Lift-off technique

Orth, R. N. Kameoka, J. Zipfel, W. R. Ilic, B. Webb, W. W. Clark, T. G. Craighead, H. G Creating biological membranes on the micron scale Forming patterned lipid bilayers using a polymer lift off technique, Biophys. J. 2003, 85, 3066 3073... 

Figure 19.5. Process sequence for the lift-off process (the planarized metalhzation process) (a) a resist film is patterned on a dielectric film (b) dielectric patterning (c) a thin catalytic film layer (PVD or CVD Ti, Al) is deposited (d) a lift-off technique removes the excess material, leaving the catalytic layer in the trench only (e) electroless Cu deposition.

Additive Approach. In the additive approach (Figure 18), a negative image of the conductor pattern is defined in a photoresist layer that must be thicker than the conductor layer. Metal is then selectively deposited in the open areas of the resist by electroplating or by a lift-off technique. For... 

Lift-Off When utilizing the lift-off technique, the photoresist is deposited and developed onto the substrate and then the metal is deposited on top of the photoresist, as shown in Figure 8.4. The excess photoresist and metal are then removed by a lift-off technique, in which the sample is submersed in a solution in which the photoresist is soluble. For example, when using the photoresist ma-P 100, submersion of the sample into acetone can complete the lift-off procedure. 

For microelectronic biosensors a further problem is the immobilization procedure of enzymes. Again different approaches were tried including drop-on techniques [61], ink-jet printing [68], spray techniques [62], electropolymerization [64], lift-off techniques [63,66] and photolithographically patterned enzyme membranes [65]. 

A miniaturized planar amperometric glucose sensor has been created on Sapphire substrates. Thin film titanium-gold electrodes are covered with an enzyme layer which is patterned by a lift-off technique [66]. This sensor exhibits a fast response time of 30 seconds but the linear measuring range is poor. 

The lift-off process is usually employed to fabricate metal electrodes. This method, as opposed to the wet-etch process, allows the dual-composition electrode to be patterned in a single step [747]. In order to achieve well-defined metal electrodes in a channel recess using the lift-off technique, the metal (Pt/Ta) will not be deposited onto the sidewalls of the photoresist structure (see Figure 2.32). This discontinuity of the deposited metal layer around the sidewalls allows metal on the resist to be removed cleanly from the surface without tearing away from the metal on the surface. Thus negative resists were used because they can be easily processed to produce negatively inclined sidewalls. To achieve this, the photoresist is subjected to underexposure, followed by overdevelopment [141]. 

FIGURE 2.32 The electrode formation on glass by metal lift-off technique without deposition on the channel sidewalls [141]. Reprinted with permission from Wiley-VCH Verlag. 

N-type Hgo.79Cdo.21Te semiconductor strips 1 are secured to a sapphire substrate 2 by a layer of epoxy adhesive. Biasing electrodes 6 and 7 are formed using ion-etching and metal lift-off technique (see EP-A-0007667). At the area of the read-out means (8 and 6) and (8 and 7) each of the strips 1 branches into two parts separated from each other by a slot 13 which extends in a direction parallel to the strip. One part provides the continuation of the ambipolar drift path and the other part supports a metal strip connection which forms the read-out electrode 8. The metal strip may not extend beyond the inner end of the slot 13 as shown in the figure or may extend right across the strip so as to form the read-out electrode beyond the slot 13. The continuation of the ambipolar drift path is narrower than the part of the drift path before the... 

A conductive pattern of lead-out conductors 12 and 13 with parts 14 and IS of reduced thickness is formed on an upper surface of an insulating substrate 10 of alumina, silicon, sapphire or beryllia. Detector elements 1 having electrodes 3 and 4 are adhered to the substrate by an epoxy resin 17, and the electrodes are connected to the lead-out conductors by a gold interconnection layer which is formed by the use of a lift-off technique. Due to the reduced thickness of the lead-out conductors and the curved edges of the detector elements no large steps or discontinuities occur in the interconnection layer. The imager is tested and defective detector elements are substituted for new ones. 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

In order to maintain the advantage of the microfabrication approach which is intended for a reproducible production of multiple devices, parallel development of membrane deposition technology is of importance. Using modified on-wafer membrane deposition techniques and commercially available compounds an improvement of the membrane thickness control as well as the membrane adhesion can be achieved. This has been presented here for three electrochemical sensors - an enzymatic glucose electrode, an amperometric free chlorine sensor and a potentiometric Ca + sensitive device based on a membrane modified ISFET. Unfortunately, the on-wafer membrane deposition technique could not yet be applied in the preparation of the glucose sensors for in vivo applications, since this particular application requires relatively thick enzymatic membranes, whilst the lift-off technique is usable only for the patterning of relatively thin membranes. 

The drain and source are based on highly conductive gallium doped zinc oxide (GZO), 150 nm thick and patterned by the lift-off technique. These depositions were also carried out at room temperature. 

As it was mentioned earlier, a much larger range of micro/nanofabrication tools are utilized in the design and fabrication of micro/nanotransducers than applied in the fabrication of microfluidic systems. Standard photolithography as described above is only one of the many techniques used. Three examples will be described here the fabrication of freestanding transducers, UV-photolithography with lift-off technique for preparation of interdigitated ultramicroelectrode arrays (IDUAs), and more traditional techniques for microtransducer preparations. 

Fig. 6.6. Inter digitated ultramicroelectrode arrays (IDUAs). (a) schematic, (b) optical micrographs with 1000 x magnification. IDUAs were fabricated using standard photolithography and lift-off techniques on silicon and were made out of gold.

The investigation of the interface of a real device contact with siuface sensitive techniques is generally not possible, since the interface is buried and these techniques penetrate only a few atomic layers. However, an interface analysis with, e.g., electron spectroscopic techniques would be highly desirable since these methods provide valuable (semi-) quantitative information on the electronic structure and ehemical interaction in the contact regime. To tackle this problem, i.e. to aecess the buried interface, we have applied a lift-off technique, whieh is illustrated in Figure 14.8a and was previously developed by our group [30]. Therefore, we prepared a sample sandwich which consists of the aetual model OFET, i.e. a gold film deposited on a DIP film that itself... 

In the present case after a rough test no major deviations from the ideal DlP/Au model system could be detected but further, more detailed studies using high-resolution electron spectroscopies with synchrotron radiation and additional morphological techniques such as AFM may reveal subtle differences. These will be performed after optimisation of the lift-off technique which is presently under way and will be applied completely under UHV conditions, thus allowing the minimisation of spurious influences. Then further investigations on buried interfaces combining XPS, UPS, IPES, NEXAFS, and AFM will be performed on the present and several other systems which are a matter of present research. 

Lift-off techniques for lithographically patterning membrane materials have been developed. A problem with lift-off for thick organic membranes is the thickness needed for the photoresist layer. A typical resist layer is about 1 pm thick. The thickness needed for chemical membranes can be as high as 50 /rm, increasing the resist layer thickness that is necessary to pattern these thick layers. Also, lift-off can only be used with materials that are resistant to the solvent used... 

Another approach in the etching of thin-film processes is the lift-off technique. In this approach, photoresist material is first deposited onto the substrate. A mask of inverse pattern is sized and the photoresist material is left on the substrate surface covering the area where metallization does not take place. Metal is then deposited over the substrate surface. The next step removes the photoresist chemically. Figure 16.5 shows the sequence of a typical lift-off... 

S. J. Kim, T. Ahn, M. C. Suh, C. J. Yu, D. W. Kim, and S. D. Lee. Low-leakage polymeric thin-fihn transistors fabricated by laser assisted lift-off technique. Japanese Journal of Applied Physics Part 2-Letters and Express Letters, 44(33-36) L1109-L1111, 2005. 

The Cu/epoxy sample used for the above analysis was prepared in a novel way, tailored to get particularly flat and easy-to-scan surfaces. In this recent approach, electron beam lithography (EBL) and the lift-off technique were employed in order to produce well-defined Cu structures [29]. The sample preparation procedure is illustrated in Fig. 8.4 a. 

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