Microstructure Patterning

The sputtering technique described in the preceding section produces a uniform thin film over the area of a substrate. This section discusses how this thin film is patterned into microstructures that will be further processed towards functional devices.

Patterning of micrometer size structures generally involves two main steps: the definition of the structures on a substrate coated with a resist layer is followed by a process that transfers the pattern to the thin film by removing the material around the defined structures. While micrometer size structures can be manufactured with standard optical lithography techniques, nanometer scale features impose more stringent constraints on resolution and alignment precision, necessitating the use of more costly and time-consuming electron-beam lithography. First, the optical lithography and electron-beam lithography techniques are introduced for both a negative and positive resist system. Secondly, the material removal processes for structure patterning are discussed, which include lift-off, ion-milling and chemical wet etching. Specific choices between equivalent techniques for the various fabrication steps are accounted for in the discussion of the full fabrication workflow in Section 4.

3.2.1. Lithographic Pattern Definition

Micrometer size structures are usually defined by optical lithography. In this process, which is depicted in Figure 3-5-A, a photosensitive polymeric resist layer is spin-coated onto a substrate and subsequently illuminated through a mask design, so that the restricted areas that are to form the structures are exposed to light. Upon development of the resist layer, either the illuminated areas (positive resist) or dark areas (negative resist) are cleared from resist. After the mask design has been transferred to the substrate in the form of a resist mask, the patterning of the deposited multilayer stack is accomplished by lift-off, chemical wet etching or ion-milling, which are discussed separately in Section 3.2.2.

As indicated in Figure 3-4-A and B, current confinement to very narrow regions of a magnetic multilayer is realized by contacting the multilayer through a nanometer scale

hole, called a ‘point contact’, etched into an insulating SiO2 layer. Commercially available optical lithography equipment is able to produce feature sizes with line widths well below 90 nm, while experimental limits up to 45 nm are obtainable.

However, high-end, state-of-the-art, high-resolution (deep UV) optical lithography equipment is costly and features only limited flexibility in experimental optimization runs, where contact lithography is preferably used. Therefore, the nanometer scale dimensions of the point contacts exclude standard optical lithography, because of its resolution limits, as a viable option for the repeatable definition of these structures.

Moreover, the definition of the nanometer scale electrode tip ends of the structures with optical access to the magnetic layer around the contact (Figure 3-4-B) requires greater alignment precision than can be obtained with manual alignment as used in the optical lithography process. This kind of high precision alignment is a prerequisite for obtaining a working device, since the slightest misalignment of the tips with respect to the underlying point contact will result in a faulty device. Therefore, the nanometer scale point contacts and top electrode tip ends are defined with high-resolution electron-beam lithography instead of conventional optical lithography.

Figure 3-4: The definition of nanometer scale point contacts and their associated electrode tips requires high resolution and precise alignment, which is obtained through the use of e-beam lithographic patterning. The magnetic thin film elements and the main part of the top electrodes are defined with regular optical lithography.

Note that a slight overlap exists between the electrode tip and the main part of the top electrode.

With electron-beam lithography, a mask design is transferred to a resist layer through a direct writing technique: the substrate surface is scanned with a focused electron-beam in a pixel-by-pixel manner. For a constant intensity electron-electron-beam, the amount of time a single pixel is illuminated determines the dose of the exposure, expressed as the amount of charge delivered per square area [C/cm²]. Due to the scanning nature of e-beam lithography, pattern definition is a rather slow and costly process, which is

Point contact diameter < 200nm

Tip in uniform Ti/Au layer Tip and top electrode overlap Optical lithography top electrode

Insulating SiO2

Point contact etched into SiO2

SiO2 (50)

why it is only employed for the definition of the critical features in the design.

However, this drawback is made up for by the greater flexibility concerning new mask designs and by the increased resolution and alignment properties compared to the optical lithography technique discussed earlier. Both optical and e-beam lithography have to be combined with a thin film removal process for the multilayer material around the transferred design, which is the topic of the next section.

Figure 3-5: Schematic illustration of the lithographic patterning process (A), lift-off (B) and (dry or wet) etching (C). In case of lift-off (B), the thin film is deposited after the pattern has been defined, while in case of ion-milling or wet etching (C), the deposition of the material precedes the lithographic patterning process.

3.2.2. Pattern Transfer 3.2.2.1. Lift-Off

In the lift-off approach to patterning (Figure 3-5-B), a mask design is transferred to a substrate in the form of a resist pattern, before the deposition of the thin film layer.

After material has been sputtered uniformly over the substrate, the wafer is immersed

Optical Mask Resist Layer Substrate

B. Lift-Off C. Ion-Mill or Wet Etch

A. Optical Lithography

Lithography and development precede deposition

Lift-Off

Deposition, followed by lithography and development

Ion-Mill or Wet Etch

Resist stripping LIGHT

Thin Film Substrate Resist

in a substance that dissolves the resist layer underneath parts of the deposited layer.

The lift-off process derives its name from the fact that it lifts these parts of material off the sample, while the designed structures are left intact. The result is a substrate that carries a series of rectangular thin film strips that will be further processed towards point contact nano-oscillators. Compared with the ion-milling technique that is to be discussed next, the lift-off process offers the advantage that no depth control is needed: the etch depth is exactly the thickness of the deposited film, i.e. all material is removed. On the other hand, the maximum layer thickness that can sustain lift-off is limited by the thickness of the resist layer and compared with shorter wavelength (e-beam) lithography the resolution is also limited. Moreover, lift-off of thicker films often results in badly defined edges which may pose difficulties in subsequent processing steps.

3.2.2.2. Ion-Milling and Wet Etching

Ion-milling is a non-selective physical process for material removal which is greatly analogous to the sputtering process described in Section 3.1.1. Within a vacuum, energetic xenon ions from a plasma are accelerated towards a target, in this case the substrate itself, and remove material in the developed resist areas. For a constant intensity beam of ions, the milling depth is controlled by the amount of time the sample is being bombarded by the ions. Note that unlike the case of a lift-off approach (Figure 3-5-B), where the lithographic step precedes the deposition of material, the ion-mill approach (Figure 3-5-C) requires the lithographic step to be performed after deposition of the magnetic stack. The same holds for the wet etching procedure that relies on the action of a chemical etching agent to remove sputtered material in the developed areas of the transferred resist pattern.