Silicon is the second most abundant solid element on Earth after oxygen, accounting for more than 25% of the Earth’s crust. However, it is rarely found in elemental form; almost all of it exists as compounds.
This lecture will answer the question of how very pure sand (SiO2) can be converted into mono-crystalline silicon and then into silicon detectors, as well as provide a description of the various mono-crystalline silicon growth procedures.
Material requirements for the manufacturing of silicon particle detectors used for high-energy physics applications have to meet three basic requirements:
- high resistivity;
- high minority carrier lifetime;
- low bulk generation current.
To fully deplete the detector bulk with a thickness of around 200 – 300 um by an acceptable voltage below about 300 V, a very high resistivity (> l Kohm/cm) is required. Instead, a high minority carrier lifetime and a very low bulk generation current are necessary to reduce detector noise.
However, for particle detectors that will be subjected to significant doses of radiation, these parameters should not be regarded too seriously because lifetimes are already reduced by orders of magnitude after moderate doses of radiation, rendering good initial lifetime qualities irrelevant.
Because of the demand for a fair price and a homogenous resistivity distribution not only across a single wafer but also over the entire ingot, Float Zone silicon is the finest option of material for detector applications today.
However, while the cost and substrate concerns associated with the epitaxial process may preclude its application as a detector material, the Czochralski method may be of interest for the creation of radiation hard material if high resistivity (> 1 KOhm/cm) CZ becomes commercially available.
The next section will briefly review the production of silicon using the two growth techniques mentioned above, with a special emphasis on high-resistivity silicon production and the possibilities of defect engineering, i.e. the controlled incorporation of impurities into the crystal.
Czochralski Silicon (Cz)
The vast majority of the commercially grown silicon is Czochralski silicon due to the better resistance of the wafers to thermal stress, the speed of production, the low cost and the high oxygen concentration that offers the possibility of Internal Gettering. The industrial standard crystals range in diameter from 75 to 200 mm, are typically l m long and have < 100> orientation. In the following, a short review is given.
Standard CZ
The Czochralski method is named after J. Czochralski, who determined the crystallisation velocity of metals by pulling mono- and polycrystals out of a melt held in a crucible against gravity. Teal and Little invented the pull-from-melt method, which is still widely used today. Figure 2.1 depicts a schematic diagram of a Czochralski-Si grower known as a puller.
The puller is made up of three major parts:
- a furnace, which includes a fused-silica crucible, a graphite susceptor, a rotation mechanism (clockwise as shown), a heating element, and a power supply;
- a crystal-pulling mechanism, which includes a seed holder and a rotation mechanism (counter-clockwise); and
- an ambient control, which includes a gas source (such as argon), a flow control and an exhaust system.
In the rotating quartz crucible, high-purity polysilicon (SGS) is melted with additional dopants as needed for the final resistivity. A single crystal silicon seed is placed on the surface and slowly drawn upward while being rotated at the same time.
This attracts molten silicon, which solidifies into a continuous crystal that extends from the seed. Temperature and pulling speed are adjusted to first narrow the crystal diameter to a few millimetres, eliminating dislocations caused by the seed/melt contact shock, and then widen the crystal to full diameter.
The quartz crucible (SiO2) gradually dissolves during the manufacturing process, releasing large amounts of oxygen into the melt. More than 99% of this is lost to the molten surface as SiO gas, but the remainder remains in the melt and can dissolve into single-crystal silicon.
Carbon is another impurity introduced into the melt by the manufacturing process, albeit in lower concentrations. Evaporating silicon monoxide from the melt surface reacts with the hot graphite susceptor, forming carbon monoxide that re-enters the melt.
As the crystal is pulled from the melt, the impurity concentration incorporated into the crystal (solid) is usually different from the impurity concentration of the melt (liquid) at the interface. The ratio of these two concentrations is defined as the equilibrium segregation coefficient k0 =Cs/c1 where Cs and C1 are the equilibrium concentrations of the impurity in the solid and liquid near the interface, respectively.
Oxygen is always the impurity with the highest concentration in CZ silicon. Typical oxygen and carbon concentrations are [O] ≈ 5 – 10 10^17cm-3 and [C] ≈ 5 – 10 10^15cm-3, respectively. The solubility of O in Si is ≈ 10^18 cm-3 at the melting point but drops by several orders of magnitude at room temperature, hence, there is a driving force for oxygen precipitation. Furthermore, the high oxygen concentration can lead to the formation of unwanted electrically active defects.
These are oxygen-related thermal double donors (TDD) and shallow thermal donors (STD) which can seriously change the resistivity of the material. However, oxygen has also good properties.
Oxygen acts as a gettering agent for trace metal impurities in the crystal (Internal Gettering) and it can pin dislocations, which greatly strengthens the crystal. Oxygen precipitates in the wafer core suppress stacking faults, and oxygen makes the Si more resistant to thermal stress during processing. This is the reason why CZ-Si is used for integrated circuit production, where there are many thermal processing steps.
However, the improved radiation hardness is the most important property of a high oxygen concentration in this work. The resistivity of CZ silicon is the main issue for the application as detector-grade material. Because of boron, phosphorus, and aluminium contamination from the dissolving quartz Crucible, the highest commercially available resistivity for n-type material is around l00 Ohmcm and only slightly higher for p-type material.
As a result, standard CZ silicon is unsuitable for detector manufacturing.
Magnetic Field Applied Cz (MGZ)
MCZ may be the future standard CZ technology since today’s approaches to solving the challenge of the 300 mm and, later on, the 400 mm crystal diameter are based on this technology. The method is the same as the CZ method, except that it is carried out within a strong horizontal (HMCZ) or vertical (VMCZ) magnetic field. This serves to control the convection fluid flow, allowing to minimise the mixing between the liquid in the centre of the bath and that at the edge.
This effectively creates a liquid silicon crucible around the central silicon bath, which can trap much of the oxygen and slow its migration into the crystal. Compared to the standard CZ a lower oxygen concentration can be obtained and the impurity distribution is more homogeneous.
This method also offers the possibility of producing detector-grade silicon with a high oxygen concentration. Since the technology is still very young, it is hard to get such material with reproducible impurity concentrations on a commercial basis. However, a first test material of 4 KΩcm p-type with an oxygen concentration of 7 – 8 l017 cm-3 and a carbon concentration below 2xl016 cm-3 was obtained.
Continuous Cz (CCZ)
With the CCZ method, a continuous supply of molten polycrystalline silicon is achieved by using a double quartz crucible. In the first one, the crystal is grown and in the second one, connected to the first one, a reservoir of molten silicon is kept, that can be refilled by new polysilicon during the growth process.
This allows for a larger crystal length and improves the throughput and operational costs of the CZ grower. Furthermore, the resulting single crystals have a uniform resistivity and oxygen concentration and identical thermal history. In combination with the magnetic field method, the Continuous Magnetic Field Applied CZ technique (CMCZ) offers the possibility to grow long and large diameter CZ. However, silicon produced by this technology has so far not been used for radiation damage experiments.
Float Zone Silicon (FZ)
Float-zone silicon is a high-purity alternative to crystals grown by the Czochralski process. The concentrations of light impurities, such as carbon and oxygen, are extremely low. Another light impurity, nitrogen, helps to control micro defects and also brings about an improvement in the mechanical strength of the wafers. It is now being intentionally added during the growth stages.
The Float Zone Method
The float Zone (FZ) method is based on the zone-melting principle and was invented by Theuerer in 1962. Fig. 2.2 shows a schematic setup of the process. The production takes place under a vacuum or in an inert gaseous atmosphere. The process starts with a high-purity polycrystalline rod and a monocrystalline seed crystal that are held face-to-face in a vertical position and rotated.
With a radio frequency field, both are partially melted. The seed is brought up from below to make contact with the drop of melt formed at the tip of the poly rod. A necking process is carried out to establish a dislocation-free crystal before the neck is allowed to increase in diameter to form a taper and reach the desired diameter for steady-state growth.
As the molten zone is moved along the polysilicon rod, the molten silicon solidifies into a single Crystal and, simultaneously, the material is purified.
Typical oxygen and carbon concentrations in FZ silicon are below 5 1015 cm-3. FZ crystals are doped by adding the doping gas phosphine (PH3) or diborane (B2H6) to the inert gas for n- and p-type, respectively. Unlike CZ growth, the silicon molten Zone is not in contact with any substances except ambient gas, which may only contain doping gas. Therefore, FZ silicon can easily achieve much higher purity and higher resistivity.
Additionally, multiple zone refining can be performed on a rod to further reduce the impurity concentrations. Once again, the effective segregation coefficient k plays an important role. Boron, for example, has an equilibrium segregation coefficient of k0 = 0.8. In contrast to this phosphorus cannot only be segregated (k0 = 0.35) but also evaporates from the melt at a fairly high rate.
This is the reason why, on the one hand, it is easier to produce more homogeneous p-type FZ than n-type FZ and on the other hand, high resistivity p-type silicon can only be obtained from polysilicon with a low boron content. Dopants with a small k0 like Sn can be introduced by pill doping – holes are drilled into the ingot into which the dopant is incorporated – or by evaporating a dopant layer on the whole ingot before the float zoning process.
References
- W. R. Leo Techniques for Nuclear and Particle Physics Experiments
