Lift-off (microtechnology)
The lift-off method (english lift-off technique) is in the semiconductor and microsystems technology, a process sequence for making a mostly metallic microstructure. In a first process step in this case be structured thin films on the surface of substrates, such as wafers, is generated. The target material is then deposited over the entire surface on this patterned sacrificial layer. The areas in which the target material is located on the sacrificial layer is then removed by a further process step, and the remaining structures form the desired microstructure. The size of the produced using the lift-off process structures ranges from tens of nanometers to centimeters, with typical structure sizes in the micrometer range. The method is used, inter alia, for the preparation of interconnect layers or contact surfaces in the manufacture of integrated circuits ( ICs) and microsystems. In contrast to this additive and building process are the subtractive method in which first the entire surface a uniform layer of the target material is deposited on the substrate and subsequent etching of this layer structure is produced.
Process Description
The lift-off process is a relatively simple and efficient sequence of different basic methods of semiconductor technology. A typical process sequence is, for example, photolithographic patterning, the Schichtabscheiden and the removal of the photoresist layer. Over time, however, different variations have developed, their capabilities depend very much on the process conditions or settings used. In the following, therefore, only the basic process steps are described.
The lift-off process begins with the whole-area deposition of the subsequent sacrifice layer (often photoresist ) on a pre-treated substrate. The pretreatment typically involves cleaning of the substrate and, if necessary, a planarization of the surface ( for example by chemical-mechanical polishing) or the application of a primer layer. Then the photolithographic patterning of the sacrificial layer is done with an inverse pattern of the later structure. The parameters of the sacrificial layer structure should be adjusted so that there are highly vertical side walls or side walls with a slight undercut (negative sidewall angle ).
After the patterning of the sacrificial layer over the entire surface of the deposition target material, for example aluminum followed by thermal evaporation. It should be no connection between the deposited target material on the substrate and the target material on the sacrificial layer, so the one that connection does not have to be separated later later, on the other hand, the side surfaces of the sacrificial layer are still uncovered and thus the removal of the sacrificial layer in the last process step is not hindered. A compound of the two areas can be avoided by two conditions:
During the deposition of the target material is also important to ensure that the victims of this layer remain unscathed. For the use of photoresist as the sacrificial layer, this means that the process temperature must not exceed the glass transition temperature of the photoresist. For this reason, the target layer is deposited usually at room temperature and is therefore frequently be amorphous or polycrystalline.
In the last step, the sacrificial layer is removed by wet chemical. For this example, the photoresist are dissolved in a solvent (eg, acetone ), optionally with the aid of ultrasound. The sacrificial layer is resolved by the side walls ( edges) ago. The target material on top of the sacrificial layer is washed away with lifted (english lift off ) and. This leaves only the target material in the regions where it had direct contact with the substrate.
Typical process error
The finished structure can after dissolving the sacrificial layer have three process-related typical errors:
The three failure modes are more or less the result of a side wall coverage of the patterned sacrificial layer by target material. It follows that a good structural quality depends crucially on the profile of the sacrificial layer and the " step coverage " of the coating process. Conveniently here are a combination of undercut (negative) edges or sacrificial layer systems, where the bottom layer was etched back, and a coating process with poor edge coverage.
Process variants
The lift-off principle for the production of metal structures was described in the 1940s, before the inception of microelectronics. Since then, many variations have been developed in the literature which differ in terms of the deposited layer, the sacrificial layer and the chemicals used, as well as by a number of process parameters and application areas. As already mentioned, use a simple method, a layer of photoresist or a polymer such as polymethyl methacrylate (PMMA ), this sacrificial layer may be patterned by conventional photolithography - in 1969 presented by Hatzakis for producing conductor tracks and source / drain contacts made of aluminum. It also process variants have been described, using the electron beam or Imprintlithografie. More advanced versions use more or less complex layer stack as a sacrificial layer, for example Fotolack/Aluminium/Fotolack-, Polyimid/Molybdän-, or polyimide / polysulfone / silicon stack, etc. The use of an additional etching step, with the side edge of the bottom sacrificial layer is etched back, this seems like a undercut edge profile and prevents the formation of a continuous layer on the side walls.
In addition to process variations with different sacrificial layers or layer systems, processes have also been described in which the lift-off effect is not due to chemical dissolution but by lifting the coated sacrificial layer using an adhesive tape (tape -assisted lift-off ) or by introduced stresses in the material ( see Carbon Dioxide Snow - technique) is performed.
Below are some process variants are described by way of example.
Photolithography with image inversion step
When using a photo-resist sacrificial layer a so-called reversing step is often used (English image reversal process), by which an undercut edge profile can be generated. This is hardly possible with a simple photolithographic patterning, as in the upper regions of the resist layer light is always absorbed more strongly and thus for the development of a profile with steep edges or over- section results, see step 2 in the adjacent figure. Reverse photoresists (English - image reversal resists ) offer the possibility of inverting the image ( engl. image reversal ) of the mask. Depending on the reversal photoresist is between a direct (acid - catalytic ) and a differentiated indirect ( basic ) reversal processes, they yield depending on the process a negative or a positive image of the mask.
Case of acid - catalytic reverse photoresists, such as a Diazonaphtoquinon ( DNQ ) / novolak photoresist in combination with an admixed säureaktivierbarem polymerizer (eg hexamethoxymethyl HMMM ) corresponds to the litigation until exposure largely a normal photolithographic patterning (paint job, soft bake, etc.). Development of the photoresist ( positive resist ) immediately after this exposure would therefore a positive image of the mask structure found. However, by an additional reverse process prior to the development of the solubility can be reversed. That is, after exposure, insoluble areas soluble, and vice versa. This is achieved mainly through two steps. On the exposure followed by a so-called First Umkehrausheizen ( engl. image reversal bake ). By the action of temperature cross-linking reaction can be effected in the exposed areas of the photoresist, and, after a rest phase in which a sufficient rehydration is ensured ( a longer storage in air is sufficient for this ), followed by the second addition step. A flood exposure of the entire wafer caused in the unexposed areas, yet the formation of 3 - indenecarboxylic acid and makes these soluble portions in relation to the alkaline developer. After development, the result is a negative image of the mask structure with an undercut edge profile.
Under basic reverse photoresists, the photoresist layer is first exposed by the exposure an amine vapor or an ammonia solution. In this case, a basic catalyst is diffused into the layer, which leads during the subsequent heating to the decomposition of the formed reversed in the exposed parts 3- indenecarboxylic acid. The resulting indene derivatives are very effective Löslichkeitshemmer. Before the development of the resist takes place as in the direct process, a flood exposure, wherein the unexposed areas initially exposed to light and thus become soluble.
In both process variants may be affected at the first exposure by varying the exposure time of the slope angle of the photoresist profile. As already mentioned, this is due to the depth-dependent absorption characteristics of the light and the resulting cross-linking degree by the reverse heating. A high exposure dose in this case leads to steep edges and a low exposure dose to strongly undercut slopes. With such a profile for the lift -off process can reduce the risk of demolition undefined edges are avoided on the side walls. In addition, the sacrificial layer is replaced by the resulting increased thermal stability up to 200 ° C.
Electron beam lithography
Another way to produce an undercut resist profile is the patterning of the sacrificial layer using electron beam lithography. In this process variant, the scattering behavior of the electrons and the energy distribution will be utilized in the photoresist layer. Depending on the output energy results in a more or less elongated, pear-shaped power distribution. For the production of the undercut profile of the upper portion of this distribution can be used. Here takes the distribution of cross section with increasing depth also increases. To achieve this profile in a few hundred nanometers thick sacrificial layer, but relatively low energies are required, which in turn affects the resolution negative. This is especially true for dense structures. Therefore, yet high energy levels are used for the production of dense features with line widths in the range below 100 nm are used, even if it is so difficult to achieve the necessary Profilhinterschneidung.
Can be remedied by the use of a double-layer resist system. Here, the lower layer has a substantially higher speed ( e.g., 50 times higher ) than the upper layer, and it can be achieved even at high Profilhinterschneidung radiation energy. Frequently used combinations of materials for such a system is a layer stack of a layer of polymethylmethacrylate ( PMMA; with high molar mass and lower electron sensitivity) and an underlying layer of one of its copolymers ( eg, P (MMA -MAA ) with low molar mass higher sensitivity). An advantage of such a material combination that both of the layers can be developed with the same solution.
Carbon Dioxide Snow- technology
A recent lift-off variant uses frozen carbon dioxide particles ( engl. carbon dioxide snow ) to remove a metal layer ( target material ) on the photoresist sacrificial layer. Solid carbon dioxide is formed at temperatures below -60 ° C, well below typical process temperatures in the coating. During cooling of the metal and the photoresist layer it comes to mechanical stresses at the interface of the two materials, leading to the rupture or detachment of the metal film due to different thermal expansion coefficients. The released metal is then removed by a carbon dioxide - beam at speeds up to 40 m / s. Usually, the back of the substrate is heated up to 60 ° C, which at the interface increases the stress again and at the same time reduces the mechanical stresses between the desired metal structures on the substrate. After removal of the metal layer from the sacrificial layer, it can be readily removed by wet-chemical or plasma ashing. The advantage of this method is that the metal particles are removed immediately and the sample surface is therefore better protected against repeated deposition.
Application
The lift-off method is a very general preparation method by which can be structured all metals and their alloys and multilayers principle. Here, since, needed unlike a subtractive patterning by etching, not tailored to the zuätzenden materials etching or etch chemistry, this is the same lift -off process can be used almost always. It must only be ensured a similar deposition of the target material.
The method offers advantages, for example, in the following cases:
Furthermore, the inclined side walls provide advantages in the deposition of films in subsequent process levels, such a higher level in the interconnect metallization of the integrated circuit. Because unlike the steep sidewalls, as they typically occur in the dry etching, the side surfaces of the patterned by lift-off material can be coated with a wider variety of procedures. The surface so prepared, among others, spin coating layers, such as photoresist layers for subsequent processes, can be applied homogeneous shows significantly fewer jumps in the topography. The process is therefore used mainly for the structuring of metal layers. It enables the production of structures with conductor tracks in the micrometer range in the manufacture of discrete components and also ( by today's standards ), relatively simple integrated circuits having up to four interconnect levels.
Despite these advantages, the lift-off process for the production of wiring levels of integrated circuits has been unable to prevail against the patterning by dry etching; currently the damascene and dual- damascene process is used to a greater extent in this area. Reasons for this include the relatively low resolution, and the elaborate production of the patterned sacrificial layer, since care must be taken during deposition and dissolution process on the compatibility of the chemical and physical characteristics of the victim and target layer.
Analogously to the preparation of conductive tracks, the lift-off can also be used for the production of bumps. It is metallic (typically a lead-tin alloy), contact elements for the direct mounting of the chips, for example, the tape-automated bonding, or flip -chip mounting. Again, there are several process variants, which may include prepared by the lift-off process copper contact surfaces including copper diffusion barriers. For further information, reference is made here to the literature.
Nowadays, lift-off is used as a common method in the fabrication of devices in the nanometer range (eg, single electron transistors or micro - SQUIDs ). This is usually an e-beam lithography in combination with a positive photoresist, usually polymethylmethacrylate (PMMA) may be used. The method is mainly limited by the limited resolution of the lithographic patterning and by the grain size of the deposited material. So may be interrupted a line structure when the grain size is in the range of the line width.