Requires a controlled process where etch rates are quantified at the submicron level
Glenn Ogura
Micro-etching, or micro-stripping, is a term used to describe the use of a laser to selectively remove a top layer to expose the underlying material or to selectively remove a portion of material to a prescribed depth. In the case of etching or stripping polymer materials, especially polymer-based life science products -- such as single-use medical devices or diagnostic consumables -- depth control, end-point detection and material type are critical attributes. This article describes a variety of micro-etching techniques developed to meet the needs of the life science industry.
By definition, micro-etching implies a highly selective material removal process with an etch rate as low as 0.1 micron per laser pulse. This means that the laser etches the material like a fine scalpel, etching the material layer-by-layer with each layer as thin as 0.1 micron. This high resolution permits the laser to create blind holes, blind wells, or channels with extremely precise depth control. As an example, if a coating 10 microns thick needs to be removed, then by pulsing the laser 100 times at an etch rate of 0.1 micron per pulse, the required coating is removed. By counting the number of laser pulses, the depth of etching can be accurately controlled. As a general rule, micro-etching does not exceed >1 mm (0.04 in.) in depth.
Important parameters
Parameters of merit of a laser micro-etching method or system vary for different applications, but for a processing catheter, for example, the following are important:
- etch feature size, including length, width, and depth,
- etch feature location relative to a pre-existing feature or location,
- submicron etch rate,
- coating thickness variation,
- asymmetry of extrusion wall thickness,
- non-concentricity of catheters,
- transition from unetched material surface to etched material surface, and
- material surface profile.
Micro-etching can be applied to a variety of material formats, including flat sheets, tubes, and 3-D devices. Flat polymer sheets or films are held by vacuum chucks with protective liners or mechanical spacers to ensure the material does not shift during laser etching. Creation of micro-wells for miniature bioreactors (FIGURE 1) or microfluidic channels for molecular diagnostics, drug-eluting wells for stents, balloon catheter delivery devices or parylene coating removal from electrodes are examples of applications. In the case of micro-bioreactors or drug-filled wells, the volume of the blind wells is often critical to the application, and the precision and high resolution of the laser-based process is an important reason why this method is chosen.
When applying selective removal of a coating on a wire, cable, or catheter, the term laser stripping is used because the selective removal occurs circumferentially around the underlying layer such as a metal wire for purposes of conductivity such as a radio frequency (RF) wave to ablate human tissue to arrest arrhythmia (irregular heart beat) or transport an electrical signal from a blood pressure sensor. The classic case involves a reel-to-cut or reel-to-reel material handling system where a laser strips a polymer coating from a wire fed from a spool and respools the stripped wire or singulates the stripped wire to the desired length. Laser stripping of tiny wires (outer diameter < 0.004 in.) for neuromodulation or cardiac rhythm management are examples of applications. However the laser stripping is not always 360 degrees around a wire, but can also involve a finite angular section that exposes multiple windows around a multilumen catheter. The cone transition, defined as the region between stripped and unstripped real estate, can also be important. In some applications, a gradual slope is desired to aid in a subsequent deposition process or a sharp transition is required to clearly define the area of the stripped region. Various scanning and masking techniques can be deployed to program the cone transition.
Challenges
One of the challenges of micro-etching is that the part geometry is not consistent part-to-part or lot-to-lot such as the coating thickness on a planar wafer or the symmetry of an extrusion wall of a catheter. If etch depth is controlled by the number of laser pulses dialed at a constant etch rate (microns per pulse), then counting the absolute number of pulses will not guarantee 100% removal of the coating and may lead to etching into the underlying material or breakthrough of a lumen wall.
There are a variety of techniques to address part geometry variation including:
- material differentiation,
- plasma end-point detection,
- plasma end-point detection with on/off control, and
- end-point detection with on/off control and concentricity compensation.
Material differentiation is the simplest way to prevent etching into an underlying substrate. To laser-strip a polymer coated wire, the ablation threshold for polymers is an order of magnitude less than the ablation threshold for metals, ceramics, or glass. Once the laser has etched away the polymer coating and reached a metal wire, then successive laser pulses do not damage the wire, no matter how many additional pulses are added. For varying polymer coatings, the process is designed to over-pulse, accounting for thicker or thinner polymer coatings, guaranteeing 100% polymer coating removal without compromising the underlying single or multiple filler metal wire.
If the underlying material is similar to the top layer such as another polymer, then the ablation threshold method cannot be relied upon. For example, in the case of a polymer coating on top of another polymer, then over-pulsing will surely lead to etching into the secondary material. If the polymer coating thickness varies, then there is no tolerance for over-pulsing. To address this concern, end-point detection can be used to ensure the laser process stops before penetrating into the underlying substrate. Every ablated material gives off a different plasma signature. As the laser ablates the top polymer coating, the intensity of the plasma signature reduces as the coating is whittled down. As the laser nears the underlying second polymer material, then a different plasma signature is emitted, telling the closed loop process control to stop the laser.
Take the case where the top layer and the underlying layer are identical materials and the coating thickness varies both spatially and lot-to-lot. FIGURE 2a shows a polymer multilumen catheter where one inner lumen must be exposed while the other lumen must not be compromised. Without closed loop compensation, the right-sided lumen wall would be penetrated since the wall thickness is thinner on that side. To achieve the results shown in FIGURE 2b, the laser is turned on and off as the catheter is rotated to ensure no over-etching, and is controlled by an endpoint detector to measure the varying thickness of the coating.
Now take the extreme case where there is spatial coating thickness variation, and the catheter is not symmetrical so that during rotation, the catheter "wobbles" in an elliptical path, causing the depth of field to change. To compensate for this geometric variation, a motorized z axis is added with an autofocus routine so that the part can be dynamically brought into the plane of focus if the end-point detector control deems that additional laser pulses are required.
For a complicated 3-D part, 9 axes (or higher) laser-etching systems are employed to remove selective portions. Surface modification of a balloon surface (FIGURE 3) to promote bonding to a catheter or other polymer component is an emerging application. Unlike the previous applications, surface modification is not a true ablation process involving material removal; instead the laser is set at the threshold of ablation and the beam is scanned across the surface to create micro- and nanostructures in the surface. The "roughening" of the balloon surface to create greater surface area is a much more controlled and consistent process than using manual techniques such as sand paper or bead blasting. The height of the surface structures can be programmed from submicron to several microns high. For very thin materials, such high resolution is critical to ensure the surface-textured balloon is not compromised in terms of passing burst pressure or tensile strength tests and guaranteeing the absence of pinholes.
In addition to surface modification of 3-D devices such as balloons, multi-axis systems can also be deployed to produce micro-blind structures in titanium alloy-based dental implants (FIGURE 4) to promote soft tissue and bone attachment. Dr. Jack Ricci at the New York University College of Dentistry says "This technique provides predictable soft tissue aesthetics and prevents crestal bone loss. Such a complex 3-D scaffold for cell and tissue attachment may have other implant applications in both polymer and metal-based devices such as in transcutaneous prosthetic fixation and control, medical sensors, orthopedic spine hardware, intraocular lens implants, and bioreactor surfaces for long-term control of cell growth and differentiation."
Summary
In summary, laser-based micro-etching or micro-stripping of polymer-based life science products is a very different process and is often more complex than etching metals. Unlike metal processing where the laser converts a metal from a solid to a liquid phase, inviting melting for either laser cutting or welding, micro-etching of softer polymers requires a very controlled process where etch rates are quantified at the submicron level (0.1 microns per laser pulse) to achieve controlled depth in 2-D and 3-D parts, complicated by geometric variations such as coating thickness and concentricity. Alternatively, laser ablation threshold can be attenuated to modify or "roughen" the surface to promote bonding of balloons and catheters. In this case, the material is displaced spatially at the ultra-thin surface level. ✺
Glenn Ogura([email protected]) is executive VP of business development at Resonetics LLC, Nashua, NH.
www.industrial-lasers.com May/June 2013 Industrial Laser Solutions
More Industrial Laser Solutions Current Issue Articles
More Industrial Laser Solutions Archives Issue Articles