Porous Silicon Multilayer Infrared Bandpass Filters

Xiang Zheng Tu

 

Nowadays many people with diabetes need to measure their blood glucose levels by pricking their fingers, squeezing drops of blood onto test strips, and processing the results with portable glucometers. The process can be uncomfortable, messy and often has to be repeated several times every day.

In order to help improve the lives of millions of people by enabling them to constantly monitor their glucose levels without the need for an implant, non-invasive measurement approaches of blood glucose concentration based on absorption measurements in the infrared region have been explored many years. Among them is a micro-optical-mechanical-electro-system (MOMES)-based non-invasive blood glucose monitor designed by the present author ten years ago, as shown in the above figure.

The monitor comprises a micromachined infrared optical filter array, a micromachined infrared mechanical modulator array, at least one micromachined infrared tunable filter, and at least one infrared detector. Each optical filter is aligned with a mechanical modulator along its optical axis direction. The optical filter continuously divides a monochromatic infrared light in a wavelength range within 0.8 to 25 micron from an infrared light. The aligned mechanical modulator turns the monochromatic infrared light into an alternating monochromatic infrared light. The tunable filter is aligned with the infrared detector along its optical axis direction. The tunable filter selects the back-diffused alternating monochromatic infrared light emitted from a measured blood subject that is illuminated by the alternating monochromatic infrared light. The infrared detector converts the back-diffused alternating monochromatic infrared light into an alternating electronic signal. Then a photo-integrated circuit (IC) combines with the infrared detector for synchronous detection and amplification of the electronic signal generated by the synchronous detection.

The micromachined infrared optical filter could be a porous silicon multilayer infrared pass band filter, reference to the above figure. The basis of the porous silicon filter is the same as in a common interference filter. The main difference is that in the porous silicon filter the difference in refractive indices is caused by different porosities of porous silicon layers, not different layers of different materials as in common interference filters. The porous silicon multilayers are produced by changing one of the etching parameters periodically. Etching parameters that affect the morphology and pores’ depth are current density, electrolyte composition, sample’s doping, etc. Once a porous silicon layer is formed anodization stops in this layer and only proceeds in pore tips. The porosity of layers depends only on current density when other etching parameters are kept fixed so that changing the current density results in layers with different porosities in depth of sample.

The porous silicon multilayer shown in the above figure can be expressed as:

(LH )^mLL (HL)^m-1 H                                                                                (1)

where L denotes a layer with low refractive index and H denotes a layer with high refractive index, m is the numbers of repeating periods. Optical thickness of L and H Layers in bandpass interference filter should be equal to one fourth of peak wavelength: 

n_Ld_L = n_Hd_H = λ_p/4                                                                                   (2)                                                               

where n_L is  refractive index and d_L physical thickness of L layers, similarly n_H and d_H correspond to H layers, and λp is a peak wavelength. 

The effective refractive index of porous silicon layer, n, depends on its porosity. The refractive index is almost a linear function of porosity. Bruggeman approximation is used to determine the effective refractive index of porous silicon layer:  

n = (1 - p) ( ɛ_si - ɛ_psi ) / (ɛ_si + 2 ɛ_psi ) + p (ɛ_air – 2 ɛ_psi ) / (ɛ_air + 2 ɛ_psi )         (3)

where p is the porosity, and ɛ_air, ɛ_si, ɛ_psi are the dielectric constants of air, silicon, and porous silicon, respectively.

The infrared light source shown in the above figure could comprise a resistive heater positioned on the top of a membrane suspending over a cavity. All theses elements of the device are constructed as a microstructure and integrated with the porous silicon multilayer infrared bandpass filter in a same silicon substrate. As can be seen in the figure the collimator is also formed in the silicon substrate and positioned along the extending direction of the light source and the porous bandpass filter. In this way the micro-optical-mechanical-electro-system (MOMES)-based non-invasive blood glucose monitor can be small in size, light in weight, compact in structure and low in power consumption.