measurement or as an essential part of a reaction chain or assay such as in photosynthesis and similar processes. Optical biochips can make use of either external or integrated light sources and detectors but typically contain integrated optical delivery components such as light or waveguides, reflectors and lenses. While there are many examples of optical integration within biochip devices, the development challenges in optical biochips are moving towards the integration of optical systems typically associated with complex optical bench configurations and high costs. To meet such challenges advances in biotechnology for assay formatting, physics and electronics for semiconductor laser development and microfabrication are required.
While many of the components of optical biochips can be manufactured using surface and bulk micromachining processes of varying formats, there is a growing necessity to create optical biochips that are easy to mass produce and hence of low cost and, for use in biomedical applications, able to be disposable. Such demands place significant restrictions on the range of materials and manufacturing processes that can be used to produce optical biochips. Typically, it is desirable to produce devices entirely from polymers. However, the need for mechanical stability often requires the use of glasses and other substrates. Additionally, if integrated sources and detectors are to be used, polymer processing must integrate with embedded semiconductor materials and associated electrical connections. This often gives rise to issues of thermal compatibility. In this work the use of laser micromachining to produce a range of optical biochip components is described.
LASER MICROMACHINING
Laser micromachining as a manufacturing technique, has emerged from the development of micro and nanotechnologies over the past two decades. While laser micromachining is still considered a new process in many areas of microengineering, it has become an established manufacturing method in niche application areas such as inkjet printer nozzle drilling [1] and flat panel display patterning [2]. Accurate laser micromachining tends to use pulsed lasers at wavelengths where heating and melting-based surface disruption is minimal. By controlling the number of laser pulses, and hence the total incident radiation, precise machining depths can be achieved while minimal thermal distortion occurs at the edge of the exposed region. In this work laser micromachining using nanosecond pulsed, ultra-violet (248nm) KrF excimer and femtoecond pulsed infrared (800nm) Ti:Sapphire lasers have been used to create a range of optical biochip components and were configured to produce microstructures using either a direct write or a mask projection machining process.
Direct write
Direct write laser micromachining, as illustrated in figure 1a, makes use of a laser beam tightly focused to a small spot which is moved over the surface of the workpiece during machining. Typically direct write machining uses a laser with a Gaussian beam profile. Good beam quality and a low M2 factor allow the beam to be focused to a small spot using simple optical components and to give a beam fluence at focus that is above the ablation threshold of the workpiece material. Control of the beam movement over the surface of the workpiece allows arbitrary 2D patterns to be machined. By overlaying machining runs it is also possible to create 3D structures. Machining depth control during workpiece movement is achieved by synchronizing laser pulse or power output with workpiece stage position. The direct write machining system used in this work was an Exitech M2000F Laser Micromachining Workstation (Exitech Ltd., UK) incorporating a Spectra Physics Hurricane Ti:Sapphire laser with a pulse duration of 120fs and a beam power density of up to 3.5Wcm-2. In this system beam delivery components allowed CNC control of beam power density and focusing. Typically the beam was focused to a 20μm spot delivering power densities of up to 0.3MWcm-2. The workpiece was held on a micropositioning stage with 1μm resolution (Aerotech Inc., USA). Movement of the workpiece and beam on/off control was through a PC based motion controller (Unidex 500, Aerotech Inc. USA).
Mask projection
Mask projection laser micromachining, as illustrated in figure 1b, uses a large area uniform intensity laser beam at a mask plane. The mask pattern is subsequently imaged onto the workpiece using a projection lens. Typically the projection lens will provide some form of demagnification to allow a low fluence beam at the mask plane and a smaller, high fluence, beam at the workpiece. Large area multi mode beams are often used for mask projection laser micromachining. Homogenizing optics are used to produce a uniform intensity over the beam area. Large area mask pattern transfer can be achieved by the synchronized movement of both the mask and workpiece using micropositioning stages. As with direct write laser micromachining, machining depth control during pattern transfer is achieved by synchronizing laser pulse output with mask or workpiece position. In this work an Exitech S8000 Laser Micromachining Workstation (Exitech Ltd., UK) was used for mask projection machining. This system incorporates a Lambda Physik Compex 110 excimer laser configurable for operation at either 248nm or 193nm and beam delivery components allowing beam shaping and homogenization to produce a uniform intensity 10mm x 10mm beam at the
mask plane. Using a 0.1NA projection lens with a demagnification of x10 a 1mm x 1mm image of the mask plane was created at the workpiece. Micropositioning stages (Aerotech Inc., USA) were used to control the position of both the mask and workpiece while beam fluence was controlled using a motorized (Aerotech Inc. USA) attenuator based on contra rotating dielectric coated quartz plates.
[attachment=30215]
ABSTRACT
Optical biochips may incorporate both optical and microfluidic components as well as integrated light emitting semiconductor devices. They make use of a wide range of materials including polymers, glasses and thin metal films which are particularly suitable if low cost devices are envisaged. Precision laser micromachining is an ideal flexible manufacturing technique for such materials with the ability to fabricate structures to sub-micron resolutions and a proven track record in manufacturing scale up.
Described here is the manufacture of a range of optical biochip devices and components using laser micromachining techniques. The devices employ both microfluidics and electrokinetic processes for biological cell manipulation and characterization. Excimer laser micromachining has been used to create complex microelectrode arrays and microfluidic channels. Excimer lasers have also been employed to create on-chip optical components such as microlenses and waveguides to allow integrated vertical and edge emitting LEDs and lasers to deliver light to analysis sites within the biochips.
Ultra short pulse lasers have been used to structure wafer level semiconductor light emitting devices. Both surface patterning and bulk machining of these active wafers while maintaining functionality has been demonstrated. Described here is the use of combinations of ultra short pulse and excimer lasers for the fabrication of structures to provide ring illumination of in-wafer reaction chambers.
The laser micromachining processes employed in this work require minimal post-processing and so make them ideally suited to all stages of optical biochip production from development through to small and large volume production.
Keywords: Laser Micromachining, Optical Biochip, Excimer, Electrokinetics, Femtosecond, Ultra Short Pulse.
INTRODUCTION
Biochips bring together the accuracy and flexibility of microfabrication processes and biological or chemical assays to create an integrated environment for biotechnological processing. The potential of biochip technology extends over a wide range of market sectors dominated by the benefits in healthcare and drug discovery but also including sectors such as food and environmental monitoring and analysis. Optical biochips extend the fundamental biochip concept to include optical principles either as part of a monitoring process such as absorbance or fluorescence measurement or as an essential part of a reaction chain or assay such as in photosynthesis and similar processes. Optical biochips can make use of either external or integrated light sources and detectors but typically contain integrated optical delivery components such as light or waveguides, reflectors and lenses. While there are many examples of optical integration within biochip devices, the development challenges in optical biochips are moving towards the integration of optical systems typically associated with complex optical bench configurations and high costs. To meet such challenges advances in biotechnology for assay formatting, physics and electronics for semiconductor laser development and microfabrication are required.
While many of the components of optical biochips can be manufactured using surface and bulk micromachining processes of varying formats, there is a growing necessity to create optical biochips that are easy to mass produce and hence of low cost and, for use in biomedical applications, able to be disposable. Such demands place significant restrictions on the range of materials and manufacturing processes that can be used to produce optical biochips. Typically, it is desirable to produce devices entirely from polymers. However, the need for mechanical stability often requires the use of glasses and other substrates. Additionally, if integrated sources and detectors are to be used, polymer processing must integrate with embedded semiconductor materials and associated electrical connections. This often gives rise to issues of thermal compatibility. In this work the use of laser micromachining to produce a range of optical biochip components is described.
LASER MICROMACHINING
Laser micromachining as a manufacturing technique, has emerged from the development of micro and nanotechnologies over the past two decades. While laser micromachining is still considered a new process in many areas of microengineering, it has become an established manufacturing method in niche application areas such as inkjet printer nozzle drilling [1] and flat panel display patterning [2]. Accurate laser micromachining tends to use pulsed lasers at wavelengths where heating and melting-based surface disruption is minimal. By controlling the number of laser pulses, and hence the total incident radiation, precise machining depths can be achieved while minimal thermal distortion occurs at the edge of the exposed region. In this work laser micromachining using nanosecond pulsed, ultra-violet (248nm) KrF excimer and femtoecond pulsed infrared (800nm) Ti:Sapphire lasers have been used to create a range of optical biochip components and were configured to produce microstructures using either a direct write or a mask projection machining process.
Direct write
Direct write laser micromachining, as illustrated in figure 1a, makes use of a laser beam tightly focused to a small spot which is moved over the surface of the workpiece during machining. Typically direct write machining uses a laser with a Gaussian beam profile. Good beam quality and a low M2 factor allow the beam to be focused to a small spot using simple optical components and to give a beam fluence at focus that is above the ablation threshold of the workpiece material. Control of the beam movement over the surface of the workpiece allows arbitrary 2D patterns to be machined. By overlaying machining runs it is also possible to create 3D structures. Machining depth control during workpiece movement is achieved by synchronizing laser pulse or power output with workpiece stage position. The direct write machining system used in this work was an Exitech M2000F Laser Micromachining Workstation (Exitech Ltd., UK) incorporating a Spectra Physics Hurricane Ti:Sapphire laser with a pulse duration of 120fs and a beam power density of up to 3.5Wcm-2. In this system beam delivery components allowed CNC control of beam power density and focusing. Typically the beam was focused to a 20μm spot delivering power densities of up to 0.3MWcm-2. The workpiece was held on a micropositioning stage with 1μm resolution (Aerotech Inc., USA). Movement of the workpiece and beam on/off control was through a PC based motion controller (Unidex 500, Aerotech Inc. USA).
Mask projection
Mask projection laser micromachining, as illustrated in figure 1b, uses a large area uniform intensity laser beam at a mask plane. The mask pattern is subsequently imaged onto the workpiece using a projection lens. Typically the projection lens will provide some form of demagnification to allow a low fluence beam at the mask plane and a smaller, high fluence, beam at the workpiece. Large area multi mode beams are often used for mask projection laser micromachining. Homogenizing optics are used to produce a uniform intensity over the beam area. Large area mask pattern transfer can be achieved by the synchronized movement of both the mask and workpiece using micropositioning stages. As with direct write laser micromachining, machining depth control during pattern transfer is achieved by synchronizing laser pulse output with mask or workpiece position. In this work an Exitech S8000 Laser Micromachining Workstation (Exitech Ltd., UK) was used for mask projection machining. This system incorporates a Lambda Physik Compex 110 excimer laser configurable for operation at either 248nm or 193nm and beam delivery components allowing beam shaping and homogenization to produce a uniform intensity 10mm x 10mm beam at the
mask plane. Using a 0.1NA projection lens with a demagnification of x10 a 1mm x 1mm image of the mask plane was created at the workpiece. Micropositioning stages (Aerotech Inc., USA) were used to control the position of both the mask and workpiece while beam fluence was controlled using a motorized (Aerotech Inc. USA) attenuator based on contra rotating dielectric coated quartz plates.