INTERFACE FOR VOLTAGE RAMP PROGRAMMERS

Abstract: A microprocessor for voltage ramp programmers having two voltage channels 2 and 3 (V₂ and V₃) and minimum varia­ tions of output voltages greater than 1 mV, including a sum­ ming amplifier having V₂, V₃ as input voltages as indicated above and V₁ as defined below, which compresses the mini­ mum output voltage variation from channel 2 (output voltage V₂) by a factor equal to or greater than the said minimum variation in channel CH₂, adds the voltages of channel 3 to the compressed voltage of channel 2, and subtracts an inserted voltage V₁, equal to the maximum output value from channel 2, so as to give an output voltage V out having voltage in­ creases of 1 mV or less.

Description: The present invention relates to an interface for voltage ramp programmers, suitable to be used in commercial voltage change programmers to obtain output signal variations of a few mV. In particular the invention relates to an interface between a voltage ramp programmer (generator) and a temperature controller to obtain temperature variations of a few tenths of a degree centigrade. One limitation of common commercial programmers is due to their high programmable minimum voltage increase which is greater than 1 mV, generally some 10-20 mV. Minimum voltage increases that are so high constitute a drawback in ovens where it is necessary to have a temperature control of some tenths of a degree centigrade. In fact, for certain applications a temperature variation of even less than 1°C is required, down to values of approximately 0.1°C, or even less. Such temperature variations correspond to minimum voltage increases of the voltage ramp generators of some 1 mV, or even less. A commercial voltage programmer normally used, for example, is the Honeywell DCP 7700. Analogous programmers are available from Messrs. Leeds and Northrup, Eurothron and others. The Honeywell voltage ramp generator has the following characteristics: voltage outputs on 3 independent channels, possibility of programming 99 temperature segments or settings, storing of 9 programmes. The nine programmes serve to pass from an initial predetermined voltage value to a final value, passing through intermediate values in established periods of time that are more or less prolonged. These commercial programmers, that have voltage variations of about 10-20 mV, have the above mentioned drawbacks, since the minimum programmable temperature variation varies approximately from 1° to 2°C. In particular, these temperature variations in the growth of semiconductor type, insulating or metallic single crystals generate a high number of lattice defects. These defects make the single crystals obtained rather unsuitable for various applications, in particular for those in the electronic field, such as emitter diodes, micro-wave devices, lasers, integrated circuits and others. The group III-V single crystals, such as for example GaP and InP, have a high dislocation density and often a high concentration of impurities. The above mentioned group III-V monocrystals are not stoichiometric and have a low structural homogeneity. In fact, inclusions, micro-precipitates, etc. are present. This represents a significant drawback when the single crystals are prepared by using a polycrystal as a precursor. In this case the single crystals will have a high number of dislocations and impurities. The oven generally used in the growth of single crystals is the CZ type (Czochralski) and growth occurs using the LEC technique (liquid encapsulation technique). Horizontal Bridgman type ovens may also be used. Another drawback encountered is that the high concentration of defects is also associated with a concentration of contaminating impurities since these in turn generally contribute to the formation of lattice defects during the growth and cooling of the single crystal. Therefore, there is a need for special programmers, capable of programming voltage variations of about 1 mV, or less, so that the single crystals obtained would have a high perfection and purity. In fact, in some stages of the growth cycle, it would be necessary to have temperature variations even of about 0.1 degrees centigrade, or less, to obtain single crystals with the high properties mentioned above. Unexpectedly, it has been found that it is possible to obtain output voltage variations even down to values of 1 mV, or less, in commercial programmers if an interface according to the present invention is used. The present invention provides an interface between a voltage ramp programmer (or generator), having two output voltage (V₂ and V₃) channels CH2 and CH3, variable between a minimum and a maximum and increasable by minimum values greater than 1 mV, and a temperature controller, including a summing amplifier having three input voltages, V₁, V₂, V₃, wherein V₂ and V₃ are the output voltages from the programmer and V₁ is a negative voltage inserted as defined hereinafter, having as output voltage V out , with minimum voltage increases less than or equal to 1 mV, defined by the equation: wherein y is an attenuation factor as defined hereinafter, the said V out being obtained: by attenuating the minimum output voltage variation from the channel CH2 of the programmer by an attenuation factor y equal to or greater than the said minimum variation of channel CH2, by adding the output voltage from the channel CH3 of the programmer, left unchanged, to the attenuated voltage of channel CH2 and by subtracting a voltage V₁, equal to the maximum output value of channel CH2, from the total output of channel CH2. Such an interface (2) between a voltage ramp programmer 1 and a temperature controller 3 is shown schematically in Figure 1 of the accompanying drawings. In particular, the interface preferably includes a summing amplifier obtained with an integrated circuit µA 725, as shown in Figure 2 of the accompanying drawings, having an output voltage V out defined by the equation: wherein x and z are the attenuation or amplification values of V₃ and V₁, also taking the value 1, and y always being equal to or greater than the minimum output voltage variation from channel CH2 of the programmer, the values of V₁, V₂, and V₃ being defined as follows: 1) V₁ is a negative voltage, stabilized by a Zener diode, necessary to maintain an output signal that fully compensates the output signal from channel CH2 of the programmer and the intrinsic output signal of the amplifier; the said voltage V₁ may also be attenuated or amplified by a factor z as a function of the reciprocal of two resistances R₁ and R F ; (2) V₂ is the output voltage supplied by the programmer at channel CH2, variable between a minimum and a maximum value with minimum increases greater than 1 mV, the said voltage V₂ being attenuated by a factor y, as a function of the reciprocal of two resistances R₂ and R F and with minimum increases, between the extreme attenuated values of the variation range of V₂, less than or equal to 1 mV. (3) V₃ is the output voltage of the second channel CH3 of the programmer, variable between a minimum and a maximum with minimum increases greater than 1 mV; the said voltage V₃ being attenuated or amplified by a factor z, as a function of the reciprocal between R₃ and R F ; the values V₁ and V₃ however being suitable amplified or attenuated, or left unvaried, and V₂ being always attenuated with the condition that x, y, z are such as to lead to V out having minimum increases being less than or equal to 1 mV, as a function of the combination of the reciprocals of the resistances indicated in 1), 2) and 3), the resistance R F being connected between the output and an input (3') of the summing amplifier and the resistances R₁, R₂, R₃ being connected respectively between the input voltage V₁, V₂, V₃ and said input (3') of the summing amplifier. An example is provided of a particular application of the interface in the growth and synthesis of single crystals with a CZ ADL oven mod. HPCZ and with a Honeywell DCP 7700 programmer and CAT series 80 temperature controller to illustrate the values that may be assumed by V₁, V₂, V₃ indicated above, to obtain output voltages V out with increases equal to 1 mV. In this particular case the values of the input voltages of the summing amplifier are: 1) V₁ = approximately -500 mV stabilized by a Zener diode, necessary to maintain an output signal that fully compensates a 500 mV signal provided by the CH2 of the programmer, as specified in 2) hereinbelow, and the intrinsic output signal of the amplifier; the amplifier gain z for the voltage is unity, since the resistances R₁ = R F (z = 1); 2) V₂ output voltage supplied by the programmer on the channel CH2, is variable between 1 V and 5 V with minimum increases of 10 mV; since R₂ = R F .10, this voltage is attenuated by the amplifier by a factor 10 (y = 10) and contributes to output voltage variations lying between 100 mV and 500 mV with minimum increases of 1 mV; 3) V₃ output voltage of the second channel CH3 of the programmer, is variable between 1 V and 5 V with minimum increases of 10 mV; since R₃ = R F , the amplifier gain is unity (x = 1). In particular, when the channel CH2 output of the programmer is kept fixed at 5 V the total contribution of V₁ and V₂ on the output voltage is zero. In this way it is possible to obtain an output voltage that is variable between 0 and -400 mV, with decreases of 1 mV. This is found to be ideal for the growth phase of a single crystal, since in this way it is possible to adopt controlled temperature drops in the oven from approximately 0.1°C to 40°C, with minimum variations even of approximately 0.1°C. Voltage programming on channel CH3 is used in all the processing phases where minimum values greater than 10 mV are required. For example, this is encountered in the synthesis phases of a polycrystal starting from the elements for semiconductors obtained with the LEC technique. In practice, in this particular case the interface performs the following functions: attenuates 10 times (divides by 10) the minimum output voltage variation of channel 2 (CH2) bringing it to the value of 1 mV. adds the output voltages of channel 3 (CH3), left unchanged, to the compressed voltage of channel 2, subtracts 500 mV from the total output of channel 2 to programme negative voltage values on CH2, which operates between 0.1-0.5 V, bringing the operating range of CH2 to between 0 and -0.4 V. In practice, in this particular case the interface connects two programmer channels to the temperature controller, with a total output voltage controlled by the following expression: V out = V₃ + V₂/10 - 500 wherein V out is the total output voltage from the summing amplifier, and V₂ and V₃ are the output voltages of channels 2 and 3 of the programmer. The interface in the particular case indicated is described in detail in the electronic diagram shown in Figure 2 of the drawings. CH1, CH2, CH3 are the output channels of the commercial programmer used: R₁, R₂, R₃, R₄, R₅, R₆, R E , R F are resistances; C₁, C₂ are capacitors; P₁, P₂, P₃ are variable resistances (potentiometers) for which the maximum value is indicated; IC₁ is a summing amplifier; IC₂ is an integrated circuit, including a Zener diode and resistance. The values of the various components are given below for the particular example considered above. R₁ = R₃ = R F = 10 K Ω, R₂ = 100 K Ω, R E = 3.9 K Ω, R₄ = 10 Ω, R₅ = 39 Ω, P₃ = P₂ = 500 Ω, P₁ = 10 K Ω, R₆ = 12 K Ω, C₁ = 0.05 µ F, C₂ = 0.02 µ F. In particular, an integrated circuit µ A 725, National make, is used as the summing amplifier IC₁. Figure 1 is a block diagram of the interface 2 between the programmer 1 and the temperature controller 3, where it is apparent that the interface has no effect on the channel CH3 of the programmer, whereas it does have an effect on the channel CH2 which is divided by a factor y = 10, and on channel CH1 the fixed applied signal of -500 mV is indicated, by referring to the particular case described. It is to be understood that, on the basis of the data of the particular case described above, a skilled person is capable of attenuating or amplifying the voltage values V₁ and V₃, and determining the attenuation of V₂ as a function of the minimum output increases from the programmers on the channel CH2, to obtain increases in V out that are less than or equal to 1 mV as a function of the different applicative requirements. The invention will he further described with reference to the following illustrative Examples. Example 1 A 500 g polycrystal of commercial InP was taken and melted in a CZ ADL type oven, mod. HPCZ at 1100°C. Voltage variations were then applied to a Honeywell DCP 7700 programmer after insertion of an interface of this invention between the programmer itself and a CAT Series 80 temperature regulator as in Figure 1, and the values of the components of the interface being those indicated above for the particular case described. It was observed that for output voltage variations of 1 mV there was a corresponding melt temperature variation of approximately 0.1°C, this temperature having been measured using precision thermocouples for high temperatures of the Platinum/Platinum-Rhodium type (13%). Comparative Example 1 Example 1 was repeated under the same conditions, but without using the interface of this invention. In correspondence with the minimum increase of 10 mV of the output voltage value, minimum temperature variation of 1°C was observed.

Claims: Interface (2) between a voltage ramp programmer (1), having at least two channels (CH2 and CH3) of output voltages V₂ and V₃, variable between a minimum and a maximum and increasable by minimum values greater than 1 mV, and a temperature controller (3), characterized in that it comprises a summing amplifier (IC1) having three input voltages V₁, V₂, V₃, wherein V₂ and V₃ are the output voltages from the programmer (1) and V₁ is a negative voltage inserted as defined hereinafter, and an output voltage V out , having minimum voltage increases less than or equal to 1 mV, defined by the equation: wherein y is an attenuation factor as defined hereinafter, the said V out being obtained: by attenuating the minimum variation of output voltage V₂ from the first channel (CH2) of the programmer by an attenuation factor y equal to or greater than the said minimum variation of the first channel (CH2), by adding the output voltage V3 from the second channel (CH3) of the programmer, left unchanged, to the attenuated voltage of the first channel (CH2), and by subtracting a voltage V₁, equal to the maximum output value of the first channel (CH2), from the total output of the first channel (CH2). An Interface as claimed in claim 1, characterized in that the summing amplifier (IC1) is obtained with an integrated circuit (µ A 725) and has an output voltage defined by the equation: wherein x and z are the amplification or attenuation values of V₃ and V₁, also taking the value of 1, y always being equal to or greater than the minimum variation of the output voltage from the first channel (CH2) of the programmer, the values V₁, V₂, V₃ being defined as follows: 1) V₁ is a negative voltage, stabilized by a Zener diode, necessary to maintain an output signal that fully compensates the output signal from the first channel (CH2) of the programmer and the intrinsic output signal of the amplifier; the said voltage V₁ may also be attenuated or amplified by a factor z as a function of the reciprocal of two resistances R₁ and R F ; 2) V₂ is output voltage provided by the programmer on the first channel (CH2), being variable between a minimum value and a maximum value with minimum increases greater than 1 mV, the said voltage V₂ being attenuated by a factor y, as a function of the reciprocal of two resistances R₂ and R F and with minimum increases, between the extreme attenuated values of the variation range of V₂, less than or equal to 1 mV; 3) V₃ is output voltage of the second channel (CH3) of the programmer, variable between a minimum and a maximum with minimum increases greater than 1 mV; the said voltage V₃ may also be attenuated or amplified by a factor z, as a function of the reciprocal of two resistances R₃ and R F ; the values V₁ and V₃ being suitably amplified or attenuated or left unchanged, and V₂ being always attenuated at the condition that x, y, z are such as to lead to V out having minimum increases less than or equal to 1 mV, the resistance R F being connected between the output and an an input (3') of the summing amplifier (IC1) and the resistances R₁, R₂, R₃ being connected respectively between the input voltage V₁, V₂, V₃ and said input (3') of the summing amplifier (IC1). Interface as claimed in claim 2, characterized in that the output channels (CH2 and CH3) from the programmer (1) supply voltages V₂, V₃ with minimum increases of 10 mV, and in that V₁, V₂ and V₃ take the following values to obtain output voltages V out with minimum increases of 1 mV: 1) V₁ = approximately -500 mV, stabilized by a Zener diode, necessary to maintain an output signal that entirely compensates a 500 mV signal supplied by the first channel (CH2) of the programmer, as defined in 2) hereinbelow, and the intrinsic output signal of the amplifier, the amplifier gain z on this voltage being unity since the resistances R₁ = R F ; 2) V₂ output voltage supplied by the programmer on the first channel (CH2), variable between IV and 5V with minimum increases of 10 mV; since R₂ = R F .10 , this voltage is attenuated by the amplifier by a factor y = 10 and contributes to output voltage variations from 100 mV to 500 mV with minimum increases of 1 mV; 3) V₃ output voltage of the second channel (CH3) of the programmer, variable between IV and 5V with minimum increases of 10 mV; since R₃ = R F , the amplifier gain x is unity. Use of an interface as claimed in any of claims 1 to 3, for the growth and cooling phases of single crystals and for the synthesis of composite polycrystals starting from the crystal elements.

Inventors: FORNARI, ROBERTO, MAGNANINI, RENATO, MATTERA, ADRIANO, PAORICI, CARLO, ZANOTTI, LUCIO, ZUCCALLI, GIOVANNI
Applicants: CONSIGLIO NAZIONALE DELLE RICERCHE, MONTEDISON S.P.A.
Application year: 1986
Main classification: G05D23
IPC codes: G05D23, G05B19, G06G7