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Diffractive Optical Elements

Diffractive optical elements are found nearly everywhere. Their application fields include

  • optical gratings (in spectroscopy),
  • industrial laser material processing (mode selection, beam delivery),
  • holographic tags (authentication of packaging material or product),
  • anti-counterfeiting holograms (for banknotes and passports),
  • LED (light emitting diode) beam extraction/collimation/shaping,
  • HUD (head-up display) systems,
  • anti-reflection structures (for solar energy systems),
  • hybrid multi-focus lenses (e.g. for CD/DVD, Blu-ray systems),
  • devices used in VR/AR (virtual and augmented reality), and
  • x-ray optics (zone plates)

– to name just a few. There are many possibilities of producing such elements, e.g. photo lithography (using masks), laser lithography and electron beam lithography.


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Why use Electron Beam Lithography (EBL) to make diffractive optical elements?

Flexibility is a key advantage of EBL. It is a maskless method of defining patterns from “computer to plate”, where digital layouts are directly converted into a resist pattern on substrate. Classical electron beam lithography using a positive or negative tone resist produces binary (two-level) patterns (surface elements with resist or without resist) after exposure and development. Further processing (e.g. etching and metallization) delivers a one-off sample or a replication template for mass fabrication, e.g. using nano imprint lithography or NIL).

Simple binary gratings as shown in Fig. 1 immediately reveal another advantage of the technique: very high resolution due to an electron beam which can be focused down to a diameter of a few nanometers. A wide range of pitches from < 100 lines / mm (>10 µm pitch) to > 33 000 lines / mm (<30 nm pitch) is covered. True digital definition provides maximum flexibility; e.g. different pitches can be established in a single device (pitch definition per coordinate, e.g. for a chirped grating). Such gratings can be made on large areas as needed in practice, e.g. to make templates for wire grid polarizers.

SEM image of a binary grating
Fig. 1a: SEM image of a binary grating with 2 500 lines / mm – made with a RAITH PIONEER Two. A SEM line scan shows a measured pitch of 402 nm.
SEM image of a grating with 30nm pitch
Fig. 1b: SEM image of a grating with 30 nm pitch (33 000 lines / mm) – written with a RAITH EBPG machine.

Focused ion beam (FIB) tools can sometimes be used as an alternative to EBL (see Raith product VELION). In the case of FIB, the focused ion beam is used to mill a structure directly into the surface, i.e. there is no resist and no further processing is necessary. Some examples shown here could also have been made using FIB technology. (Picture 2a shows part of a device produced with FIB technology.)

Any shape

EBL- (or FIB-) written structures can naturally have any shape (defined by a mathematical function or numerically defined). Examples of elements with curved elements are Fresnel zone plates, which are widely in use in x-ray applications (see Fig. 2a), and Bragg grating couplers, which are used in integrated optics to launch or extract light into or from a photonic integrated circuit (see Fig. 2b).

SEM image of Circular zone plate for x-ray applications
Fig. 2a : Circular zone plate for x-ray applications (courtesy of MPI Stuttgart)
SEM image of a Bragg grating coupler
Fig. 2b: Bragg grating coupler (Centech Münster Germany)

Example for complex multilevel structures: Computer Generated Holograms (CGH)

Electron beam lithography can be used to build devices for which any element (“cell”) over a matrix of cells has a different height: in other words, a defined phase shift per coordinate. An example is the computer-generated hologram (CGH) as shown in Fig. 4. It is not possible to “see” the content of this element; only reconstruction (by shining a laser beam on the CGH) reveals the content – in this example, a computer keyboard.
There are numerous further applications for such devices like (laser) beam splitters, shapers, focusers and diffusers.

SEM image of “grayscale phase hologram”
Fig. 4a: SEM image of “grayscale phase hologram” exposed in resist.
3D SEM picture showing a cross-section of a Fresnel lens
Fig. 5a: 3D SEM picture showing a cross-section of a Fresnel lens (the product actually made was an array of such lenses covering many square centimeters) – written with a Raith VOYAGER tool.
Reconstructed image of the hologram
Fig. 4b: Reconstructed image of the hologram from the left, comprising a keyboard. Example produced with RAITH ELPHY by PIDC, Taiwan.
SEM picture of a blazed grating cross-section
Fig. 5b: SEM picture of a blazed grating cross-section, structure written with a Raith VOYAGER tool.

Grayscale (3D) electron beam lithography

Computer-generated holograms comprise typically a number of fixed height levels (phase shifts), the same is true for multi-level diffractive optics.
In grayscale (3D) lithography the height of residual resist after exposure and development is determined by an individual, continuously defined electron dose per pixel, therefore defining a three-dimensional landscape of resist after one step of electron beam lithography and development. This method could be regarded as a production method of multi-level diffractive optics with an infinite number of height levels. For real-world usage (requiring e.g. stamps for replication) the 3D resist structure is converted into other materials such as glass or nickel using metallization or etching.

Examples of use of grayscale electron beam lithography include blazed gratings, (“echelette” gratings) which deliver maximum diffraction efficiency in a given diffraction order, and Fresnel-lenses, which in their simplest form comprise a set of curved annular elements replacing an ordinary curved (spherical) lens. Examples of such diffractive optical elements are shown in Figs. 5a and 5b.

Optical variable devices (OVDs) are another type of diffractive optical elements for which 3D electron beam lithography is the manufacturing method of choice. OVDs change their optical appearance depending on illumination and viewing angle (hence the name). This effect is due to (blazed) grating elements for which the pitch and orientation are a function of the coordinate. Elements like these are familiar from banknote security features.

SEM image of an OVD test element
Fig. 6: OVD test element made with an EBPG tool, illuminated with white light (Courtesy of Institute of Scientific Instruments of the Czech Academy of Sciences (ISI CAS) Brno, Czech Republic). The element, actual size 16mm*26mm, is a performance pattern to test machine capabilities and generated with a computer algorithm. The resolution used goes down to sub-15 nm pixel size.

Commercial production of diffractive optics 

Commercial production of diffractive optical elements (typically via master fabrication) comes along with challenges. Requirements include not only fidelity (i.e. the result must match the design as much as possible), but also a reasonable throughput (writing speed). The Raith EBPG Plus series and Raith VOYAGER have been proven as possible solutions. Raith offers to evaluate your application as your partner. Specialist expertise in the fields of optical variable devices (OVD) used as security labels (security “holograms”) and virtual and augmented reality devices is available. Please contact us for more information.

Academic research and education on the topic of diffractive optical elements

As the examples above show, manufacturing diffractive optics is feasible with various (in fact with all) Raith electron beam lithography tools. Simple diffractive optical elements could even be made using a SEM and the Raith lithography kit ELPHY. The ELPHY kit comes with an optional tool box for grayscale lithography and includes a calculator for Fresnel lenses with defined focus length for a given wavelength. Raith provides both the software and the necessary hands-on training by experienced lithography engineers to get off to a smooth start in the subject.