METHOD AND ARRANGEMENT FOR FLASHLAMP CONTROL

Abstract

A method for flashlamp control, in which a main pulse of the lamp current, producing a flash, is generated, and a pre-pulse of the lamp current is previously generated by application of a bias voltage includes a flashlamp with an ignition electrode, a bias voltage source, a main voltage source and a control system. The load of the flashlamp is minimized during the production of a main pulse by a pre-ignition. A pre-pulse is generated by applying a plasma voltage which is higher than the bias voltage, as an electrode voltage, and igniting a plasma in the flashlamp by means of an ignition electrode and maintaining same by means of the bias voltage during the pre-pulse.

Claims

1. A method for flashlamp control in which a main pulse of a lamp current generating a flash is effected by applying an electrode voltage to the electrodes of a flashlamp comprising: prior to the flash generation, a pre-pulse of the lamp current is generated by applying a bias voltage, wherein the pre-pulse is generated in that a plasma voltage higher than the bias voltage is applied as an electrode voltage, and a plasma is ignited in the flashlamp by an ignition of an ignition electrode and is maintained by the bias voltage during the pre-pulse.

2. The method according to claim 1, wherein: the bias voltage from a bias voltage source with a first capacitance (C.sub.1) is applied as electrode voltage, in parallel to the bias voltage, but nonreactive with respect to the bias voltage source, the plasma voltage from a plasma voltage source with a second capacitance (C.sub.2), which is at least one order of magnitude smaller (C.sub.2<<C.sub.1) than the first capacitance (C.sub.1), is applied as additional electrode voltage, the pre-pulse is generated by applying an ignition voltage effecting an ignition pulse generated with the plasma voltage to the ignition electrode, wherein a breakdown is effected by the plasma voltage and is maintained by the bias voltage.

3. The method according to claim 1, wherein a main voltage source, which is in parallel to but disconnectable from the bias voltage source and the plasma voltage source, applies a main voltage as an electrode voltage for generating the main pulse.

4. The method according to claim 1, wherein the main voltage source is disconnected from the electrodes during the pre-pulse and is connected to the electrodes only during the main pulse.

5. An apparatus for flashlamp control comprising: a flashlamp which is provided with two electrodes between which an electrode voltage can be applied; an ignition electrode which is operatively connected to the flashlamp; a bias voltage source operative to generate a bias voltage; a main voltage source and a control system; wherein a plasma voltage source operative to generate a plasma voltage, the effect of which can be controlled by the ignition electrode, is nonreactively connected in parallel to the bias voltage source, and wherein the plasma voltage is greater than the bias voltage.

6. The apparatus according to claim 5, wherein: the bias voltage source has a first capacitor which is connected to a first charging unit to charge the first capacitor, the plasma voltage source is connected in parallel to the bias voltage source via a first decoupling means, the plasma voltage source has a second capacitor which is connected to a second charging unit to charge the second capacitor and which has a capacitance (C.sub.2) which is at least one order of magnitude smaller than the capacitance (C.sub.1) of the first capacitor, and the ignition electrode of the flashlamp is connected to an ignition voltage generator which is connected to and controllable by the control system.

7. The apparatus according to claim 5, wherein the main voltage source comprises a third capacitor which is connected to a third charging unit to charge the third capacitor, and that the main voltage source is connected to the electrodes via a switch which can be controlled by the control system.

8. The apparatus according to claim 7, wherein the switch is designed as a thyristor.

9. The apparatus according to claim 6, wherein at least one of the charging units is connected to and controllable by the control system.

10. The apparatus according to claim 9, wherein the absence of reaction of the plasma voltage source with respect to the bias voltage source is implemented in that both the bias voltage source and the plasma voltage source are connected to an electrode of the flashlamp via in each case one diode poled in the flow direction.

11. The apparatus according to claim 6, wherein the main voltage source comprises a third capacitor which is connected to a third charging unit to charge the third capacitor, and that the main voltage source is connected to the electrodes via a switch which can be controlled by the control system.

12. The apparatus according to claim 11, wherein the switch is designed as a thyristor.

13. The apparatus according to claim 7, wherein at least one of the charging units is connected to and controllable by the control system.

14. The apparatus according to claim 8, wherein at least one of the charging units is connected to and controllable by the control system.

Description

[0061] The invention will be explained in more detail below with reference to an exemplary embodiment. In the associated drawings:

[0062] FIG. 1 shows an arrangement according to the prior art,

[0063] FIG. 2 shows a profile of the lamp current with a pre-pulse according to the prior art,

[0064] FIG. 3 shows an arrangement for the flashlamp control according to the invention,

[0065] FIG. 4 shows an illustration of the flash generation according to the invention with three partial pulses, and

[0066] FIG. 5 shows a simulation of pre-pulse and main pulse and of the resulting total pulse,

[0067] FIG. 6 shows an arrangement for the flashlamp control with a secondary circuit,

[0068] FIG. 7 shows a diagram with the current profile with the secondary circuit, and

[0069] FIG. 8 shows a diagram with an enlarged illustration of the current increase.

[0070] FIGS. 1 and 2 have already been described in the presentation of the prior art.

[0071] FIG. 3 now shows an arrangement with the features according to the invention. It comprises a flashlamp 1 which is provided with two electrodes 2; 3. Furthermore, an ignition electrode 4 is provided, which is arranged immediately adjacent to the flashlamp 1 and is thus operatively connected thereto, since an electric field at the ignition electrode 4 can influence the ignition behavior of the flashlamp 1. Such an ignition electrode can be implemented by a simple wire on the outside of the lamp body or by a reflector or the like.

[0072] Between the electrodes 2 and 3, an electrode voltage can be applied which, according to the invention, can be configured as a plasma voltage, bias voltage or a main voltage. Accordingly, a bias voltage source generating a bias voltage, a plasma voltage source generating a plasma voltage, and a main voltage source 7 generating a main voltage are provided.

[0073] The bias voltage source 5 has a first capacitor 8 connected to a first charging unit 9 charging the first capacitor 8. The first capacitor 8 together with a first inductor 10 forms an oscillating circuit.

[0074] The plasma voltage source 6 has a second capacitor 11 connected to a second charging unit 12 charging the second capacitor 11.

[0075] The capacitance C2 of the second capacitor 11 is at least one order of magnitude smaller than the capacitance C1 of the first capacitor 8.

[0076] The plasma voltage source 6 is connected in parallel to the bias voltage source 5 via a first decoupling means. This first decoupling means makes the parallel connection nonreactive. This first decoupling means comprises a first diode 13, which is connected in series with the first capacitor 8 and the first inductor 10 and is poled in the flow direction of the lamp current IL flowing to the flashlamp 1. Similarly, the first decoupling means has a second diode 14 which is connected in series with the second capacitor 11 and is likewise poled in the flow direction of the lamp current IL flowing to the flashlamp 1.

[0077] The second charging unit 12 charges the second capacitor to the plasma voltage, which is selected to be high enough that reproducible ignition takes place, preferably higher than the bias voltage with which the first capacitor 8 is charged. Thus, the voltage applied to the cathode of the first diode 13 is higher than that applied to the anode thereof and blocks it. On the other hand, the second diode 14 is forward biased because the voltage applied to its anode is higher than that applied to its cathode.

[0078] When the second capacitor is discharged, its voltage is less than the bias voltage and the second diode 14 blocks and the first diode 13 is forward biased.

[0079] The action of the plasma voltage source 6 is controllable by the ignition electrode 4. For this purpose, the ignition electrode is connected to an ignition voltage generator 15, which is connected to and controllable by a control system 16.

[0080] The main voltage source 7 has a third capacitor 17 which is connected to a third charging unit 18 which charges the third capacitor 17 to main voltage. The third capacitor 17 together with a second inductor 19 forms an oscillating circuit.

[0081] The main voltage source 7 is connected to the electrodes 2 and 3 via a second decoupling means. The second decoupling means consists of a thyristor 20 and a third diode 21. The main voltage source 7 can be controlled by the control system 16 via the thyristor 20 which acts as a switch.

[0082] The charging units 9; 12 or 18 can also be connected to and controllable by the control system 16. As symbolized by the switches 22; 23 and 24, they can be switched off by the control system 16 at certain times, for example, the first charging unit 9 and/or the second charging unit 12 can be switched off during the generation of the main pulse.

[0083] However, controlling is not mandatory if the charging units detect the timing of the flash independently or as soon as the voltage has reached the set point.

[0084] The assignment of the control inputs of the switches 20; 22; 23 and 24 to the control system 16 is symbolized by the letters A through D.

[0085] The function of the arrangement can now be seen in the fact that the bias voltage from the bias voltage source 5 with the first capacitor 8 and its first capacitance C1 is applied as electrode voltage. In parallel to the bias voltage, but—as described above, nonreactive with respect to the bias voltage source—the plasma voltage from the plasma voltage source 6, i.e., from the charged second capacitor 11 with its second capacitance C2, which is at least one order of magnitude lower (C2<<<C1) than the first capacitance C1, is applied as additional electrode voltage. The electrode voltage now corresponds to the plasma voltage, which is significantly higher than the bias voltage.

[0086] The pre-pulse 25 shown in FIG. 4 is now generated by applying a plasma voltage higher than the bias voltage as the electrode voltage and by generating an ignition pulse of the lamp current IL by ignition by means of the ignition electrode 4, whereby a plasma is ignited in the flashlamp 1 and maintained by means of the bias voltage during the pre-pulse 25. At the end of the ignition pulse 26, the second capacitor 12 with its small capacitance is discharged. Its function of providing an initial high plasma voltage is fulfilled.

[0087] Through the main voltage source 7, which is in parallel to but disconnectable from the bias voltage source 5 and plasma voltage source 6, a main voltage is applied as an electrode voltage during the pre-pulse 25 to generate the main pulse 27. The main pulse 27 is generated in that a main voltage generating the main pulse 27 is applied by the capacitor 17 by switching the thyristor 20.

[0088] Analogous to the concepts of the prior art already described, a plasma is ignited before the actual flash. In addition to the oscillating circuit of the actual flash (main pulse), another oscillating circuit (independent with respect to all parameters) is connected in parallel, which serves for generating a pre-pulse 25. At the time of the main pulse 27, the plasma has already reached the maximum possible plasma volume.

[0089] A third parallel circuit of the ignition voltage generator 15 supports the reproducibility of the ignition of the pre-pulse 25, which takes place through the ignition electrode 4. The ignition pulse 26 generates with the comparatively small capacitance C2 and a high voltage (compare with the other circuits) a short boost current. If several lamps (electrically independent) are to be operated synchronously, the ignition pulse is crucial to implement simultaneous ignition.

[0090] As shown in FIG. 2, the flash now consists of three independent superimposed partial pulses, which, however, are all based on a capacitor discharge. This means that it is possible to use only cost-efficient and long-lasting electronics.

[0091] Substantial changes of the plasma resistance occur depending on the lamp length and/or the lamp diameter. The total flash can only be optimal by freely combining the electrical parameters (I and inductance (L) 10 and 19) of pre-pulse 25 and main pulse 27.

[0092] In FIG. 5, the lamp current during the pre-pulse 25 is shown in a dotted line, during the main pulse 27 in a dashed line and the addition 28 of both is shown in a solid line.

[0093] In the illustration according to FIG. 5, the capacitances are set accordingly in such a manner that the discharge takes place completely at ideal current profiles (with respect to lamp load). In particular in the case of long lamps (large plasma resistance), the damping of the oscillating circuit is very high (long time, capacitor does not discharge completely). To reduce the damping, the capacitance can be reduced or/and the inductance can be increased (higher inductance, however, implies a longer pulse duration). A smaller capacitance in turn means lower energy at the same voltage. To counteract this, the voltage must be increased, which technically limits the component variety and safety of the systems and economically causes higher costs. The aim is therefore to be able to supply sufficient energy at moderate voltages.

[0094] As can further be seen from FIG. 5, the energy of the pre-pulse 25 is matched to the energy of the main pulse 27. If the energy of the pre-pulse 25 is too low, the lamp would be destroyed, for example (pressure increase and current increase too high). The flashlamp 1 has already almost reached the minimum resistance at the time of generation of the main pulse 27, which reduces the damping of the main pulse 27 or shortens the pulse time (compared to without pre-pulse 25). Tuning of the electronic parameters is mandatory for operation in the illustrated variant. Incorrectly tuned networks would, for example, cause the plasma to go out even though the capacitors are not yet substantially discharged.

[0095] In addition, there are technological possibilities which have not yet been implemented by the invention, which are described in the following.

[0096] By a clever combination of pre-pulse and main pulse, a substrate irradiated by the flashlamp or an arrangement of multiple flashlamps can be heated in two stages, but within only one pulse. Thus, the preheating, as it is known, e.g., by IR-emitters, can be completely or partially omitted. Preheating substantially serves to achieve a necessary temperature rise while preventing the destruction of the substrate.

[0097] A reduction of conventional preheating by the invention reduces the overall thermal budget and suppresses, for example, diffusion processes in the substrate or substrate layers.

[0098] Moreover, in combination with conventional preheating, higher maximum temperatures can be implemented by the two-stage pulse. This makes it possible to produce novel materials that cannot be implemented with previous processes.

[0099] Another possible embodiment when using the invention is the combination of two flashlamp arrays, one from the substrate back side and one from the substrate top side. The pre-pulse is applied synchronously for both lamp fields. Furthermore, the ignition of the main pulse is additionally performed from the top side. With this type of application, the substrate is effectively preheated from both sides (reduction of thermal stress) with the top side being additionally brought to the required target temperature with a short main pulse.

[0100] In another exemplary embodiment, the basic principle of the previous arrangement remains and is referred to below as the primary circuit. As shown in FIG. 6, the extension consists of an additional fourth capacitor 29, preferably with a capacitance in the range 100 μF-2000 μF, as well as a second thyristor 30 and a fourth diode 31, a fifth diode 32 and a sixth diode 33. The extension is referred to below as secondary circuit 34. In it, the fourth capacitor 29 in series with the fifth diode is connected to the electrode 2. Inserted into the line to electrode 2 is the sixth diode 33, to the anode of which a series connection of the fourth diode 31 and the second thyristor is connected, which series connection is in parallel with the fourth diode 31.

[0101] The essential elements first capacitor 8, coil 35 and flashlamp 1 remain and, furthermore, form an oscillating circuit. The pulse is shaped (current curve) by the inductance I of coil 35, the capacitance C of the first capacitor 8 and the resistance R of flashlamp 1. The increase of capacitance C and resistance R leads to damping of the oscillating circuit, and the increase of inductance I counteracts this. The spectrum emitted by the flashlamp 1 as well as the efficiency of the flashlamp

[00001]$1\left(\mathrm{conversion}\mathrm{of}\mathrm{electrical}\mathrm{energy}=\frac{c}{2}.Math.U^2\mathrm{into}\mathrm{electro}\text{-}\mathrm{magnetic}\mathrm{radiation}\right)$

are substantially influenced by the current IL flowing through the flashlamp 1. It is assumed here that all other factors for influencing the yield have already been optimally selected. A damping that is too high causes a prolonged discharge process, which results in a non-optimal current flow; i.e., the pulse time is widened and the current that flows is too low (with regard to the efficiency). For this reason, among other things, the capacitance C of capacitor 8 cannot be selected to be arbitrarily high.

[0102] During operation of the flashlamp 1, the plasma diameter reaches the inner diameter of the quartz tube, which delimits the plasma. Until this degree of expansion of the plasma is reached, the energy supplied to the flashlamp 1 is required to generate the plasma (no yield in the form of electromagnetic radiation). For this reason, the diameter of the flashlamp 1 cannot be selected to be arbitrarily large. The resistance of the flashlamp can be given in simplified form by

[00002]$K=1.28.Math.l.Math.{\left(\frac{p}{g}\right)}^2.Math.\frac{1}{d\left(t\right)}$

wherein the following designations are used: [0103] K flash lamp impedance [0104] p filling pressure [0105] l lamp length [0106] d(t) time-dependent plasma diameter [0107] g gas-type-dependent constant
At the time of maximum plasma expansion, the formula is simplified with d(t)=d.sub.quarz:

[00003]$K^{*}=1.28.Math.l.Math.{\left(\frac{p}{g}\right)}^2.Math.\frac{1}{d_{\mathrm{quartz}}}$

[0108] The resistance increases with the increase of the filling pressure and the resistance decreases with the increase of the inner diameter of the quartz glass tube. The filling pressure and the inner diameter of the quartz glass tube are selected optimally.

[0109] Increasing the lamp length also causes the resistance to increase.

[0110] To increase the range of applications, there is a need to use longer flashlamps. However, with increasing lamp length, the resistance of the lamp plasma increases, without having a decisive possibility of influence to counteract it. The value is defined by the lamp geometry. In addition, with a longer lamp length, more energy must be provided to generate the correspondingly required energy density. The increase in voltage is limited to 5 kV for cost reasons. Therefore, the energy increase is also achieved by increasing the capacitance in the primary circuit to increase the energy. However, this results in more damping of the oscillating circuit.

[0111] At the time of ignition, the flashlamp has an infinitely large resistance, which decreases to a lamp-dependent minimum value K* by generating charge carriers. The current generated by the discharge of the capacitor 8 is mainly limited by the lamp resistance or correlates with the lamp impedance K* in the dependence shown above. For longer lamps (greater resistance), this results in a partial discharge of the capacitor 8, up to the preferred time of ignition of the thyristor 20. Due to the residual charge of capacitor 8, there is also a partial discharge of the capacitor 17 when the residual voltage of capacitor 8 is reached and the thyristor goes out since no potential difference and no current flow occurs. Capacitors 8 and 17 are not reproducibly discharged or are only partially discharged.

[0112] The extension with the secondary circuit 34 leaves the function of the elements 4, 6 and 15 for ignition of the plasma unaffected.

[0113] After the ignition process, first, the capacitor 8 is discharged in that a current flows through flashlamp 1 and, in addition, the additional fourth capacitor 29 of the secondary circuit 34 is charged. With the extension, the discharge current increases or the complete discharge of the first capacitor 8 is achieved in a shorter time. Thus, igniting the thyristor 20 is possible at a shorter time with the capacitor 8 fully discharged. The energy of the first capacitor 8 is thus temporarily stored, with the discharge of the fourth capacitor 29 of the secondary circuit 34 taking place at the same time as the discharge of the capacitor 17.

[0114] The resulting advantages of the secondary circuit 34 as shown in the diagrams according to FIGS. 7 and 8 are

[0115] a) a higher peak current (1),

[0116] b) a lower current increase (lamp current) during the discharge of capacitor 8,

[0117] c) a complete discharge (3) of both capacitors, and

[0118] d) a shorter pulse width (4)/higher peak current.

[0119] To a)

The lamp has an optimum operating point with respect to the current. The higher the current, the higher the efficiency (conversion of electrical energy into radiant energy) and the greater the UV component (for the operating range shown).

[0120] To b)

A small increase in current (importantly achieved here without additional inductance) causes less damage to the lamp electrodes until full plasma expansion is reached. A comparable effect could be achieved by increasing the inductance, but this increases the pulse time. This results in a lower peak current.

[0121] To c)

When the capacitors of the primary circuit are completely discharged, the entire stored energy is used.

[0122] To d)

Shorter pulse times are advantageous for certain applications and also increase the surface temperature on the substrate (with the same energy), or the energy can be reduced (energy saving).

[0123] The extension by the secondary circuit is particularly advantageous in the case of increased lamp impedance K>20. This value is achieved in the case of optimal lamp parameters with a lamp length greater than 200 mm. The same relationship is effective in the case of a series connection of a plurality of lamps which, for example, are arranged geometrically in parallel to cover larger areas.

[0124] In another embodiment of the invention, an additional thyristor, not shown in more detail, is integrated to determine the time of the discharge process of capacitor 8 (previously only diode). With the additional thyristor, the temporarily stored energy can be supplied to the lamp plasma in a temporally controlled manner for optimum pulse shaping.

REFERENCE LIST

[0125] 1 flashlamp [0126] 2 electrode [0127] 3 electrode [0128] 4 ignition electrode [0129] 5 bias voltage source [0130] 6 plasma voltage source [0131] 7 main voltage source [0132] 8 first capacitor [0133] 9 first charging unit [0134] 10 first inductor [0135] 11 second capacitor [0136] 12 second charging unit [0137] 13 first diode [0138] 14 second diode [0139] 15 ignition voltage generator [0140] 16 control system [0141] 17 third capacitor [0142] 18 third charging unit [0143] 19 second inductor [0144] 20 thyristor [0145] 21 third diode [0146] 22 switch [0147] 23 switch [0148] 24 switch [0149] 25 pre-pulse [0150] 26 ignition pulse [0151] 27 main pulse [0152] 28 addition [0153] 29 fourth capacitor [0154] 30 second thyristor [0155] 31 fourth diode [0156] 32 fifth diode [0157] 33 sixth diode [0158] 34 secondary circuit [0159] 35 coil