GB2148515A

GB2148515A - Capacitive mask aligner

Info

Publication number: GB2148515A
Application number: GB08425831A
Authority: GB
Inventors: Armand P Neukermans; James H Boyden; Garrett A Garrettson; Steven G Eaton
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1983-10-12
Filing date: 1984-10-12
Publication date: 1985-05-30
Also published as: GB2180351A; GB8624008D0; DE3437502A1; GB8425831D0; NL8403128A; FR2553532A1; GB2180351B; GB2148515B

Abstract

An aligner for aligning a mask and a wafer during photolithography of a semiconductor chip uses detection of the differential capacitance between two sets of conductive fingers on the mask and ridges on the wafer 3. An A.C. signal is coupled between the ridges and the fingers and the phase or amplitude of the signals is detected. An aligner utilizing multiple groups of ridges and fingers allows rotational alignment or two axis lateral alignment. An aligner having reference ledges to which the mask and the wafer are capacitively coupled allows alignment when the distance between the mask and the wafer is too great to permit meaningful capacitive coupling between the mask and the wafer to occur. The finger spacing may be regular, chirped or random. <IMAGE>

Description

SPECIFICATION Capacitive mask aligner During the processing of an electronic chip a semiconductor wafer is exposed to a radiation source in order to develop a photo-resist layer on top of the wafer. A mask is used between the source and the wafer to selectively block the radiation and, thereby, to develop a desired design in the photoresist which controls subsequent etching of the chip. At various processing stages different masks may be used to develop different desired designs on the wafer. It is essential that the various masks be correctly aligned with the wafer so that tight tolerances, allowing, for example, the fabrication of submicron width lines, may be maintained.

Optical aligners have been used in the prior art to manually align wafers and masks. Prior art optical aligners have been slow and subject to operator error because of the need to visually align reference marks lying in different planes. Other techniques using Fresnel lenses or diffraction gratings have been proposed but have proved to be adversely sensitive to reference mark variations and have not allowed dynamic control during wafer exposure.

According to the invention there is provided an element aligner as set out in claim 1 of the claims of this specification. The invention is also expressed in the other independent claims of the claims of this specification.

In accordance with the illustrated preferred embodiment of the present invention, alignment is accomplished by detecting a differential capacitance between ridges located on the wafer and sets of interdigitated fingers located on the mask. An electrical signal applied to the wafer is capacitively coupled from the wafer ridges to the overlying mask fingers.

The wafer and the mask are aligned when the coupled signals observed on each set of interdigitated fingers are equal in amplitude. Since signal coupling occurs between numerous fingers and ridges, errors due to variations in the fabrication of individual fingers or ridges are averaged if the finger and ridge repetition patterns (constant, chirped or random) are kept substantially identical. Further, rotational or orthogonal alignment may be achieved with the use of multiple finger/ridge sets and alignment may be automated by using the measured signals to control positioning equipment.

In accordance with another preferred embodiment of the present invention, conductive shields are placed between the wafer and interconnecting side lines of the fingers. The shields ensure that coupling only occurs between the ridges and the fingers and not between the ridges and the side lines so that lateral movement of the wafer does not cause variations in the differential capacitance to occur. This allows independent single-dimension alignments to be made.

In accordance with an additional preferred embodiment of the present invention, two sets of fingers are driven by a single driving signal.

A phase shifter shifts the driving signal applied to one finger set so that the signal on the finger set is 180 degrees out of phase with the signal on the other finger set. A voltmeter detects a coupled signal on the wafer and a null is observed when alignment is ahieved.

In accordance with a further preferred embodiment of the present invention, alignment is achieved with the wafer grounded. A single driving signal is applied to two finger sets through a transformer and a diode bridge and is coupled to ground through the wafer. A recharge path is provided through conductive shields overlaying the finger sets. Alignment is achieved when a D.C. current null is observed.

In accordance with still another preferred embodiment of the present invention, alignment is performed on a mask and a wafer which are separated by a substantial distance as required in some optical lithography or metrology applications. In these applications the distance between the mask and the wafer is too great for meaningful capacitive coupling to occur between the mask and the wafer. Two groups of references, one for the mask and the other for the wafer, are attached to a backbone frame and are initially aligned together. The mask is aligned to one reference and the wafer is aligned to the other reference by measurement of the differential capacitances involved.

Figure 1 is a side view of a wafer and mask which are aligned in accordance with the preferred embodiment of the present invention.

Figure 2 is a detailed view of the wafer array which is used on the wafer shown in Fig. 1.

Figures 3A-B provide detailed views of the interdigitated fingers which are located on the mask shown in Fig. 1.

Figures 4A-B are exploded side views of the mask and wafer shown in Fig. 1.

Figure 5 is a perspective view of the preferred embodiment of the present invention.

Figure 6 is a schematic diagram of the detector shown in Fig. 5.

Figure 7 is a perspective view of another preferred embodiment of the present invention in which the mask is driven with a signal.

Figure 8 is a schematic diagram of another preferred embodiment of the present invention in which the wafer is grounded.

Figure 9 shows a rotational aligner which uses four aligners which are constructed in accordance with the preferred embodiment of the present invention shown in Fig. 5.

Figure 10 is a side view of another preferred embodiment of the present invention in which the mask and the wafer are separated by a substantial distance.

Figure. 1 shows a mask 1 aligned with a wafer 3 (mounted on a chuck 5) so that the wafer 3 is irradiated by a source 7 and a desired design is developed on a photoresist coating of wafer 3. Source 7 may generate, for example, visible light or X-rays. Alignment is accomplished by measuring a differential capacitance between a finter region 9 and a wafer array 11.

The effect of specific locational errors of individual fingers is minimized by the averaging effect over the total number of fingers.

Fig. 3B shows a detailed view of finger region 9 including grounded shields 37, 39.

Typically, mask 1 comprises a 3 micron thick boron nitride substrate having a polyimide coating. The hands 21, 25 comprise a gold layer deposited on the polyimide. In order to ensure that capacitive coupling occurs only between ridges 13 and fingers 23, 27 (and not between ridges 13 and side lines 31, 35) grounded shields 37, 39 overlay all side lines 31, 35. Thus, grounded shields 37, 39 are interposed between side lines 31, 35 and wafer 3 to eliminate capacitive coupling between side lines 31, 35 and any portion of wafer 3. Shields 37, 39 may be fabricated by depositing an insulating photoresist layer over hands 21, 25 and then depositing shields 37, 39 as a one micron thick conductive layer (e.g. aluminum) which is then grounded.

Fig. 4A shows an exploded side view of a portion of mask 1 and wafer 3. The repetition pattersn of fingers 23, 27 and ridges 13 are constant and substantially identical. Mask 1 and wafer 3 are in sufficiently close proximity, as is typical in X-ray photolithography, for example, that meaningful capacitive coupling between ridges 13 and fingers 23, 27 occurs.

A block 29 may be used to cover finger region 9 on mask 3 so that inadvertent processing of array 11 on wafer 3 does not occur during irradiation of wafer 3. Block 29 may comprise a material which absorbs the radiation generated by source 7.

Fig. 4B shows an exploded side view of a portion of mask 1 and wafer 3 in which the repetition patterns of the ridges 13 and fingers 23, 27 are chirped in spatial frequency.

If the repetition patterns of fingers 23, 27 and ridges 13 are substantially identical and aperiodic there will be one unique position at which the ridges 13 are centered between pairs of fingers 23, 27. This permits determi nation of a single unique alignment. It should be noted that a unique alignment may also be obtained by using identical repetition patterns which are random.

Fig. 5 shows a perspective view of the preferred embodiment of the present invention including finger region 9 and array 11 shown in Figs. 1-4. For the sake of illustrative clarity, mask 1 itself and the remainder of wafer 3 outside of array 11 are not shown in Fig. 5. An oscillator 41 is connected to one side of wafer 3 and an opposite side is grounded. Oscillator 41 impresses a sine wave or other signal across array 11. Side lines 31, 35 of hands 21, 15 are connected to a detector 43 which compares the air coupled capacitance of ridges 13 to fingers 23 and the air coupled capacitance of ridges 13 to fingers 27. Detector 43 may, for example, measure a relative signal amplitude or a relative signal phase.

Fig. 6 is a schematic diagram of the detector 43 shown in Fig. 5 which is operative for measuring a relative signal amplitude. Capacitors 53 and 57 represent the capacitive coupling between ridges 13 and fingers 23 and 27, respectively. Bridge 51, comprising matched Schottky barrier diodes, rectifies the signals coupled by capacitors 53, 57 and applies them to amplifier 55 which utilizes a feedback resistor (R) 59. The output of detector 43 is proportional to the difference in capacitance of capacitors 53 and 57 and the output (Vo) is zero when the two capacitances are equal.

An aligner incorporating the preferred embodiment of the present invention shown in Figs. 1-6 has been used in conjunction with X-ray lithography to allow fabrication of one micron wide lines on a silicon wafer. (100) orientation silicon was used and KOH was -utilized as the orientation dependent etchant to create ridges 13 and valleys 15 on wafer 3. The gap between mask 1 and wafer 3 was 30 microns. Array 11 on wafer 3 was 3 by 3 millimeters in size although the size and location of array 11 may be varied as dictated by the particular geometry of the wafer being fabricated. The valleys 15 were approximately 80 microns deep, the tops of ridges 13 were 40 microns wide and the repetition pattern had a constant period of 150 microns. The fingers 23, 27 were 60 microns wide and the constant period of the repetition pattern yielded a 15 micron spacing between adja cent fingers. The sine wave output of oscillator 41 was 100 volts peak-to-peak at 500 KHz.

Using the above-described X-ray lithography aligner, it was found that a 0.07 micron misalignment of mask 1 and wafer 3 created a a measurable capacitance differential of ap proximateiy 0.28 femtofarad. The relationship between misalignment and capacitance differ ential was linear since capacitance is inversely proportional to distance. In Fig. 5, the output of detector 43 was zero when each of ridges 13 was centered between a finger 23 and a finger 27. When centering occurred the dis tances between each of ridges 13 and the nearest fingers, 23, 27 were equal and, hence, the capacitances were equal. Since multiple fingers on each of hands 21, 25 were used, individual errors were averaged.

When relative movement of mask 1 and wafer 3 occurred, as shown by the arrow in Fig. 5, one capacitance increased while the other decreased and the output of detector 43 deviated from zero. It should be noted that shields 37, 39 shown in Fig. 3B ensured that relative movement in a direction orthogonal to the arrow shown in Fig. 5 did not produce a change in the differential capacitances or a deviation in the output of detector 43. For optimal alignment sensitivity it was found that the period of the repetition pattern should be roughly 5 to 6 times the size of the gap.

Fig. 7 shows an aligner which is constructed in accordance with another preferred embodiment of the present invention with which alignment may be performed without applying a high voltage to wafer 3. An oscillator 101 provides a sine wave signal to side line 31 and to a non-attenuating phase shifter 103. The phase shifter 103 provides a sine wave signal to side line 35 which is of the same amplitude and frequency as the signal applied to side line 31 but which is phase shifted by 180 degrees. In order that the two signals have identical amplitudes and opposite phases, a Blumlein transformer may be used in place of the phase shifter 103. The Blumlein transformer,which is well known to persons of ordinary skill in the art, provides two outputs having identical amplitudes and opposite phases. The two outputs may be coupled to side lines 31 and 35, respectively.A voltmeter 105 detects a summation of the two signals which are coupled to wafer 3.

When alignment is achieved the shifted and unshifted signals are coupled equally to wafer 3. the two signals cancel and a null is detected by voltmeter 105. Time or frequency multiplexing may be used if more than one aligner is utilized on a single wafer 3.

Fig. 8 shows an aligner which is constructed in accordance with another preferred embodiment of the present invention with which alignment may be performed with wafer 3 grounded. A transformer 113 having matched windings provides a sine wave from driver 111 to fingers 23, 27 and to shields 37, 39 (shown in Fig. 3B) at the same amplitude. The aligner shown in Fig. 8 may be viewed as having an air gap capacitor (C1) between ridges 13 and fingers 27, and another air gap capacitor (C2) bet between ridges 13 and fingers 23. Charge is delivered to capacitor C1 from storage capacitor 131 during the positive portions of the sine wave and is returned to storage capacitor 131 from capacitor C2 during the negative portions.

Any difference in capacitance between capacitors C1 and C2 (caused by misalignment) causes a net D.C. voltage across storage capacitor 131 which is detected by amplifier 123 and voltmeter 125. A null occurs when alignment is achieved.

Fig. 9 shows a rotational aligner which is constructed in accordance with another preferred embodiment of the present invention.

Since the aligner shown in Fig. 5 does not provide alignment sensitivity in a direction which is orthogonal to the arrow depicted in Fig. 5 it is necessary to use two mutually orthogonal sets of ridges and fingers to provide simultaneous alignment in both an and a "y" direction. Further, if four sets of ridge/fingers 81, 83, 85, 87 are used as shown in Fig. 9, rotational misalignment of 10 to 20 microradians can be detected and corrected. For the sake of illustrative clarity, only twelve ridges 89 and twenty-four fingers 91, 93 are shown while in reality the number will be dependent upon the space available and ridges 89 and fingers 91, 93 will be similar to ridges 13 and fingers 23, 27 shown in Figs. 2-5. Shields as discussed above with reference to Fig. 3B should be used to avoid misalignment caused by unwanted capacitive coupling.

Fig. 10 shows another preferred embodiment of the present invention in which a mask 1 and a wafer 3 are spaced a substantial distance apart as is required by various optical lithography methods. Thus, the capacitance between mask 1 and wafer 3 can not easily be measured. Instead, a rigid backbone 71 is equipped with two upper reference ledges 61, 63 and two lower references ledges 65, 67 which are initially aligned together to provide a benchmark. Ledges 61, 63 include conductive ridges 73 which perform the same functions as do ridges 13 in Fig. 2. Thus, mask 1 can easily be aligned to ledges 61, 63 in the manner discussed above with reference to Figs. 1-6 by driving ridges 73 with oscillator 41 and connecting detector 43 to finger region 9. In a like manner, ledges 65, 67 include fingers 75 and mask 3 can be aligned to ledges 65, 67 by driving wafer 3 with oscillator 41 and connecting detector 43 to fingers 75. Thus, alginemtn can be achieved without requiring that mask 1 be in close priximity with wafer 3.

Claims

1. An aligner for aligning a primary element with a secondary element across a gap, the aligner comprising: an array located on the primary element; first and second hands located on the secondary element and capacitively coupled to the array; an oscillator, connected across the array, for driving the array with an electrical signal; and a detector, connected to the first and second hands, the detector being operative for measuring a first component of the signal at the first hand and for measuring a second component of the signal at the second hand.

2. An aligner as in claim 1, wherein: the first hand comprises a plurality of substantially parallel conductive fingers disposed in a first repetition pattern; the second hand comprises a plurality of substantially parallel conductive fingers disposed in a second repetition pattern, the second hand fingers being interdigitated with the first hand fingers; and the array comprises a plurality of substantially parallel ridges disposed in a ridge repetition pattern.

3. An aligner as in claim 2, wherein the number of first hand fingers and the number of second hand fingers are both equal to the number of array ridges.

4. An aligner as in claim 3, wherein the first hand fingers and the second hand fingers are substantially parallel to the array ridges.

5. An aligner as in claim 4, wherein the first, second and ridge repetition patterns are substantially identical.

6. An aligner as in claim 5, wherein periods of the first, second and ridge repetition patterns are substantially constant.

7. An aligner as in claim 5, wherein the first, second and ridge repetition patterns are random.

8. An aligner as in claim 5, wherein the first, second and ridge repetition patterns are chirped.

9. An aligner as in claim 4, wherein the primary element comprises a semiconductor wafer and the secondary element comprises a mask.

10. An aligner as in claim 9, wherein the ridges on the wafer are separated by valleys which are etched into the wafer.

11. An aligner as in claim 5, wherein the ridges comprise conductive traces on the wafer.

12. An aligner as in claim 10, wherein a period of the ridges is between five and six times the gap size.

13. An aligner as in claim 2, wherein: the first hand fingers are interconnected by a side line; and the second hand fingers are interconnected by another side line.

14. An aligner as in claim 2, wherein a radiation block is attached to the mask underlying, and on a side opposite to, the first and second hands.

15. An aligner as in claim 1, wherein the detector is further operative for presenting a null indication when the first and second signal components are equal in amplitude.

16. An aligner as in claim 1, wherein the detector is further operative for presenting a null indication when the first and second signal components are equal in phase.

17. An aligner for aligning a primary element with a secondary element, the aligner comprising: a rigid backbone frame; an upper ledge attached to the frame; a lower ledge attached to the frame at a point lower than the upper ledge, the lower ledge being aligned with the upper ledge; an upper array located on the upper ledge; first and second hands located on the secondary element and capacitively coupled across an upper gap to the upper array; third and fourth hands located on the lower ledge; a lower array located on the primary element and capacitively coupled across a lower gap to the third and fourth hands; an upper oscillator, connected across the upper pattern, for driving the upper array with an upper electrical signal; a lower oscillator, connected across the lower pattern, for driving the lower array with a lower electrical signal;; an upper detector, connected to the first and second hands, the upper detector being operative for measuring a first component of the upper signal at the first hand and for measuring a second component of the upper signal at the second hand; and a lower detector, connected to the third and fourth hands, the lower detector being operative for measuring a first component of the lower signal at the third hand and a second component of the lower signal at the fourth hand.

18. An aligner as in claim 17, wherein: the first, second, third and fourth hands each comprise a plurality of substantially parallel conductive fingers disposed in first, second, third and fourths repetition patterns; the first hand fingers are interdigitated with the second hand fingers; the third hand fingers are interdigitated with the fourth hand fingers; the upper array comprises a plurality of substantially parallel ridges disposed in an upper repetition pattern; and, the lower array comprises a plurality of substantially parallel ridges disposed in a lower repetition pattern.

19. An aligner as in claim 18, wherein the number of first hand fingers and the number of second hand fingers are both equal to the number of upper array ridges.

20. An aligner as in claim 19, wherein the first and second repetition patterns are substantially identical to the upper repetition pattern.

21. An aligner as in claim 20, wherein the number of third hand fingers and the number of fourth hand fingers are both equal to the number of lower array ridges.

22. And aligner as in claim 20, wherein the third and fourth repetition patterns are substantially identical tothe lower repetition pattern.

23. An aligner as in claim 22, wherein the first hand fingers and the second hand fingers are substantially parallel to the upper array ridges.

24. An aligner as in claim 23, wherein the third hand fingers and the fourth hand fingers are substantially parallel to the lower array ridges.

25. An aligner as in claim 24, wherein the primary element comprises a semiconductor wafer and the secondary element comprises a mask.

26. An aligner as in claim 25, wherein the lower array ridges on the wafer are separated by valleys which are etched into the wafer.

27. An aligner as in claim 25, wherein the lower array ridges comprise conductive traces on the wafer.

28. An aligner as in claim 25, wherein the upper repetition pattern has a period which is substantially constant and which is between five and six times the upper gap size.

29. An aligner as in claim 25, wherein the lower repetition pattern has a period which is substantially constant and which is between five and six times the lower gap size.

30. An aligner as in claim 25, wherein: the first hand fingers are interconnected by a first side line; the second hand fingers are interconnected by a second side line; the third hand fingers are interconnected by a third side line; and the fourth hand fingers are interconnected by a fourth side line.

31. An aligner as in claim 17, wherein the upper detector is further operative for presenting a null indication when the first and second components of the upper signal are equal in amplitude.

32. An aligner as in claim 31, wherein the lower detector is further operative for presenting a null indication when the first and second components of the lower signal are equal in amplitude.

33. An aligner as in claim 17, wherein the upper detector is further operative for presenting a null indication when the first and second components of the upper signal are equal in phase.

34. An aligner as in claim 33, wherein the lower detector is further operative for presenting a null indication when the first and second components of the lower signal are equal in phase.

35. An aligner for aligning a primary element with a secondary element across a gap, the aligner comprising: an array located on the primary element; first and second hands located on the secondary element and capacitively coupled to the array; shifting means for shifting a signal received at an input by 180 degrees and for presenting a shifted signal at an output, said shifting means having the output connected to the second hand; oscillator means, connected to the first hand and to the input of the shifting means, for presenting thereat an electrical signal; and detection means, connected between the primary element and ground, for measuring a coupled signal at the primary element.

36. An aligner as in claim 35, wherein the signal is sinusoidal.

37. An aligner as in claim 36, wherein: the first hand comprises a plurality of sub stantially parallel conductive fingers disposed in a first repetition pattern; the second hand comprises a plurality of substantially parallel conductive fingers dis posed in a second repetition pattern, the sec ond hand fingers being interdigitated with the first hand fingers; and the array comprises a plurality of substan tially parallel ridges disposed in a ridge repe tion pattern.

38. An aligner as in claim 37, wherein the number of first hand fingers and the number of second hand fingers are both equal to the number of array ridges.

39. An aligner as in claim 38, wherein the first hand fingers and the second hand fingers are substantially parallel to the array ridges.

40. An aligner as in claim 39, wherein the first, second and ridge repetition patterns are substantially identical.

41. An aligner as in claim 40, wherein periods of the first, second and ridge repetition patterns are substantially constant.

42. An aligner as in claim 41, wherein the first, second and ridge repetetition patterns are random.

43. An aligner as in claim 42, wherein the first, second and ridge repetition patterns are chirped.

44. An aligner as in claim 43, wherein the primary element comprises a semiconductor wafer and the secondary element comprises a mask.

45. An aligner for aligning a primary ele ment with a secondary element across a gap, the aligner comprising: an array located on the primary element; first and second hands located on the sec ondary element and capacitively coupled to the array; oscillator means for generating an electrical signal; transformer means, having an input con nected to the oscillator means and first and second matched output pairs, said transformer means being operative for coupling the signal to the first and second output pairs; a shield located between the first and sec ond hands and the array, said shield being connected to a high port of the first output pair, a low port of which is grounded; detector means, connected to a low port of the second output pair, for detecting a current null; and rectifying means, connected to the first and second hands, the high ports of the first and second output pairs and to the shield, said rectifying means being operative for coupling a positive portion of the signal to the second hand and for coupling a negative portion of the signal to the first hand.

46. An aligner as in claim 45, wherein: the first hand comprises a plurality of substantially parallel conductive fingers disposed in a first repetition pattern; the second hand comprises a plurality of substantially parallel conductive fingers disposed in a second repetition pattern, the second hand fingers being interdigitated with the first hand fingers; and the array comprises a pluralityof substantially parallel ridges disposed in a ridge repetition pattern.

47. An aligner as in claim 46, wherein the first, second and ridge repetition patterns are substantially identical.

48. An aligner as in claim 47, wherein periods of the first, second and ridge repetition patterns are substantially constant.

49. An aligner as in claim 48, wherein the first, second and ridge repetetition patterns are random.

50. An aligner as in claim 49, wherein the first, second and ridge repetition patterns are chirped.

51. An aligner as in claim 50, wherein the primary element comprises a semiconductor wafer and the secondary element comprises a mask.

52. An aligner as in claim 51, wherein the signal is sinusoidal.

53. An aligner as in claim 52, wherein the ridges on the wafer are separated by valleys which are etched into the wafer.

54. An aligner as in claim 53, wherein the ridges comprise conductive traces on the wafer.

55. An aligner as in claim 54, wherein a period of the ridges is between five and six times the gap size.

56. An aligner as claimed in any one of claims 1 to 16, wherein the signal is sinusoidal.

57. An aligner as claimed in any one of claims 1 to 16 or claim 56 wherein said first hand comprises a plurality of subatantially parallel conductive fingers capacitively coupled to the array and said second hand comprising a plurality of substantially parallel conductive fingers interdigitated with the fingers of the first hand and capacitively coupled to the array; the aligner further comprising a first side line located on the secondary element and interconnecting the fingers of the first hand; a second side line located on the secondary element and interconnecting the fingers of the second hand; first and second insulating layers covering the first and second side lines; and first and second conductive shields covering the first and second insulating layers.

58. An aligner as in claim 57, wherein the first and second conductive shields are grounded.

59. An aligner as claimed in any one of claims 1 to 16 comprising an orthogonal array located on the primary element and orthogonal to the array; first and second orthogonal hands located on the secondary element, said first and second orthogonal hands being orthogonal to the first and second hands and capacitively coupled to the orthogonal array; said oscillator being also connected across the orthogonal array, for also driving the orthogonal array with an electrical signal; the aligner further comprising an orthogonal detector, connected to the first and second orthogonal hands, the orthogonal detector being operative for measuring a first orthogonal component of the signal at the first orthogonal hand and a second orthogonal component of the signal at the second orthogonal hand.

60. An aligner as claimed in claim 59 when dependent on claim 2, wherein: the first orthogonal hand comprises a plural- ity of substantially parallel conductive fingers which are orthogonal to the first hand fingers; the second orthogonal hand comprises a plurality of substantially parallel conductive fingers which are interdigitated with the first orthogonal hand fingers; and the orthogonal array comprises a plurality of substantially parallel ridges which are orthogonal to the array ridges.

61. An aligner as claimed in claim 60 when dependent on claim 3, wherein: the number of orthogonal first hand fingers and the number of orthogonal second hand fingers are both equal to the number of orthogonal array ridges.

62. An aligner as claimed in claim 61 when dependent on claim 4, wherein: the orthogonal first hand fingers and the orthogonal second hand fingers are substantially parallel to the orthogonal array ridges.

63. An aligner as claimed in any one of claims 60 to 62 when dependent on claim 10, wherein the ridges of the orthogonal array are separated by valleys which are etched into the wafer.

64. An aligner as claimed in any one of claims 59 to 63 when dependent on claim 5, wherein the repetition patterns of the first and second orthogonal hands are substantially identical to a repetition pattern of the orthogonal array.

65. An aligner as claimed in any one of claims 59 to 64 when dependent on claim 12, wherein the periods of the repetition patterns of the first and second orthogonal hands and the orthogonal array are substantially constant and are between 5 and 6 times the size of the gap.

66. An aligner as claimed in any one of claims 59 to 65 when dependent on claim 13, wherein the orthogonal first hand fingers are interconnected by an orthogonal first hand side line; and the orthogonal second hand fingers are interconnected by an orthogonal second hand side line.

67. An aligner as claimed in any one of claims 59 to 66 when dependent on claim 14, wherein another radiation block is attached to the mask underlying, and on a side opposite to, the orthogonal first and second hands.

68. An aligner as claimed in any one of claims 59 to 67 when dependent on claim 1 5, wherein the orthogonal detector is further operative for presenting a null indication when the first and second orthogonal signal components are equal in amplitude.

69. An aligner as claimed in any one of claims 59 to 68 when dependent on claim 16 wherein the orthogonal detector is further operative for presenting a null indication when the first and second orthogonal signal components are equal in phase.

70. An aligner for rotationally aligning about a center of rotation a primary element and a secondary element across a gap, the aligner comprising: first, second, third and fourth arrays located on the primary element; first through eighth hands located on the secondary element, the first and second hands being capacitively coupled to the first array, the third and fourth hands being capacitively coupled to the second array, the fifth and sixth hands being capacitively coupled to the third array and the seventh and eighth hands being capacitively coupled to the fourth array; an oscillator, connected to the arrays, for driving the arrays with an electrical signal; a first line connected to the first, third, fifth and seventh hands; a second line connected to the second, fourth, sixth and eighth hands; and a a detector, connected to the first and second lines, the detector being operative for measuring a first component of the signal at the first line and a second component of the signal at the second line.

71. An aligner as in claim 70, wherein: the hands each comprise a plurality of substantially parallel conductive fingers; the first hand fingers are interdigitated with the second hand fingers; the third hand fingers are interdigitated withthe fourth hand fingers; the fifth hand fingers are interdigitated with the sixth hand fingers; the seventh hand fingers are interdigitated with the eighth hand fingers; the fingers of the first, second, fifth and sixth hands are substantially parallel; the fingers of the third, fourth, seventh and eighth hands are substantially parallel and are orthogonal to the fingers of the first, second, fifth and sixth hands; and the fingers are radially aligned with the center of rotation.

72. An aligner as in claim 71. wherein: the arrays each comprise a plurality of substantially parallel ridges; the number of first hand fingers, second hand fingers and first array ridges are equal; the number of third hand fingers, fourth hand fingers and second array ridges are equal; the number of fifth hand fingers, sixth hand fingers and third array ridges are equal; and the number of seventh hand fingers, eighth hand fingers and fourth array ridges are equal.

73. An aligner as in claim 72, wherein.

the first hand fingers, second hand fingers and first array ridges are substantially parallel; the third hand fingers, fourth hand fingers and second array ridges are substantially parallel; the fifth hand fingers, sixth hand fingers and third array ridges are substantially parallel; and the seventh hand fingers, eighth hand fingers and fourth array ridges are substantially parallel.

74. An aligner as in claim 73, wherein the primary element comprises a semiconductor wafer and the secondary element comprises a mask.

75. An aligner as in claim 74, wherein the ridges of each array are separated by valleys which are etched into the wafer.

76. An aligner as in claim 75, wherein: repetition patterns of the first and second hands and the first array are substantially identical; repetition patterns of the third and fourth hands and the second array are substantially identical; repetition patterns of the fifth and sixth hands and the third array are substantially identical; and repetition patterns of the seventh and eighth hands and the fourth array are substantially identical.

77. An aligner as in claim 76, wherein the repetition patterns have a period which is substantially constant.

78. An aligner as in claim 77, wherein the period is between five and six times the gap size.

79. An aligner as in claim 75, wherein a radiation block is attached to the mask underlying, and on a side opposite to, the hands.

80. An aligner as in claim 79, wherein the detector is further operative for presenting a null indication when the first and second signal components are equal in amplitude.

81. An aligner as in claim 80, wherein the detector is further operative for presenting a null indication when the first and second signal components are equal in phase.

82. An aligner for aligning a primary element with a secondary element substantially as herein described with reference to and as illustrated in Figs. 1 to 6 or any of Figs. 7 to 10 of the accompanying drawings.