Great success of large diameter
ASTROHN Technology Ltd specialists have developed a large-diameter infrared (IR) lens. This caused the need to obtain a single crystal of germanium of a sufficiently large diameter (about 200 mm) for the manufacture of an input lens. In addition, the problem of high structural excellence of the workpiece was critical. Therefore, the workpieces must be single-crystal, so that internal defects do not weaken the passage of the beam. Such blanks can only be obtained using the Czochralski method.
There is no need to talk about the importance of the diameter of lenses in security thermal imagers. A small-diameter lens is unable to collect the necessary amount of infrared radiation from the distant object for the reaction of the bolometric pixel. The sensitivity of the entire thermal imager will depend on how many rays get from the object to the bolometer. Therefore, to use microbolometers at long distances, larger germanium optics are required. Germanium, as is known, transmits a 2–16 μm radiation spectrum and has a high refractive index, which makes it possible to obtain high optical power of IR devices in the range of 8–12 μm. Thermal imaging modules use specially designed lenses to detect people and objects in poor visibility conditions over long distances. The experience of our experts shows that the current level of Czochralski growing of single crystals of germanium with high structural perfection allows for mass production of the necessary lenses.
Single-crystal optical germanium growing plant
Recently, our company has developed a large-diameter IR lens. It caused the urgent need to grow germanium crystals of extremely large diameter (about 200 mm). Therefore, the workpieces must be single-crystal, so that internal defects do not weaken the passage of the beam. Such blanks can only be obtained using the Czochralski method.
Growing single-crystal optical germanium
The single-crystal germanium obtained by this method is known to be gradually pulled out of the melt during the initial seed. In the melt, the temperature decreases from the periphery to the center – a radial temperature gradient occurs, and due to heat removal by the growing single crystal, an axial gradient develops. Growing single crystals with a perfect structure and shape requires to observe very stringent requirements for the symmetry of the thermal field, constant temperature over time, etc. This is achieved, firstly, by the heating unit of the growing plant, and secondly, by choosing the growing modes of the single crystal and crucible with a melt and temperature control of heaters. The growth rate in our case is limited by heat removal from the crystallization front. Thermal stresses cause plastic deformation in the crystal. The conditions become especially harsh for germanium, which especially easily develops large temperature differences.
200 mm diameter single-crystal germanium (Lytkarino)
The values of axial temperature gradients in our case are especially dependent on the cooling intensity of the grown crystal – on the intensity of heat removal along the growth axis. At significant and noticeably anisotropic thermal stresses, dislocations occur in slip planes and are grouped into small-angle boundaries (SAB). When the crystallization front is convex in the melt, the growth of the crystal face occurs most often as a result of the growth of a single two-dimensional nucleus arising in the coldest central part of the face. In the case of a concaved or planar front crystallization, the growth of a face can occur from several simultaneously growing two-dimensional nuclei. Thus, the sources of small-angle boundaries are all the same thermal stresses in the crystal and the simultaneous growth of several centers of the new phase. We managed to achieve a flat shape of the crystallization front by reducing heat loss from the surface of the grown crystal, directing heat by a uniform flow along its length. In practice, this is achieved by using the original screening system of a growing crystal.
200 mm diameter single-crystal lens blank (Lytkarino)
Vast practical experience and deep scientific knowledge of our specialists helped us to successfully solve this problem.