DE102019201699A1 - Optical sensor for a projection exposure apparatus for semiconductor lithography and projection exposure apparatus - Google Patents

Abstract

The invention relates to an optical sensor (30) for a projection exposure apparatus (1) for semiconductor lithography, comprising a light source (31) and a sensor head (32), wherein the light source (31) is a superluminescent light emitting diode (50).

Description

The invention relates to an optical sensor for a projection exposure apparatus for semiconductor lithography and to a projection exposure apparatus.

For such projection exposure systems, there are extremely high demands on the imaging accuracy in order to be able to produce the desired microscopically small structures as error-free as possible. For this purpose, the optical elements of the imaging optics, such as lenses or mirrors, must be positioned with high precision, for which purpose almost all optical elements are arranged on manipulators. These can shift the optical element in up to six degrees of freedom or rotate around the three main axes of an orthogonal coordinate system. The manipulators have actuators and sensors connected by a control and regulation unit. The sensors are designed, for example, as optical sensors such as interferometers or optical encoders. The light sources commonly used for the sensors emit heat in addition to the useful light, ie the light which the sensor uses to determine the position of the component to be measured. Heat or thermal radiation can heat the optical elements or other components of the system, which in turn has the consequence that they expand due to the heat absorbed and, for example, optical elements change their surface shape. This can have a negative effect on the imaging properties of the optical elements. For this reason, the light sources are often placed in an area of the projection exposure apparatus in which the heat can be easily dissipated. The useful light is then guided by means of an optical waveguide to the actual sensor in the immediate vicinity of the optical element, wherein on this route usually at least one coupling point is present, which are arranged for example at the transition to the projection optics.

In the patent application WO 2013/004403 A1 is disclosed a projection exposure apparatus for semiconductor lithography with an array of frames and mirrors, the position of which can be measured directly or indirectly with linear scales or any other type of non-contact measuring sensors.

Non-contact measuring sensors usually comprise two sensor parts, wherein a first sensor part comprises a reference surface and a second sensor part comprises a measuring surface; In this case, a measuring path is formed between the reference surface and the measuring surface.

The US patent US 4,676,645 B2 discloses an optical sensor which uses as a light source a semiconductor laser in so-called "multi-mode operation", ie a laser which emits light with different discrete frequencies or modes. The laser light is guided to the sensor head. In this context, the sensor head is the part of the optical sensor which comprises the start of the measuring path of the sensor and into which the laser light is coupled, the sensor head usually being fastened to a stationary reference or a component. The light is split at a beam splitter into two measuring beams. Both light beams are diffracted at a diffraction grating, each reflected at a mirror and in turn diffracted at the diffraction grating, wherein the diffraction grating are arranged on a second movable member. In the beam splitter, the beams interfere and are imaged on two detectors via a semi-transparent mirror. These detect the intensity of the interference, from which the position is subsequently determined.

The disadvantage of the described laser is that back reflections occur, in particular at the interfaces of the optical contact points between the laser and the sensor head, which can lead to a time-dependent jumping of the emitted modes of the semiconductor laser (also referred to as "mode hopping"). As a result, this leads to position errors in the optical sensor whose measurement accuracy depends on the frequency stability and intensity stability of the laser. This can lead to positional jumps in the subsequent control unit of the imaging optics of the projection exposure apparatus.

The object of the present invention is to provide a device which solves the above-described disadvantages of the prior art.

This object is achieved by a device having the features of the independent claim. The subclaims relate to advantageous developments and variants of the invention.

An inventive optical sensor for a projection exposure apparatus for semiconductor lithography comprises a light source and a sensor head, wherein the light source is a superluminescent light-emitting diode (SLED). A SLED is a semiconductor device that emits incoherent high intensity broadband light. SLEDs are described, for example, in the US patent US 7,119,373 B2 described or produced by the company Exalos AG. The SLED is a hybrid of a light-emitting diode (LED) that emits broadband light in all directions and a laser diode (LD) that emits narrow-band light in a laser beam. The SLED includes in the Contrary to a semiconductor laser, no light amplification by reflection in a cavity, but uses power supply to amplify the emitted light, the light passes through the semiconductor only once. The lack of cavity and broadband light make a SLED less susceptible to back-reflection than a laser diode.

Furthermore, the sensor may comprise a light feed from the light source in which the light is generated to the sensor head, wherein the light feed may comprise at least one coupling point. The light supply allows the light source to be located outside the vacuum of the EUV projection system. This has the advantage that no vacuum-compatible materials have to be used for the light source and that the heat generated during the generation of the light can be dissipated more easily.

In particular, the light feed may comprise an optical waveguide. Fiber optic cables have the advantage that they can transmit light over relatively large distances, even in the space limitations usually given in EUV projection exposure systems, without requiring a direct line of sight between the light source and the sensor.

In a variant of the invention, the sensor can be designed as an optical encoder.

Furthermore, the sensor head can be made vacuum-suitable. The sensor head is in this context the part of the optical sensor into which the laser light is coupled. It is usually attached to a fixed reference. In addition, the sensor head often also includes the detectors that detect the measuring light, from which the position information can subsequently be determined.

In a further variant of the invention, the light source can be frequency-stabilized by an external reference. Frequency stabilization can advantageously increase the accuracy of the position determination. The frequency stabilization of a SLED is well known in the art, such as the European patent EP 01 946 388 B1 known and will not be discussed here.

• 1 den prinzipiellen Aufbau einer EUV-Projektionsbelichtungsanlage, in welcher die Erfindung verwirklicht sein kann,
• 2 ein Ausführungsbeispiel der Erfindung,
• 3 eine schematische Darstellung einer SLED, und
• 4a-c eine schematische Darstellung von emittierten Frequenzspektren verschiedener Lichtquellen.

Embodiments and variants of the invention will be explained in more detail with reference to the drawing. Show it

• 1 the basic structure of an EUV projection exposure apparatus in which the invention can be realized,
• 2 an embodiment of the invention,
• 3 a schematic representation of a SLED, and
• 4a-c a schematic representation of emitted frequency spectra of different light sources.

1 shows an example of the basic structure of an EUV projection exposure system 1 for microlithography, in which the invention can find application. An illumination system of the projection exposure apparatus 1 points next to a light source 3 an illumination optics 4 for illuminating an object field 5 in an object plane 6 on. One by the light source 3 generated EUV radiation 14 as optical useful radiation is by means of a light source in the 3 integrated collector aligned so that they are in the area of a Zwischenfokusebene 15 undergoes an intermediate focus before moving to a field facet mirror 2 meets. After the field facet mirror 2 becomes the EUV radiation 14 from a pupil facet mirror 16 reflected. With the aid of the pupil facet mirror 16 and an optical assembly 17 with mirrors 18 . 19 and 20 become field facets of the field facet mirror 2 in the object field 5 displayed.

Illuminated is a in the object field 5 arranged reticle 7 that of a schematically represented Retikelhalter 8th is held. A merely schematically illustrated projection optics 9 serves to represent the object field 5 in a picture field 10 into an image plane 11 , The optical elements of the projection optics 9 are usually arranged on manipulators, which may comprise a sensor according to the invention. A structure is shown on the reticle 7 on a photosensitive layer in the area of the image field 10 in the picture plane 11 arranged wafers 12 , by a wafer holder also shown in detail 13 is held. The light source 3 can emit useful radiation, in particular in a wavelength range between 5 nm and 120 nm.

The invention can also be used in a DUV system, which is not shown. A DUV system is in principle like the EUV system described above 1 constructed, in a DUV system mirrors and lenses can be used as optical elements and the light source of a DUV system emits a useful radiation in a wavelength range of 100 nm to 300 nm.

2 shows a schematic representation of an optical element designed as a mirror 40 in a partially shown projection optics 9 which is on a manipulator 42 is arranged. The manipulator 42 includes several actuators 43 and a sensor 30 , wherein the optical element 40 over the actuators 43 with a frame 44 connected is. The manipulator 42 can the optical element 40 and thus its optically active surface 41 , that is, the area that usually with Nutzlicht during operation of the plant 14 is positioned in six degrees of freedom. The position is over several sensors 30 measured, in 2 for clarity, only one sensor 30 and not all actuators 43 are shown. The example designed as an optical encoder sensor 30 includes a light source 31 , a light feeder 34 , a sensor head 32 and a diffraction grating 33 , The light source 31 is designed as a SLED. She is in a control unit 37 arranged, for example, in an area outside of the in the projection optics 9 prevailing vacuum is arranged. This has the advantage that the requirements for the materials used for the control unit 37 and also for the light source 31 are lower and waste heat of the light source 31 is easier to dissipate. The light is from the light source 31 through an optical fiber 36 to the sensor head 32 guided, with a coupling point 35 , For example, at the transition to the vacuum is present. The sensor head 32 is also on the frame 44 arranged, with the sensor head 32 For better mechanical decoupling, it can also be arranged on a second frame. The diffraction grating 33 is at the height of the sensor head 32 on the optical element 40 arranged. The sensor 30 so can the movement of the optical element 40 measure and the optical element 40 through the actuators 43 relative to the frame 44 be positioned.

3 shows the basic structure of a super-luminescent light-emitting diode (SLED) 50 which, like any other diode, also has a positive doped region 53 and a negatively doped area 54 includes. The positively endowed area 53 is via a cathode contact 51 and the negatively doped region 54 over an anode contact 52 with an energy source 57 connected. The applied voltage leads to a current flow from the positive one 53 to the negative area 54 of the semiconductor. At the contact area and in its immediate vicinity, the so-called Active Zone 55 , between the positive 53 and negatively doped area 54 For example, the light generated by spontaneous emission is amplified by stimulated emission. The Active Zone 55 between positive 53 and negative 54 Range of the semiconductor is designed so that a variety of frequencies and power to light 58 be emitted and amplified in the course. The light 58 becomes the edge of the SLED 50 led and further reinforced. There the light enters 58 without reflection as incoherent focused light 58 from the SLED 50 out.

4a . 4b and 4c each show a diagram in which a frequency spectrum is shown, in which the power (P) over the wavelength (λ) for different light sources at the same input power is plotted qualitatively.

4a shows the frequency spectrum of a laser diode (LD), which is operated in a mode with several frequencies emitted, the so-called "multimode" operation. The emitted light is emitted at different frequencies, which are represented here by the vertical lines, with different power.

4b Figure 4 shows the frequency spectrum of a super-luminescent light-emitting diode (SLED) in which the power over the frequency runs in a continuous curve, the maximum power level being lower than that of the laser diode.

4c shows the frequency spectrum of a light emitting diode (LED), which also gives a continuous power over the frequency, but in an even lower power level than the SLED.

LIST OF REFERENCE NUMBERS

1

Projection exposure system

2

facet mirror

3

light source

4

illumination optics

5

object field

6

object level

7

reticle

8th

reticle

9

projection optics

10

field

11

image plane

12

wafer

13

wafer holder

14

EUV radiation

15

Between field focal plane

16

Pupil facet mirror

17

module

18

mirror

19

mirror

20

mirror

30

Optical sensor

31

light source

32

sensor head

33

diffraction grating

34

light supply

35

coupling point

36

optical fiber

37

control unit

40

optical element

41

optically active surface

42

manipulator

43

actuator

44

frame

50

SLED

51

cathode contact

52

anode contact

53

positively doped area

54

negatively doped area

55

activation zone

57

energy

58

light

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2013/004403 Al [0003]
• US 4676645 B2 [0005]
• US 7119373 B2 [0009]
• EP 0 1946388 B1 [0014]

Claims (7)

Optical sensor (30) for a projection exposure apparatus (1) for semiconductor lithography, comprising a light source (31) and a sensor head (32), characterized in that the light source (31) is a superluminescent light emitting diode (50). Optical sensor (30) after Claim 1 , characterized in that the sensor (30) comprises a light feed (34) from the light source (31) to the sensor head (32), wherein the light feed (34) comprises at least one coupling point (35). Optical sensor (30) after Claim 1 or 2 , characterized in that the light feed (34) comprises an optical waveguide (36). Optical sensor (30) according to one of the preceding claims, characterized in that the sensor (30) is designed as an encoder. Optical sensor (30) according to one of the preceding claims, characterized in that the sensor head (32) is designed to be vacuum-compatible. Optical sensor (30) according to one of the preceding claims, characterized in that the light source (31) is frequency stabilized by an external reference. A projection exposure apparatus (1) for semiconductor lithography, characterized in that it comprises an optical sensor (30) according to one of the preceding claims.

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