Development and design of a diffractive optical element (DOE)

Description

Diffractive optical elements (DOEs) are computer generated holograms that can be used for laser beam shaping and sampling. In order to design a DOE it is very important to apply the light propagation laws. The scalar diffraction theory is the most used to determine the transmittance or reflectance of a DOE designed to generate a certain light structure by transmission or by reflection respectively. However, if our desired light pattern has just a simple form, we can use a procedure to design phase DOE based not on diffractive optics, but on geometrical optics (ray tracing). At last we shall apply diffraction theory to prove the designed DOE.




A DOE having the same phase function as the refractive element, but wrapped to modulo 2π values, i.e. in the range [0, 2π) is equivalent to the refractive element, i.e. it has the same action to the incident beam as the refractive element itself. Therefore one must fabricate a transparent structure able to realize this phase modulation. The main restrictive factor in producing the DOE is the phase quantization, which decrease the quality of the image. A 16-level phase quantisation would give satisfactory results. Our technologies allow us to realize only binary DOEs (two quantization levels).

Technology

A very good control of the tracks localizations and depths is required to create DOEs. We prefer to realize reflective DOEs. Reflective DOEs must be created onto a very flat substrate with very small roughness. Moreover the substrate must be covered with a strong reflective layer for the working wavelength. The phase modulation amplitude must be of π and the track depth must be δ = λ/4. To create reflective DOEs, a thermally grown SiO2 layer on a Si wafer, with a precise thickness, has been structured by optical lithography, using the Cr on glass master mask. Then this structure must be uniformly metallized.

The reflective DOEs technological steps are given as follows: We chose an appropriate Si substrate. Firstly the substrate must be covered with a SiO2 layer of a certain thickness (λ/4). For this purpose we kept the Si substrate in an oven at a temperature of 1100°C in dry oxygene atmosphere for a time of 140 minutes. We wanted to get a 160 nm thick SiO2 layer, but after the oxydation process, the layer thickness resulted to be of 180 nm, as measured by a Talystep device. In the second step, a positive photoresist (HPRD 402- type), was spun by spinning at 2000 rpm on the Si wafer and then thermally dried in oven for 40 minutes.

After UV exposure trough the chrom photomask the photoresist was developed. The next step consisted in etching the SiO2 layer left uncovered by photoresist. To do this we used a mixture of ammonium chloride (NH4Cl) and fluorhidric acid (HF). Then the resist has been removed in acetone solvent.

At last we deposited an uniform Al layer by electron beam evaporation. The Al layer thickness must be of about 150 nm to ensure a good reflectivity. Because the etching process is isotropic, the etched regions become larger than the corresponding regions on the lithographic mask. That is why supplimentar precautions must be taken into account when designing the lithographic mask.