Fabricate Nanofluide Devices with Doped Silicon Wafers
A researcher requested help with the following project.
I’m in the process of searching for silicon wafers to be used in the generation of nanofluidic devices. Our initial thought is to try to use silicon wafers with the exact thickness of the channel depth we wish to create lithographically (5 nm). It seems to be the usual minimum thickness is 10 nm. If this is unfeasible or has a sizeable additional cost, we’re looking at using 10 nm and 100 nm thermal oxide thickness layers and working out the etch time to provide the desired depth. I think 100 mm diameter seems reasonable, but I’m unsure about what dopant we should use. The final step will be anodic bonding of a glass layer to the top of the oxide surface if that makes a difference.
Also, we’re looking at using anodic bonding to seal these chips, and I was wondering if the borofloat 33 glass would work? If so I was looking at an order of 10x of the below silicon wafers and 10x of the matching borofloat glass. Also, If you have any recommendations on anodic bonding temperature, voltage, and applied force it would help us in minimizing troubleshooting.
UniversityWafer, Inc. Answered:
You probably want Silicon wafers with either (100) or (110) crystallographic orientation {not (111)}. (110) wafers let you etch deep trenches with almost vertical walls.
4"Ø wafers are actually 100.0±0.5mm in diameter. Standard thickness is 500 or 525±25µm. Wafers thicker than that or thinner than 380µm cost more.
p-type, Boron doped Silicon wafers are most common, although for your application the dopant is immaterial. Wafer resistivity should be (1-1,000)Ohmcm. Lower resistivity implies more impurities and higher resistivity costs more.
Silicon is crystallized by CZ or FZ process. CZ Silicon invariably contains about 20ppma of Oxygen which can affect activity of some etchants. FZ costs more.
Wafers can be one-side-polished or double-side-polished.
If you are dealing with structures 10nm deep then you want extremely well polished wafers. All Silicon wafers are polished to a surface roughness of about 0.5nm rms (as determined by AFM over an area of about 2×2µm). However, wafer thickness variation across the entire wafer is much larger. SEMI standard calls for Total Thickness Variation (TTV) of 10,000nm. Wafers can be polished to a TTV of 1,000nm but at significant extra cost.
DRY Thermal Oxide is the Silicon Oxide of choice for your application. We can grow 5nm of SiO2 (allowing for native Oxide layer) but with thickness tolerance of ±15%. At Oxide thickness of 50nm we can guarantee tolerance of ±5%. Surface roughness and TTV of Silicon wafers with Thermal Oxide layer are essentially those of the underlying Silicon wafers.
Item Qty. Description GV69. 10/25 Silicon wafers, per SEMI Prime, OxP/POx 4"Ø×500±25µm, FZ n-type Si:P[100]±0.5°, Ro=(50-70)Ohmcm,
both-sides-polished (both with 1,000ű5% oxide),, SEMI Flat (one),
Sealed in Empak or equivalent cassette, MCC Lifetime>1,000µs.
Reference #254311 for specs and pricing.
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What is Doped Silicon?
Doped silicon is a semiconductor with one or more impurities, usually silicon. Intentional doping is the introduction of impurities into a semiconductor intentionally with the intention of modulating its properties. The semiconductor is then classified as either an intrinsic or an extrinsic semiconductor. There are two main types of doped silicon: N-type and trivalent impurity doped silicon. In this article, we will discuss the difference between the two types and their properties.
Trivalent impurity doped silicon
In semiconductors, trivalent atoms are present in the crystal structure of silicon and germanium. When silicon is doped with these atoms, it forms a p-type semiconductor. This type of semiconductor has electron-hole pairs rather than conduction-band electrons, which are a minority carrier in a p-type material. To understand the p-type semiconductor's properties, we should first understand what a trivalent atom is.
A semiconductor contains a valence band and a conduction band, and an energy gap between them. Impurities add electrons to semiconductors, and by doing so, make the material more electrically conductive. The addition of trivalent impurities to silicon significantly changes the semiconductor's electronic properties. The added impurity is a donor to the semiconductor, while the remaining one is an acceptor.
The three elements, Boron, Gallium, and Indium, are trivalent impurities. The three valence electrons in trivalent atoms allow them to fit perfectly into the crystal of silicon. These elements are often used to dope silicon and germanium. When silicon is doped with a trivalent atom, the intrinsic semiconductor transforms into a P-type semiconductor. But how do you know which impurity to use?
The process of doping semiconductors involves introducing impurities into the crystal's lattice structure. These impurities change the semiconductor's conductivity due to either excess or deficient electrons. This process is known as "partial" silicon doping. Trivalent impurities have three valence electrons and accept one electron from the doped material. Therefore, they are called ACCEPTOR impurities.
Unlike n-type semiconductors, trivalent semiconductors are electrically neutral. Adding an additional outer electron to a p-type semiconductor creates a hole in the valence band of the atoms. It also creates a hole in the valence band, which is the opposite direction to electrons. The energy needed to lift an electron into a p-type semiconductor is only one percent of the energy in silicon.
When adding trivalent impurities to a p-type semiconductor, the concentration of holes becomes higher than the concentration of electrons. This makes the material more conductive than a pure semiconductor. The extra electrons are known as 'donor impurities' and they are part of the fifth group of the periodic table. Adding trivalent impurities to a p-type semiconductor creates a deficiency of valence electrons.
N-type semiconductor
A doped semiconductor is a material that is enhanced in electrical conductivity by adding an impurity to the crystal. Common examples include silicon that has been doped with boron and phosphorus, both of which have three valence electrons, as well as with a wide variety of other elements. In both cases, the dopants are integrated into the lattice structure of the semiconductor crystal, which alters its properties. By introducing these impurities into a silicon crystal, the conductivity of the material can increase by as much as 10 times.
The process of doping silicon involves adding small amounts of an element to silicon, usually phosphorus or arsenic. The donor element accepts the extra electron and forms four covalent bonds with the silicon. This results in an N-type semiconductor. A positive hole will form at the center of the doped semiconductor, which is a semiconductor that is enhanced in electrical conductivity. This process is also known as n-type doping.
Doping semiconductors has two advantages. Doped semiconductors are higher in conductivity than p-type materials. A doped semiconductor will have a comparatively narrow depletion band. This is due to the fact that a p-type semiconductor has three valence electrons, while an n-type semiconductor has one. An N-type semiconductor will have an extra electron, which is called a donor impurity.
A semiconductor that is doped with an acceptor will be p-type. The electrons of the dopant are fixed in the crystal lattice, allowing only the positive charges to move around. P-type semiconductors are made of silicon with phosphorus, because the dopant has more valence electrons than the host silicon atoms. Thus, semiconductors that contain doped silicon are p-type.
Similarly, doped semiconductors have a hole in the middle. This hole is a result of trivalent impurities such as boron and phosphorus. These substances act as dopants to create electron holes. These holes can then be transferred from one atom to another. This is why the p-type semiconductors are able to transfer energy from one atom to another. This is the basic difference between an N-type and a p-type semiconductor.
Another method for doping silicon is to introduce an aluminum or nickel into the crystal. The latter is known as annealing, and it involves the use of a chemical reaction that allows these impurities to form ionic bonds with the silicon atoms in the crystal. Doping silicon is a process that can lead to a better electrical conductivity than the former. In addition, doped semiconductors are also referred to as degenerate semiconductors.
Non-phononic mechanisms in doped silicon
The lifetimes of minority carriers in doped silicon are studied in detail at dopant concentrations of 1018-1020 cm-3. We review the major theories of carrier recombination in silicon and discuss the contributions they are expected to make. We also describe and critically evaluate methods of minority carrier lifetime measurement in doped silicon. Using the lifetime data, we propose four different mechanisms to explain these lifetimes: shallow donor/acceptor states, SRH-type phononic recombination, and trapped Auger recombination.
The measurements indicate that the mobility of electrons near the B-Si interface is high, with an effective mass component a few orders of magnitude higher than the free electron. This result indicates that the mobility of a single electron near the interface is high, and the measured sheet resistance is 60 kO/square, which is much higher than that of photodiodes with a p++ top layer.
While thermal conduction in bulk silicon is dominated by phonon transport, non-phononic mechanisms in doped silicon reduce thermal conduction. These scattering mechanisms are important at low temperatures, and are exacerbated by imperfections introduced during thin-film fabrication. In addition, impurities in doped silicon lower thermal conductivity. This phenomenon is termed as "phonon-boundary scattering," and it has been shown that impurities in doped silicon significantly decrease thermal conductivity.
The hot LO phonon population is the result of HCs losing excess energy through electron-phonon coupling. After the relaxation of the hot LO phonon, the HCs lose energy through the generation of daughter phonons with smaller wave vectors. Moreover, the relative excess LO phonon population is calculated at three different delay times. The HC-cooling effect is a common phenomenon observed in highly excited polar semiconductors.
The electrons in doped silicon move to the interface. This movement is mediated by Coulomb interactions. Electrons move from c-Si to the p-zone, while electrons from n-Si migrate to the interface. This transfer of charge is crucial for forming Si-B junctions. Therefore, non-phononic mechanisms in doped silicon can explain the observed behavior of doping.
In addition to enhancing valence band dispersion and decreasing the hole effective mass, b-Ga2O3 exhibits a delocalization effect. It also exhibits reduced hole effective mass and a decrease in acoustic deformation potential scattering. A theoretical study also demonstrates the p-type conductivity of b-Ga2O3 films. Hyperdoping induces the broadening of dopant energy levels and separation of impurity bands. It can even exhibit superconductivity.
