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. 

MEMS Optical Fabry–Pérot Switches

Xiang Zheng Tu

 

Ten years ago, the present author designed a MEMS optical switch array for DNA synthesis and detection, as shown in the figure 1.  A MEMS optical Fabry-Perot switch consists of a silicon substrate, a cavity and a driving circuit. The cavity is formed by a deflectable plate and a fixed plate which are separated by an air gap. The plates are constructed by dielectric thin films coated with a metal film on their opposite surfaces. The dielectric plates are transparent in the wavelength ranges 350 nm – 14000 nm. The metal films are used as both optical reflecting mirrors and electrodes connecting to the driving circuit. The air gap can be changed by applying the voltage between the two plates resulting in an electrostatic force which pulls the plates closer.

 

The principle of operation of the optical switch is illustrated in the figure 2. The input signal is incident on the left surface of the cavity. After one pass through the cavity a part of the light leaves the cavity through the right facet and a part is reflected. A part of the reflected wave is again reflected by the left facet to the right facet. If the air gap is equal to half an even multiple of the wavelength in the cavity a round trip through the cavity will be an integral multiple of the wavelength. In this case all the light waves will transmit through the right facet add in phase. Such wavelengths are called the resonant wavelengths of the cavity and the optical switch is in “on” state. Similarly, if the air gap is equal to half an odd multiple of the wavelength all the light waves will reflect by the cavity and the optical switch is in “off” state.

Optical Fabry-Perot cavities based on micro electro-mechanical systems (MEMS) are an enabling technology for hyper spectral images and micro spectrometer. MEMS optical switches are high pass filters that block the visible light and pass ultraviolet light. They are characterized by their bandwidth at which maximum transmission is 50%. A MEMS optical switch array consists of a Cartesian grid of switches. This can be used chiefly to map or "encode" the coordinate of each switch to its function. Switches in these arrays typically use a universal signal ling technique (e.g. fluorescence), thus making coordinates their only identifying feature.

Additional features of the MEMS optical switch array for DNA synthesis and detection are combination of DNA synthesis and detection, high probe density and low fabrication cost. Such DNA probes with a MEMS optical switch array can help to dramatically accelerate the identification of the estimated 80,000 genes in human DNA, an ongoing world-wide research collaboration known as the Human Genome Project. The DNA probes can quickly sequence DNA. In addition to genetic applications, the DNA probes can be used in toxicological, protein, and biochemical research. The DNA probes can also be used to rapidly detect chemical agents used in biological warfare so that defensive measures can be taken.

Thermopile Natural Gas Mass Flow Meters

Xiang Zheng Tu

  

Natural gas flow meters are used at residential, commercial, and industrial buildings that consume gas supplied by a gas utility. Several different types of gas flow meters are in common use, depending on the flow rate of gas to be measured, the range of flows anticipated, the type of gas being measured and other factors.

Diaphragm gas flow meters are one type of the most common and oldest gas meters. The advantage of these gas meters is simplicity of construction and, therefore, low cost but their limits are as follows:

(i)             presence of moving parts subject to wear;

(ii)           high pressure losses;

(iii)         mechanical output and

(iv)          inability to indicate an instantaneous flow rate value.

Nowadays new safety-related and consumption-control-related functions have brought about the development of better performing flow meters, with features such as:

(i)             distribution network and user-connection/disconnection blockage valves remote control;

(ii)           remote consumption reading; (iii) overflow and minimum level flow rate alarms;

(iii)         self-control and diagnostics;

(iv)          metrological performance improvement (accuracy, rangeability, stability and thermodynamic condition compensation); (vi) size reduction; and

(v)           advanced computing functions (prepayment and time bands).

Ultrasonic flow meters could represent a good solution in such better performing flow meters. There are two leading types, the transit-time and Doppler style meter. In the transit-time ultrasonic flow meter, the transducers are upstream and downstream of each other and each act as a transmitter and receiver. One transducer, of course, emits the ultrasound signal with the flow, while the other emits it against the flow. The meter measures the difference in transit time between the two transducers, and the velocity difference is used to calculate volume flow.

Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. They vary greatly in purchase price but are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.

Actuary, gases are more difficult to measure than liquids, as measured volumes are highly affected by temperature and pressure. Gas meters measure a defined volume, regardless of the pressurized quantity or quality of the gas flowing through the meter. Temperature, pressure and heating value compensation must be made to measure actual amount and value of gas moving through a meter.

To solve these problems thermal mass flow sensors could be the best choice. As shown in the above figure, thermal flow meters measure mass flow, not volumetric flow, and use heat disperse to compute the measurement. The primary reason thermal mass flow meters are popular in many applications is their particular features including no moving parts, nearly unobstructed straight through flow path, require no temperature or pressure corrections and retain accuracy over a wide range of flow rates.

The thermal natural gas mass sensors provided by POSIFA Microsystems are manufactured using an US patented technology. The sensor comprises a porous silicon wall with numerous vacuum-pores which is created in a silicon substrate, a porous silicon membrane with numerous vacuum-pores which is surrounded and supported by the porous silicon wall, and a cavity with a vacuum-space which is disposed beneath the porous silicon membrane and surrounded by the porous silicon wall.

Compared to the other thermal natural gas flow sensors, the vacuum-cavity-insulation flow sensor presents superior properties in many aspects.  Among them are easiness of fabrication, perfection of thermal isolation, strength of membrane structure, and lower cost of manufacturing. They have additional performance such as auto-diagnostic, data-recording, block and other functions that can be integrated in an electric output sensor. In the near future they could be an excellent replacement for the ultrasonic flow meters.