Integrated sensitive on-chip ion field effect transistors based on wrinkled InGaAs nanomembranes
- Stefan M Harazim†1Email author,
- Ping Feng1,
- Samuel Sanchez1,
- Christoph Deneke1,
- Yongfeng Mei1 and
- Oliver G Schmidt1
© Harazim et al; licensee Springer. 2011
Received: 1 November 2010
Accepted: 14 March 2011
Published: 14 March 2011
Self-organized wrinkling of pre-strained nanomembranes into nanochannels is used to fabricate a fully integrated nanofluidic device for the development of ion field effect transistors (IFETs). Constrained by the structure and shape of the membrane, the deterministic wrinkling process leads to a versatile variation of channel types such as straight two-way channels, three-way branched channels, or even four-way intersection channels. The fabrication of straight channels is well controllable and offers the opportunity to integrate multiple IFET devices into a single chip. Thus, several IFETs are fabricated on a single chip using a III-V semiconductor substrate to control the ion separation and to measure the ion current of a diluted potassium chloride electrolyte solution.
The integration of ion field effect transistors (IFETs) into solid state micro-/nanofluidic systems for accurate transport control of charged species such as ions, proteins, or DNA in "Lab on a Chip" systems (LoC) is highly important in life sciences [1–5]. Recently, there is an increasing interest in IFETs due to the high demand in micro total analysis system (μ-TAS) devices dealing with bio-health, bio-sensing, and bio-physical applications [3, 6, 7]. Those μ-TAS devices should automate the entire analytical process, from sample processing and preparation to sensing and analysis within a small, cheap, and easy to handle system. To realize structures for simultaneous sample treatment on a single chip, it is crucial to combine (1) a semiconductor material as substrate material for microelectronic components and (2) a fabrication technique which ensures the integration and alignment of the nanofluidic channels within the chip .
Over the last few years, different production techniques have been developed to prepare nanochannels for ion control such as nanotubes [9–12], porous membranes  or ion channels  to name a few. The permeability of nanofluidic channels for ions can be modified by the channel size, surface charge distribution within the channel, and external electrical fields [2, 5]. For extrinsic ion current control within the nanochannel, a field effect transistor electrode structure design has to be integrated [1, 5, 14]. New technological methods are necessary to achieve the challenging goal of fabricating accurately aligned single nanochannels, including the microelectronic integration into a μ-TAS device. Other fabrication techniques have not succeeded in the combination of all of these μ-TAS requirements. Furthermore, the fabrication of nanochannels with only one dimension in the nanometer range does not provide easy single molecule treatment. The random alignment to the substrate of some nanochannel fabrication methods leads to difficulties in the reproducibility of IFET production. As an alternative solution or to overcome these limitations, self-deterministic wrinkling in the nano regime on semiconductor materials were investigated recently and successfully tested for their fluidic capabilities. Due to the high integration state and their good alignment to the substrate, it was predicted that they could eventually be potential candidates in new lab on a chip applications . Herein, we present a new method of manufacturing nanofluidic channels for IFET devices on a semiconductor material by combining our recently developed technique called "release and bond back of layers" (REBOLA) with microfluidic fabrication techniques [8, 15, 16]. The original REBOLA process relied on wrinkling large strained nanometer thick membranes onto semiconductor substrate materials, which allowed us to fabricate complex networks of nanochannels. We improved the REBOLA process to fabricate aligned single nanochannels of different types, instead of producing networks, to be utilized in IFET devices. Consequently, the nanochannels are integrated into a microfluidic device unit which includes the photolithographically defined electrodes for ion detection as well as the microchannel system for liquid transport. A self-limiting atomic layer deposition (ALD) of Al2O3 isolates the electrodes electrically from the substrate and fine-tunes the inner diameter of the nanochannel to match the channel height with that of the Debye length of the liquid. The Debye length expresses the thickness of the electrical double layer at the solid-liquid interface, where the electrical neutrality of an electrolyte is broken and is equal to approximately 30 nm for a 10-4 M ion solution . The integration of wrinkled nanomembranes into a LoC system is presented. Furthermore, the nanochannels are suitable for ion current manipulation and IFET applications, which was tested with a 10-4-M KCl solution as electrolyte with differing electrode parameters (from +1 V to -1 V).
The device structure consists of three assembling parts: (1) the nanofluidic channel fabrication, (2) integration of electrical components, and (3) microfluidic channel fabrication for the liquid reservoirs on either side of the nanochannel, including a top poly-dimethylsiloxane (PDMS) sealing layer.
Versatility of wrinkling
The sample was mounted onto a chip carrier which was plugged into the measurement stage. The measurement itself was controlled by a PC software and was operated by the semiconductor parameter analyzer 4156C (Agilent, Santa Clara, CA, USA). All measurements have been done at room temperature. Prior to the ion current measurement, the system was successfully tested to have no electrical or liquid leakage, which was carried out without liquid in the system to ensure electrical isolation between each electrode and between the electrodes and the substrate. Al2O3 is stable using the current measurement conditions with a maximum electrical field strength of about 2 V/μm. The breakdown voltage for similar systems was investigated recently and is more than one order of magnitude higher . The liquid leakage tests have been investigated with pure DI water and the same KCl solution as for the ion current measurement. The conductivity was always in the expected range so that no leakage was present .
The ion current manipulation had been driven in a sweep mode starting with V G = -1 V to V G = +1 V in 0.5-V steps. The ion permeability of the nanochannel for a specifically charged species can be tuned by the gate electrode while keeping all other electrode parameters constant. The measurement took place always at the drain electrode. Figure 6b shows the ion current modulation of Cl- (red circular data points) and K+ (black square data points) ions with changing of the gate potential. In case that there is no gate effect (V G = 0 V), the same ion current is expected, independently to the chosen source and drain parameters, since potassium and chloride ions have almost the same mobility in a liquid (76.2·10-7 m2/sV for K+ and 79.1·10-7 m2/sV for Cl-) . Figure 6b shows that at V G = 0 V, the ion current for Cl- and K+ is in the same order at about 28 pA. By increasing the gate potential to V G = +1 V, positive ions will be repelled and negative ions will be attracted to move through the nanochannel. This gate potential modification results in a lower K+ and an increased Cl- ion current. A mirrored behavior can be observed when the gate potential is decreased to V G = -1 V. The weak effect on the chloride ion current modulation at lower gate potentials can be explained by the permeability of PDMS for water molecules during the measurement. During this time, water molecules will diffuse into the PDMS, which can slightly increase the ion concentration in the reservoirs. Therefore, the Debye length decreases, and the gate potential loses the efficiency for ion current manipulation.
The presented IFET devices have a comparatively fast ion current detection capability in the lower pA regime . The feasibility of accurate nanochannel alignment on semiconductor substrates, and the high integration state of all components are strong criteria for future continued investigations on wrinkled nanomembranes for more complex IFET systems.
The fast and easy fabrication of nanochannels by the combination of the versatile REBOLA technique and standard photolithography leads to semiconductor films which form nano-sized wrinkles with useful fluidic capabilities. Different nanomembrane shapes have been prepared to investigate the most suitable wrinkling structure for IFET devices. Rectangular-shaped membranes wrinkle into straight two-way channels with a fixed orientation relative to the crystal structure of the substrate. Indeed, a circular-shaped membrane creates a higher variation of channel types, which can be used to fabricate more complex fluidic circuit, on-chip structures, but the square-shaped structures wrinkle always the same, which is more useful for reproducible IFET assembling. To demonstrate the feasibility of the integrated nanochannels for IFET on-chip devices, samples with several IFETs have been fabricated and were successfully tested for ion separation using KCl as a model electrolyte solution. The usage of a semiconductor material as a substrate and the highly integrated state of all components, including accurate channel positioning and definable channel orientation, might be highly demanded for the next integration level of "Lab on a Chip" devices. The near future approach is to adapt the system for single molecule detection, which should find a huge number of applications in bio-analytic μ-TASs. Later on, light can be used for controlling the wrinkling behavior of the nanomembrane in order to obtain more complex and deterministic wrinkle structures .
Film growth, structure reproducibility
The III-V semiconductor layers were epitaxially grown on GaAs(001) substrates. After the growth of a 200-nm GaAs buffer, 80-nm AlAs and 20-nm In0.2Ga0.8As were grown. Several samples were lithographically patterned to ensure the reproducibility of nanochannel formation. The sample dimension is always 7 by 7 mm in lateral dimension. A sample contains 12 of the so-called nanochannel arrays with 165 wrinkling structures each (see also Figure 3a). This is to increase the chance to have the same-shaped nanochannel with the same fluidic capabilities on every new sample. For reproducibility, all samples are cut along the <110> GaAs direction and the photolithographic pattern has been always aligned to the substrate structure in the same way.
Wet etching risks and safety
The HF etchant is highly toxic. Special safety clothes are strongly recommended. The local waste disposing procedure is to be obeyed.
Electrical measurement device and conditions
The semiconductor parameter analyzer Agilent 4156C has been used under standard environment conditions, such as room temperature and normal pressure. The measurement device has been driven in the "sweep measurement mode." Light impact on the sample as well as acoustic vibration has to be avoided at all times because of its semiconductor behavior and the high electrical sensitivity of the measurement device.
The authors thank Elliot Smith for the fruitful discussions and Emica Coric for the FIB cut and SEM images. This work was funded by the Volkswagen Foundation (I/84072).
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