By Dr. Petra Göring, Halle
Using photolithography and electrochemical processing allows metal deposition into silicon pores over a wide range of diameters from a few hundred nanometres to some tens of microns. By exploiting the crystalline structure of silicon, a pore structure with excellent orientation and complete permeation of the substrate is possible. The resulting silicon carriers with thicknesses between 15 μm and 500 μm with diameters of 130 mm are ideal for separation of a range of species, as for example is often the requirement in medical technology.
Makroporöse Siliziumträger für multifunktionale Anwendungen
Unter Einsatz der Photolithographie und elektrochemischer Verfahrenstechnik lassen sich in Silizium Poren mit einem relativ breiten Spektrum des Durchmessers im Bereich von einigen Hundert Nanometer bis zu einigen Zehn Mikrometer erzeugen. Durch Ausnutzung der Kristallstruktur des Siliziums entstehen so Porenstrukturen mit einer exzellenten Orientierung und vollständiger Durchdringung der Substrate. Die erzeugten Siliziumträger mit Dicken zwischen 15 µm und 500 µm sowie Durchmessern von 130 mm eignen sich sehr gut zur Trennung unterschiedlicher Stoffe, wie sie beispielsweise in der Medizintechnik benötigt werden.
Macro porous silicon produced by electrochemical etching of lithographically pre-structured silicon wafers [1, 2] is a promising material for a variety of novel devices. Potential applications of macro porous silicon have been proposed in the fields of micro-processing, gas measurement, photonic crystals, biotechnology and many others. Examples are short-time optical filters [3], 2D and 3D photonic crystals [4-6] and optical and capacitive sensors [7, 8].
The etching of silicon and therefore the growth of pores with a certain diameter covers several orders of magnitude. According to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature for porous materials [9], structures with a pore width below two nanometers are called micro porous. The meso porous structures range from 2 to 50 nanometers. Bigger pores are referred to as macro pores (Fig. 1).
Fig. 1: Overview of the porous silicon types
This kind of pores can be obtained under a variety of conditions and with differing morphologies. The key parameters are the electrolyte type (aqueous, organic, oxidant), the HF concentration, the surfactant, the silicon doping type and level (n, n+, p, p+) and in some cases the illumination (backside or front side illumination). Detailed reviews about their formation are available, e. g. Föll, Lehmann 2002 [1] or Chazalviel and Ozanam 2005 [10].
In this article, focus are on electrochemically etched macro pores in n-type silicon.
1 Electrochemical macro pore formation in n-type silicon
For the release of anodic-poled silicon at the semiconductor electrolyte boundary layer the existence of holes is required [2]. In the case of photo-electrochemical etching the silicon is therefore additionally illuminated from the rear side (i. e., the side facing away from the electrolyte) in order to produce electron-hole pairs by absorption of the photons. The generated electrons are directly sucked off by the anodic polarity while the holes diffuse through the semiconductor and thus reach the boundary layer. In addition, the current-voltage characteristic is shifted by the generation of holes in the illuminated n-type silicon, so that in the case of maximum illumination it assumes the form of the characteristic curve for p-doped silicon.
The illumination of n-doped silicon also results in a controllable degree of freedom for the etching process since the current density at the semiconductor-electrolyte junction can now be regulated independently from the applied voltage via the illumination intensity. Figure 2 shows the structure for photo-electrochemical etching according to Lehmann et al. [1].
Fig. 2: Etching of macro pores in n-type silicon under backside illumination. The holes generated by absorption of the light at the wafer backside diffuse through the wafer to the etching front. There they are consumed at the pore tips for the etching process (left). REM picture of a silicon pore bottom. At the lowest region of the pore bottom electro polishing takes place due to the high hole density while the outer edge of the pore bottoms is divalent dissolved (right)
The (100)-oriented n-type silicon wafer is in contact with HF at one side. The positive pole of the voltage source is placed on the back side of the n-type silicon wafer (anode). The negative pole forms the cathode made by platinum wire in the HF. The back side of the wafer is illuminated with light. Thereby electrons at the wafer backside will raise from the valence band into the conduction band and generate simultaneously holes in the valence band. The electrons of the conduction band are extracted to the positive pole of the voltage source while the holes drift through the wafer to the opposite etching front. The development of the space charge region (SCR) of the interface between silicon and hydrogen fluoride acid follows the geometry of the pore tips.
Therefore the lithography has to match the intrinsic material parameters. However, it is possible to initiate pore growth selectively at defined positions via artificial preparation of suitable ordered nucleation on the silicon surface. For this purpose, an anisotropic etching solution is used to generate etching pits in the form of inverted pyramids in the (100)-silicon surface (Fig. 3).
Fig. 3: Macro porous silicon by photo electrochemical etching, prepared by lithography and KOH pre-structuring; A – plane Si, B – photo resist + silicon dioxide, C – photolithography, D – HF dip, E – KOH etching
A defined top surface photo mask determines the structure and arrangement of the required pits. For this purpose, first the silicon wafer is coated with a photo resist (Fig. 3 A, B). Photolithography is used to expose the mask (Fig. 3 C) and to transfer the structure into the silicon surface (Fig. 3 E). In addition to the periodically ordered trigonal or cubic pore arrangements by specifically omitting certain nuclei, point or line defects can also be produced for the generation of resonant structures or photonic waveguides.
Starting material is a pre-patterned single crystal silicon wafer as shown in Figure 3. It is brought into contact with the structured front side with hydrofluoric acid and illuminated at the back. In addition, an anodic voltage is applied between silicon - contacted at the backside - and the electrolyte (contact by means of a platinum wire electrode). Depending on the intensity, the backlighting generates different electron-hole pairs. Due to the externally applied anodic voltage and supported by the back contact, the electrons are immediately sucked off. The generated defect electrons, driven by the concentration gradient, flow through the entire silicon wafer to the interface between silicon and hydrofluoric acid.
Figure 4 shows typical example of such two-dimensional macro pores, which can be produced, with lattice constants of 0.5 μm to 12 μm. They are characterized by a very flat etching front, i. e. deep pores, pores with practically identical diameters, low surface roughness and great perfection and precision.
The silicon standards available at 6 inch wafer scale at SmartMembranes are listed in Table 1. The structure parameters have some degrees of freedom, but the maximum or minimum main values are fixed as listed.
The standard spacings (a = pitch) are currently 1.5 µm, 4.2 µm and 12 µm. Other pitches are reasonable as well but in needs of a new pre-patterned surface and thus a new lithographic mask. Additional various post processing steps are possible, such as substrate lift-off to generate membranes thinner than the bulk material (Fig. 4 A), anisotropic or isotropic pore shaping (Fig. 4 B, C) and laser dicing in membranes sizes regarding the customer needs. All processes are applicable on 6” wafer size, but also scalable in future.
Fig. 4: SEM pictures of silicon membranes: anisotropic pore shape (left), isotropic pore shape (middle) and lifted membrane (right)
2 Potential applications of the porous silicon material systems
Based on SmartMembranes’ experience the membranes have a large application portfolio in many areas of materials science as well as in nano and micro production (Tab. 2). Nowadays the main focus of our membrane production lies in the needs of defined market segments - so-called niche market segments (Fig. 5).
Fig. 5: Application fields of the nano and macro porous membranes of SmartMembranes
Applications of macro porous silicon have been shown in the fields of micro system technology, gas sensors, photonic crystals, biotechnology and many others. Examples include short-pass optical filters, 2D and 3D photonic crystals, optical and capacitive immuno-sensors for monitoring immune complex formation. Macro porous silicon provides a platform for lots of interesting and innovative applications mostly in biotechnology. Silicon membranes with straight passages of different dimensions (pore size and length) have been proposed for instance for selective bio-organism detection [11]. Here an initial use of macro porous silicon is the Ratchet membrane [12], where the pores are characterized with asymmetrically modulated diameter in depth. This system can be suitable for efficient and selective continuous separation of sensitive biological materials such as viruses and cell fragments. Miniaturized biochips with involved high ordered macro porous silicon arrays have also proved to be advantageous for the use of the detection and recognition of molecular binding events [13, 14]. All of the above examples utilize several unique features of macro porous silicon, e. g. the ability to generate perfect periodic pore arrays of defined arrangements and high aspect ratio, large area-to-volume ratio and full process compatibility with silicon micro technology. Most applications benefit from the remarkable periodicity and straightness of macro pore arrays which are essentially 3D systems.
3 Elected applications of macro porous silicon membranes and templates
One of the innovative application examples is the multifunctional biochip sensor TipChip which was developed in cooperation with the company Axela Inc. (Canada). This flow-through chip is based on a lab diagnostic method for the determination of DNA and protein species. In this case, complex elaboration methods are replaced by substance-selective separations in the pores (transition from 2D to 3D structures with defined surface modification of the inner pore walls). The single-use consumable has been designed for routine and focused multiplex analysis for nucleic acids or proteins. The TipChip is a disposable device consisting of a 6.5 mm square chip mounted on a plastic tube. The chip is made of porous silicon with > 200.000 micro channels incorporated in that area. A single capture probe site occupies approximately 70 micro channels. This approach facilitates the interaction between target molecules and immobilized probes, resulting in 3 to 4 times faster hybridization of oligonucleotides or protein binding (Fig. 6). A highly selective multifunctional biosensor system could be realized by using a electro-sensory measurement technology.
Fig. 6: 3D Flow-through TipChip Technology (Axela Inc.); mode of operation with transfer from 2D to 3D substrate (above); TipChip – functionalized macro porous silicon membrane (below)
4 Conclusion
A basic introduction to the fabrication as well as applied aspects of macro porous silicon were presented. These outstanding material properties make the system ideal candidates for great number of applications. The highly reproducible geometries can be fabricated using an inexpensive and well-controllable etching process.
The morphology can be tuned over a broad range (0,5 µm up to a few microns) by modification of the anodization conditions as well as defined material properties.
Characteristics which occur due to the nano structuring of bulk material, for example luminescence, dependence of the refractive index on the porosity, biodegradability and bioactivity render porous silicon a material which can be exploited in optics, sensor technology, biomedicine and many more.
The focus of product development in the actual market in recent years are sensing and biomedical applications (gas-, biosensors, tissue engineering, controlled drug delivery and diagnostics).
Literature
[1] V. Lehmann, H. Föll: Formation mechanism and properties of electrochemically etched trenches in n-type silicon; J. Electrochem. Soc. 137 (1990), pp. 653–659
[2] V. Lehmann: The physics of macropore formation in low doped n-type silicon; J. Electrochem. Soc. 140 (1993), pp. 2836–2843
[3] V. Lehmann, R. Stengl, H. Reisinger, R. Detemple, W. Theiss: Optical shortpass filters based on macroporous silicon; Appl. Phys. Lett. 78 (2001), pp. 589–591
[4] A. Birner, R. Wehrspohn, U. Gösele, K. Busch: Silicon-based photonic crystals; Adv. Mater. 13 (2001), pp. 377–388
[5] J. Schilling, R.B. Wehrspohn, A. Birner, F. Müller, R. Hillebrand, U. Gösele, S.W. Leonard, J.P. Mondia, F. Genereux, H.M. van Driel, P. Kramper, V. Sandoghdar, K. Busch: A model system for two-dimensional and three-dimensional photonic crystals: macroporous silicon; J. Opt. A: Pure Appl. Opt. 3 (2001), pp. 121–132
[6] S. Matthias, F. Müller, C. Jamois, R.B. Wehrspohn, U. Gösele: Large-area three-dimensional structuring by electrochemical etching and lithography; Adv. Mater. 16 (2004), pp. 2166–2170
[7] C. Betty,R. Lal, D.K. Sharma, J.V.Yakhmi, J.P. Mittal: Macroporous silicon based capacitive affinity sensor-fabrication and electrochemical studies; Sens. Actuator B 97 (2004), pp. 334–343
[8] R. Angelucci, A. Poggi, L. Dori, A. Tagliani, G.C. Cardinali, F. Corticelli, M. Marisaldi: Permeated porous silicon suspended membrane as sub-ppm benzene sensor for air quality monitoring; J. Porous Mater. 7 (2000), pp. 197–200
[9] D. H. Everett: IUPAC, Manual of Symbol and Terminology for Physicochemical Quantities and Units, Appendix, Definitions, Terminology and Symbols in Colloid and Surface Chemistry, Part I. Pure Appl. Chem. 31 (4), 577 (1972)
[10] Macropores in p-type silicon; in: Wehrspohn RB (ed) Ordered porous nanostructures and applications, Chapter 2., Springer, New York, pp. 15–35
[11] S.E. Letant, B.R. Hart, A.W. van Buuren, L.J. Terminello: Functionalized silicon membranes for selective bio-organism capture; Nat. Mater. 2 (2003), pp. 391–395
[12] S. Matthias, F. Müller: Asymmetric pores in a silicon membrane acting as massively parallel brownian ratchets; Nature 424 (2003), pp. 53–57
[13] V. Lehmann: Barcoded molecules; Nat. Mater. 1 (2002), pp. 12–13
[14] V. Lehmann: Trends in fabrication and applications of macroporous silicon; Phys. Stat. Sol. A 197 (2003), pp. 13–15
