Method for controlling the temperature of MEMS micromirrors

Abstract

The invention relates to a method for controlling the temperature of surfaces of MEMS micromirrors (24) which are arranged within a micromirror field (10) and exposed to EUV irradiation (12). Detection of a surface temperature (20) and heating of a MEMS micromirror (24) are implemented by means of a temperature-control unit (26) that is integrated into the MEMS micromirror (24) as a constituent part thereof. A desired temperature T0 (60) at a surface of the MEMS micromirror (24) is set on the basis of an output voltage UH (58) or on the basis of a present heating current IH (92), either of which is supplied to a temperature-dependent resistor RT (42). The temperature at the surface of the MEMS micromirror (24) is kept substantially at the desired temperature T0 (60) during the operation of the micromirror field (10).

Description

[0001] Description

[0002] title

[0003] Method for tempering MEMS micromirrors

[0004] Technical area

[0005] The invention relates to a method for controlling the temperature of MEMS micromirrors arranged within a micromirror array and exposed to EUV irradiation. Furthermore, the invention relates to the use of the method for controlling the temperature of MEMS micromirrors in a micromirror array that is operated under vacuum and exposed to EUV irradiation during operation.

[0006] State of the art

[0007] The use of devices with, for example, matrix-arranged, displaceable micromirrors, so-called micromirror arrays or micromirror actuators, is nowadays found in a variety of devices, for example in smartphones, projectors, head-up displays, barcode readers, mask exposure units in semiconductor manufacturing, and in microscopes. Corresponding micromirror arrays are known, for example, from the documents DE 10 2013 208 446 A1, EP 0 877 272 A1, and WO 2010 / 049076 A1. Disclosures regarding suitable actuator devices for displacing the individual mirrors of a micromirror array of micromirrors are, for example, from

[0008] DE 10 2013 206529 Al, DE 10 2013 206531 Al and DE 10 2015 204874 Al.

[0009] DE 10 2019 205 251 A1 relates to a projection exposure system, in particular for EUV semiconductor lithography, having a heating device for a plurality of individual components, wherein the heating device comprises inductively acting heating elements. The individual components, which may be mirrors, may comprise temperature sensors for monitoring the temperature. EP 4 124 910 A1 relates to EUV lithography and discloses a micromirror array having a substrate, a plurality of movable micromirrors, an actuator system for controlling the position of the micromirrors, with a measuring system for measuring the position of the micromirrors, and furthermore a control unit on a processor, wherein the micromirrors have temperature sensors for measuring the temperatures of the micromirrors.

[0010] Disclosure of the invention

[0011] According to the invention, a method for temperature control of MEMS micromirrors arranged within a micromirror array is proposed, wherein the individual micromirrors are exposed to EUV irradiation and the following method steps are carried out: a) detection of a surface temperature and heating of a MEMS micromirror by means of a temperature control unit integrated into the MEMS micromirror as a component thereof, wherein b) a desired temperature To is set on a surface of the MEMS micromirror using an output voltage UH or using a heating current IH which is applied to a temperature-dependent resistor RT of the temperature control unit and c) maintenance of the temperature on the surface of the MEMS micromirror substantially at the desired temperature To during operation of the micromirror array.

[0012] The method proposed according to the invention advantageously makes it possible to reduce the installation space and the number of electrical connections between a micromirror and a control ASIC due to the fact that the heating devices and the temperature detection devices act on one and the same electronic components.

[0013] In an advantageous development of the method proposed according to the invention, the temperature control unit integrated into the MEMS micromirror comprises a wire-shaped conductor with a temperature-dependent resistance RT.

[0014] The method proposed according to the invention is characterized in that the output voltage UH is generated by means of a voltage source Us, such that resistors RMS, RMH connected in series with the temperature-dependent resistor T represent a resulting resistor RM, across which a resulting voltage UM drops.

[0015] The method proposed according to the invention is further characterized in that the resulting voltage UM is proportional to the current flowing through the temperature-dependent resistor RT and is applied to a control algorithm as an input variable.

[0016] In the method proposed according to the invention, the temperature-dependent resistance RT is advantageously determined according to with

[0017] RT: temperature dependent resistance RT

[0018] UH: Output voltage

[0019] UM: resulting voltage

[0020] In this context, the output voltage UH can advantageously be switched between three voltage values: UHI, UHO, and 0 volts. UHI and UHO are either regulated (constant voltage) or measured values. The current switching state and the current voltage value are known to the control algorithm. UHI is much higher than UHO. For the heating strategy with a variable heating voltage, UHI is generated from Us using a voltage regulator. For the pulse-width modulation-based heating strategy, Us is passed during heating and appears at the output as UH. UHO is generated from a small voltage source. This small voltage source can, for example, be a buffered voltage scaled down from Us. Scaling can be implemented using a voltage divider or pulse-width modulation. It is also possible for UHO to be obtained as a reference voltage from the low-voltage domain.

[0021] In an advantageous development of the method proposed according to the invention, a temperature of the surface TM of a micromirror is estimated using the following relationship:

[0022] TM = f (RTC T ) with

[0023] RT: temperature-dependent resistance T

[0024] CT: temperature-dependent coefficient of the wire-shaped conductor

[0025] In an advantageous development of the method proposed according to the invention, the control algorithm adapts its output voltage UH such that errors between the desired temperature To and the estimated mirror temperature TM are minimized.

[0026] Advantageously, the control algorithm either varies a first voltage level UHI of the output voltage UH during a defined heating period tp, or delivers a constant output voltage UH over a variable duration of the heating period. To achieve the same heating output, the two parameters can therefore be varied differently.

[0027] The method proposed according to the invention is further characterized in that the desired temperature To, the resulting voltage UM and either an output voltage of a voltage source Us or an output current of a current source Is are supplied to the control algorithm as input variables and the control algorithm supplies an output voltage UH or a current heating current IH.

[0028] Alternatively, it is possible to design the method proposed according to the invention in such a way that, instead of a voltage source Us, a current source Is supplies an output current to the control algorithm, from which the control algorithm determines a current heating current IH taking into account the desired temperature To and the resulting voltage UM.

[0029] In a further advantageous embodiment of this alternative method, for example, a value for the temperature-dependent resistance RT is determined according to the following relationship: furthermore I RT — IH — IN and with

[0030] UM: resulting voltage M: resulting series resistance

[0031] RMO: Resistance in series, either with RMH or RMS

[0032] Furthermore, the invention relates to the use of the method for tempering ME MS micromirrors of a micromirror array which is operated under vacuum and is exposed to EUV irradiation during operation.

[0033] Advantages of the invention

[0034] The method proposed in the invention allows the heating elements and the temperature-measuring elements to use the same electronic components, using either a voltage source Us or a current source Is. This drastically reduces the installation space or size of such a temperature control unit, as well as the number of electronic components that must be installed and interconnected. Finally, the number of electrical connections between each micromirror of the micromirror array and the control ASIC is reduced.

[0035] The solution proposed according to the invention allows temperature monitoring to be carried out at any time, regardless of the operating state, ie whether heating is on or not being carried out.

[0036] The method proposed by the invention enables heating that enables highly reliable control of the temperature or temperature gradients, thus compensating for mechanical characteristics such as thermal expansion. Furthermore, the method proposed by the invention advantageously allows for final testing of the micromirror array and for compensating for mechanical deformations by controlling individual MEMS micromirrors in the micromirror array.

[0037] Advantageously, the resolution of the measured voltage UM can be optimized by dynamically adjusting the series resistance RM. Thus, the temperature monitoring has a very high degree of accuracy and thus reflects the real-world conditions extremely accurately. Furthermore, the method proposed by the invention can advantageously be implemented in an electronic component, for example, within an ASIC.

[0038] Short description of the drawings

[0039] Embodiments of the invention are explained in more detail with reference to the drawings and the following description.

[0040] They show:

[0041] Figure 1 shows an arrangement of a micromirror array with a number of MEMS micromirrors exposed to EUV irradiation,

[0042] Figure 2 shows the structure and integration of a temperature control unit proposed according to the invention including a control device,

[0043] Figure 2.1 shows a possible design of the course of a wire-shaped conductor for heating,

[0044] Figure 3 shows a control circuit including control algorithm, fed by a voltage source Us,

[0045] Figure 4 shows the implementation of a possible first heating strategy for the arrangement according to Figure 3,

[0046] Figure 5 shows an alternative embodiment of the control electronics or the control algorithm, which is fed via a current source, and Figure 6 shows a heating strategy that can be implemented for the arrangement according to Figure 5.

[0047] Embodiments of the invention

[0048] In the following description of the embodiments of the invention, identical or similar elements are designated by the same reference numerals, whereby a repeated description of these elements is omitted in individual cases. The figures only schematically illustrate the subject matter of the invention.

[0049] Figure 1 shows an exemplary arrangement of a micromirror array 10. This array is exposed to EUV irradiation 12 and operates under the influence of a vacuum 14. The individual mirrors 16 indicated in Figure 1, which can have an individual inclination 18, are MEMS micromirrors 24. These are microelectromechanical mirrors that can be tilted independently of one another via actuators not shown in detail. The individual surfaces of the MEMS micromirrors 24 have surface temperatures 20, which can each be individual. They are designated in Figure 1 by Ti to TN according to the number of MEMS micromirrors 24 shown.

[0050] Figure 2 shows the structure and integration of a temperature control unit proposed according to the invention including control device,

[0051] According to the invention, each of the MEMS micromirrors 24 is provided with a temperature control unit 26 formed as a component thereof. For example, the plate-shaped temperature control unit 26 can be arranged between a carrier plate 28 below the MEMS mirror 24. Electrical connections 34 can run through a spring structure 30, which connects the carrier plate 28 to the base plate 32. The individual inclinations 18 of individual MEMS micromirrors 24 shown in Figure 1 can be realized via the spring structure 30. The electrical connections 34 extend from the temperature control unit 26, which is a component of the MEMS micromirror 24, to a control unit 36.

[0052] The temperature control unit 26 can be designed as a wire-shaped conductor 38, as shown in Figure 2.1. The wire-shaped conductor 38 can, for example, have the meander shape 40 shown in Figure 2.1. The wire-shaped conductor 38 represents a temperature-dependent resistor RT 42. Instead of the meander shape 40 shown in Figure 2.1, other conductor geometries or conductor paths are also possible for generating a heating surface, for example, circular, rectangular, elliptical, polygonal, square, or any other geometries that have a planar character.

[0053] The illustration in Figure 3 shows a first embodiment of the control unit 36, which includes a control algorithm 68. The control unit 36 ​​in its embodiment according to Figure 3 is powered by a voltage source Us 44. An output voltage is impressed into the control algorithm 68, as schematically indicated in Figure 3, via the voltage source Us 44. Furthermore, a desired temperature To 60 and a resulting voltage UM 52 are applied to the control algorithm 68 as input variables. The voltage source Us 44 provides a constant supply voltage for the heating process. Two resistors, a first resistor RMS 46 and a second resistor RMH 48, are provided and are connected in series with the temperature-dependent resistor RT 42.While the first resistor RMS 46 is of a comparable size to the resistance range of the temperature-dependent resistor RT 42, the temperature-dependent resistor RT 42 is several orders of magnitude larger than the second resistor RMH 48. The corresponding values ​​are determined such that the resulting voltage UM 52 is of a comparable order of magnitude and can be applied to the control algorithm 68.

[0054] A resulting series resistance RM 54 is provided by either the first resistor RMS 46 or the second resistor RMH 48. The resulting voltage UM 52, which drops across the resulting series resistance RM 54, is measured. The resulting voltage UM 52 is proportional to the current flowing through the temperature-dependent resistor RT 42 and is fed to the control algorithm 68 as an input. Since the voltage across the temperature-dependent resistor RT 42 is known, a value for the temperature-dependent resistor RT 42 results according to the following relationship: The output voltage UH 58 is switched between three voltage values ​​UHI 64, UHO 66, and 0 volts. UHI 64 and UHO 66 are either regulated (i.e., constant) or measured values. The current switching state and the current voltage value are known to the control algorithm 68. UHI 64 is much higher than UHO 66. For the heating strategy with a variable heating voltage, UHI 64 is generated from Us 44 using a voltage regulator. For a pulse width modulation-based heating strategy, Us 44 is passed during heating and appears at the output as UH 58. UHO 66 is generated from a small voltage source. This small voltage source can, for example, be a buffered voltage scaled down from Us 44. The scaling can be implemented using a voltage divider or by means of pulse width modulation. Furthermore, there is the possibility that UHO 66 can also be a reference voltage from the low-voltage domain.

[0055] The temperature of the surface of the MEMS micromirror 24 is estimated according to the following relationship:

[0056] TM = f(R T , CT) with

[0057] CT: known temperature coefficient of conductor material of the temperature-dependent resistance.

[0058] Thus, f (RT, CT) is a known function of the values ​​of the temperature-dependent resistance RT 42 and the temperature coefficient CT of the wire-shaped conductor 38.

[0059] The control algorithm 68, as shown in Figure 3, now adjusts the output voltage UH 58 so that an error between the desired temperature To 60 and the estimated mirror temperature TM 20 is minimized.

[0060] The illustration according to Figure 4 shows a possible first heating strategy 70, with which a temperature control or heating of individual MEMS micromirrors 24 of the micromirror array 10 can be carried out according to the schematic illustration in Figure 1. In the illustration according to Figure 4, the output voltage UH 58 and the resulting series resistance M 54 are plotted over time.

[0061] The output voltage UH 58 is switched between three different levels, namely a first level UHI 64, a second level UHO 66, and 0 volts. The first level UHI 64 is higher than the second level UHO 66 > 0 volts. The first level UHI 64 is used to heat the ME MS micromirror 24 and at the same time to provide an electrical voltage for measuring the temperature-dependent resistance RT 42 of the wire-shaped conductor 38 of the temperature control unit 26. The second level UHO 66 is used to measure the temperature-dependent resistance RT 42 when the temperature control unit 26 is switched off. The second level UHO 66 is dimensioned such that its contribution to heating or its influence on the accuracy of the heating is negligible. The second level UHO 66 can be set to 0 volts in case the temperature-dependent resistance RT 42 is measured only during a heating period tp 72.

[0062] The control algorithm 68, as indicated in Figure 3, determines the energy required for heating either by influencing a heating duration at a voltage level, namely the first level UHI 64, or by adjusting the first level UHI 64 over a specific period of time. The first heating strategy 70 shown in Figure 4 operates similarly to pulse width modulation. The control algorithm 68 triggers heating at each heating period tp 72. A heating duration tn 74 designates the duration of the heating. The resulting series resistance RM 54 is then formed by the second resistor RMH 48. In a first phase ts 76 of the heating period tp 72, the value of the temperature-dependent resistance RT 42 is measured. The error between the real and the reference temperatures is calculated once the temperature-dependent resistance RT 42 and a surface temperature TM 56 of the MEMS micromirror 24 are determined.

[0063] The control algorithm 68 determines a remaining heating time tn 74 - ts 76. After the heating phase, the output voltage UH 58 is reduced to the second level UHO 66, which occurs during the interval tp 72 - tn 74. Within this interval 80, the resulting series resistance RM 54 is applied to the first resistor RMS 46, and the temperature-dependent resistance RT 42 is measured by applying the lower second voltage level UHO 66. The combination of the second level UHO 66 with the relatively high first resistor RMS 46 is necessary to ensure a low influence of the second level UHO 66 on the heating accuracy and a sufficiently large resolution for the resulting voltage UM 52. The first phase ts 76 can be set to zero if the temperature is only to be measured when the temperature control unit 26 is switched off.

[0064] Figure 5 shows an alternative embodiment of the control unit 36, in which a current source Is 90 is used instead of the voltage source Us 44.

[0065] In the illustration according to Figure 5, the current source Is is designated 90. An actual heating current IH 92 represents the actual current made available for heating by the control algorithm 68. A voltage drop across the temperature-dependent resistor RT 42 is designated as voltage U RT 94, and the current flowing through the temperature-dependent resistor RT 42 is designated as I RT 96. These values ​​are measured to estimate the respective value of the temperature-dependent resistor RT 42. A resistor RMO 98 is connected in series with either the first resistor RMS 46 or the second resistor RMH 48 and is required to determine the voltage U RT 94 and the current I RT 96. It should be noted that the value of the second resistor RMH 48 is several orders of magnitude greater than the temperature-dependent resistor RT 42 and the value of the second resistor RMH 48 is comparable to the resistance value of the resistor RMO 98.In this context, it is assumed that the sum of the resistances, ie the first resistance RMS 46 and the resistance RMO 98, is of a comparable order of magnitude to the temperature-dependent resistance RT 42.

[0066] The following relationships arise: In this context, the resulting series resistance M 54 corresponds, depending on the circuit according to Figure 5, either to the first resistance RMS 46 or to the second resistance RMH 48.

[0067] The control algorithm 68 according to Figure 5 is comparable to that shown in Figure 3.

[0068] Figure 6 shows a heating strategy 70 that can be implemented with the control unit 36 ​​according to the configuration shown in Figure 5 using a current source Is 90. The current heating current IH 92 is switched between three levels, namely a first level IHI 102, a second level IHO 104, and 0 amperes. The first level IHI 102 is above the second level IHO 104. Furthermore, the second level IHO 104 is above 0 amperes. The current heating current IH 92 is used to heat the respective ME MS micromirror 24 and at the same time to provide an electrical current to measure the temperature-dependent resistance RT 42 during heating. The second level IHO 104 is used to measure the temperature-dependent resistance RT 42 while the temperature control unit 26 is switched off. The second level IHO 104 is so small that its contribution to heating or its influence on accuracy is negligible.The second level IHO 104 can be set to 0 amperes in case the temperature-dependent resistance RT 42 is only measured during the heating phase.

[0069] The control algorithm 68 shown schematically in Figure 5 determines the heating energy either by influencing the heating duration with a constant first level IHI 102 or by varying the first level IHI 102 over a certain period of time.

[0070] The control algorithm 68 initiates heating for all heating periods tp 72, as shown in Figure 6. Heating occurs during a heating period tn 74. The resulting series resistance RM 54 is applied to the second resistor RMH 48. Within the first phase ts 76 of the heating period tp 72, the value of the temperature-dependent resistance RT 42 is measured. The error between the actual temperature value and the reference temperature is calculated once the temperature-dependent resistance RT 42 and thus the surface temperature of the MEMS micromirror 24 TM 56 are determined. Based on a temperature error, the control algorithm 68 determines a remaining heating time TH 74 - Ts 76. After the heating phase TH 74, the current heating current IH 92 is reduced to the second level IHO 104, which occurs for the interval tp 72 - tn 74.Within this interval 80, the resulting series resistance M 54 is applied to the first resistor MS 46 (low resistance), ensuring that the heating accuracy is only slightly influenced by the second level IHO 104 and that there is sufficient resolution for the resulting voltage UM 52. The first phase ts 76 can be set to zero in the event that the temperature is to be measured when the temperature control unit 26, as part of the ME MS micromirror 24, is switched off.

[0071] The proposed method can be used for tempering, in particular for heating, individual micromirrors within a micromirror field 10 operated under vacuum, wherein the micromirror field 10 is under EUV irradiation 12.

[0072] The solution proposed according to the invention further comprises a device for temperature control of MEMS micromirrors 24, which are arranged within a micromirror array 10 and exposed to EUV irradiation 12. Individual mirrors 16 of the micromirror array 10 each have a temperature control unit 26 integrated into the individual mirrors 16, which detects their surface temperature 20 and heats the individual mirror 16, and a desired temperature To 60 on a surface of the individual mirror 16 is controlled by a control algorithm 68. The temperature control unit 26 is designed as at least one wire-shaped conductor 38 with a temperature-dependent resistor RT 42. In the device proposed according to the invention, the control algorithm 68 is integrated into the control unit 36 ​​such that either an output voltage from a voltage source Us 44 or an output current from a current source Is 90 is applied to the control algorithm 68.Furthermore, the control unit 36 ​​is constructed such that it has a first resistor RMS 46 and a second resistor RMH 48. The resulting voltage UM 52 can be adjusted via a switch 62, which switches either the first resistor RMS 46 or the second resistor RMH 48.

[0073] The invention is not limited to the embodiments described here and the aspects highlighted therein. Rather, numerous modifications are possible within the scope of the claims, which are within the scope of one skilled in the art.

Claims

Claims 1. Method for temperature control of MEMS micromirrors (24) which are arranged within a micromirror field (10) and exposed to EUV irradiation (12), with the following method steps: a) detecting a surface temperature (20) and heating a MEMS micromirror (24) by means of a temperature control unit (26) integrated into the MEMS micromirror (24) as a component thereof, wherein b) a desired temperature To (60) is set on a surface of the MEMS micromirror (24) using an output voltage UH (58) or using a heating current IH (92) which is applied to a temperature-dependent resistor RT (42) of the temperature control unit (26), and c) maintaining the temperature on the surface of the MEMS micromirror (24) substantially at the desired temperature To (60) during operation of the micromirror field (10).

2. Method for temperature control according to claim 1, characterized in that the temperature control unit (26) integrated into the MEMS micromirror (24) comprises a wire-shaped conductor (38) with a temperature-dependent resistance RT (42).

3. Method according to claims 1 and 2, characterized in that a voltage source Us (44) is used to generate the output voltage UH (58), such that resistors RMS, RMH (46, 48) connected in series (50) with the temperature-dependent resistor RT (42) represent a resulting resistor RM (54) across which a resulting voltage UM (52) drops.

4. Method according to claims 1 to 3, characterized in that the resulting voltage UM (52) is proportional to the current and the temperature-dependent resistance RT (42) and is fed to a control algorithm (68) as an input variable.

5. Method according to claims 1 to 4, characterized in that the temperature-dependent resistance RT (42) results in 6. Method according to claims 1 to 5, characterized in that a temperature of the surface TM (56) is estimated according to TM = f ( TCT) with T: temperature-dependent resistance RT (42) CT: temperature-dependent coefficient of the wire-shaped conductor (38) 7. Method according to claims 1 to 6, characterized in that the control algorithm (68) adapts its output voltage UH (58) such that errors between the desired temperature To (60) and the estimated temperature of the surface of the micromirror (24) TM (20) are minimized.

8. Method according to claims 1 to 7, characterized in that the control algorithm (68) either varies a first voltage level UHI (64) of the output voltage UH (58) during a defined heating period (72) tp, or delivers a constant output voltage UH (58) over a variable duration of the heating period (72) tp.

9. Method according to claims 1 to 8, characterized in that the desired temperature To (60), the resulting voltage UM (52) and either an output voltage of a voltage source Us (44) or an output current of a current source Is (90) are supplied to the control algorithm (68) as input variables, and the control algorithm (68) provides an output voltage UH (58) or a current heating current IH (92).

10. The method according to claim 1, characterized in that a current source Is (90) supplies an output current to the control algorithm (68) from which it determines a current heating current IH (92) taking into account the desired temperature To (60) and the resulting voltage UM (52).

11. Method according to claim 10, characterized in that a value for the temperature-dependent resistance RT (42) is determined according to with UM: resulting voltage (52) RM: resulting series resistance (54) RMO: resistance in series, either with RMH (48) or RMS (46) 12. Device for temperature control of ME MS micromirrors (24) which are arranged within a micromirror field (10) and are exposed to EUV irradiation (12), characterized in that individual mirrors (16) of the micromirror field (10) each have a temperature control unit (26) integrated into the individual mirrors (16), which detects its surface temperature (20) and heats the individual mirror (16), wherein a desired temperature To (60) on a surface of the individual mirror (16) is controlled by means of a control algorithm (68).

13. Device according to claim 12, characterized in that the temperature control unit (26) comprises at least one wire-shaped conductor (38) with a temperature-dependent resistor RT (42).

14. Device according to claims 12 and 13, characterized in that the control algorithm (68) is integrated into a control device (36) which is suitable for applying to the control algorithm (68) either an output voltage source Us (44) or an output current of a current source Is (90).

15. Use of the method according to claims 1 to 11 for tempering ME MS micromirrors (24) of a micromirror array (10) which is operated under vacuum (14) and is exposed to EUV irradiation (12) during operation.

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