Method and apparatus for endpoint detection in a semiconductor wafer etching system

• US 4,953,982 A
• Filed: 07/20/1988
• Issued: 09/04/1990
• Est. Priority Date: 07/20/1988
• Status: Expired due to Fees

• Current Assignee: Applied Materials Incorporated
• Original Assignee: Applied Materials Incorporated
• Inventors: Birang, Manoocher, Ebbing, Peter
• Primary Examiner(s): McGraw, Vincent P.
• Assistant Examiner(s): TURNER, SAMUEL A
• Application Number:
  US07/221,979
• Time in Patent Office: 776 Days
• Field of Search: 356/354, 356/355, 356/357, 356/358, 356/382, 156/626
• US Class Current: 356/504
• CPC Class Codes: B24B 37/013 Devices or means for detecting lapping completion
• CPC Class Codes: B24D 7/12 with apertures for inspecting the surface to be abraded
• CPC Class Codes: G01B 11/026 by measuring distance between sensor and object G01B11/0608 takes precedence
• CPC Class Codes: G01B 11/0683 measurement during deposition or removal of the layer
• CPC Class Codes: G02B 7/32 using active means, e.g. light emitter including both an active and a passive focus detecting device G02B7/285; using ultrasound G02B7/40
• CPC Class Codes: G03F 7/70358 Scanning exposure, i.e. relative movement of patterned beam and workpiece during imaging
• CPC Class Codes: H01L 21/67069 for drying etching
• CPC Class Codes: H01L 21/67242 Apparatus for monitoring, sorting or marking testing or measuring during manufacture H01L22/00, marks per se H01L23/544; testing individual semiconductor devices G01R31/26
• CPC Class Codes: H01L 21/67253 Process monitoring, e.g. flow or thickness monitoring
• CPC Class Codes: H01L 22/26 Acting in response to an ongoing measurement without interruption of processing, e.g. endpoint detection, in-situ thickness measurement endpoint detection arrangements in CMP apparatus B24B37/013, in discharge apparatus H01J37/32

Abstract

A method for endpoint detection in a semiconductor wafer etching system characterized by the steps of: (1) scanning a semiconductor wafer with a narrowly focussed laser beam; (2) analyzing a reflected portion of the beam to determine a preferred parking spot on a preferred flat area of the wafer; (3) parking the beam at the preferred spot; and (4) analyzing the reflected portion of the beam to determine when the preferred flat area has been etched through. The beam spot of the laser beam is smaller than the width of the preferred flat area to eliminate noise generated at the transition boundaries of the flat area. Preferably, the wafer is scanned several times along the same beam path to permit the comparison of several scans to determine the preferred parking spot. The apparatus includes a beam forming assembly; a scanning assembly which causes the laser beam to scan across the wafer; a detection assembly reponsive to a portion of the laser beam which is reflected off of the wafer; and a controller which operates the laser and the scanning assembly and which is responsive to an output of the detection assembly. When output of the detection assembly indicates a cessation of the characteristic etching curve, the controller develops an endpoint detection signal which can automatically shut down the etching system.

Claims

1. A method for endpoint detection in a semiconductor wafer etching system comprising the steps of:

making multiple scans of a semiconductor wafer surface along a substantially identical path with a narrowly focussed beam of radiant energy and detecting a reflected portion of said beam;

firstly analyzing said reflected portion to determine a preferred parking spot on a preferred flat area of said surface, said preferred flat area having a minimum transverse dimension greater than a spot size of said beam, said step of firstly analyzing said reflected portion including analyzing a region of a first scan and a corresponding region of a second scan;

parking said beam at said preferred spot; and

secondly analyzing said reflected portion of said beam to determine when said preferred flat area has been etched through.

2. A method as claimed in claim 1 wherein said step of firstly analyzing said reflected portion includes analyzing a series of regions of said first scan and a corresponding series of regions of said second scan.

3. A method as claim in claim 1 wherein said step of secondly analyzing said reflected portion includes comparing an actual etching curve with a projected etching curve.

4. A method for finding a preferred parking spot for a laser beam of a laser beam interferometer comprising the step of:

making multiple scans along a scan path on the semiconductor wafer surface with a narrowly focussed laser beam and detecting a reflected portion of said beam;

analyzing said reflected portion of said beam to determine a preferred parking spot within a preferred flat area having a minimum transverse dimension which is larger than a spot size of said laser beam, wherein said analyzing said reflected portion of said beam develops a set of data values for each of said multiple scans including a plurality of data values taken within said preferred flat area, thereby minimizing the effects of transistors and steps on said semiconductor wafer surface which may surround said preferred flat area, said analyzing said reflected portion of said beam further including the step of comparing a first subset of data values of first scan S1 to a corresponding first subset of data values of second scan S2.

5. A method as claimed in claim 4 wherein said first subsets are compared to find the minimum difference S1S2_min between any two corresponding data values.

6. A method as claimed in claim 5 wherein a second subset of scan S1 is used to calculate a factor S1_max corresponding to the maximum difference between any two data values within said second subset of scan S1, and wherein a second subset of scan S2 is used to calculate a factor S2_max corresponding to the maximum difference between any two data values within said second subset of scan S2;

wherein said second subsets are not necessarily different from said first subsets.

7. A method as claimed in claim 6 wherein a relative value R is calculated as follows:

space="preserve" listing-type="equation">R=S1S2 .sub.min -[S1.sub.max +S2.sub.max ]

where if R<

0 then R=0;

where said relative value R represents the relative desirability of said spot.

8. A method as claimed in claim 7 wherein a plurality of R values are calculated for scans S1 and S2, the maximum of which corresponds to a quality factor Q for scans S1 and S2.

9. A method as claimed in claim 8 wherein more than one pair of scans are compared to determine a plurality of Q values, the maximum of which corresponds to said preferred parking spot.

10. A method for endpoint detection in a semiconductor wafer etching system comprising the steps of:

parking a beam of radiant energy on a spot of a semiconductor wafer surface;

detecting a reflected portion of said beam to determine an actual etching curve; and

analyzing said reflected portion to determine when said surface at said preferred spot has been etched through by comparing said actual etching curve to a projected etching curve, wherein endpoint is detected when a series of points on the actual etching curve are of a greater magnitude than a corresponding series of points on said projected etching curve by at least a predetermined fraction of the difference between the magnitude of a prior peak value of said actual etching curve and said series of points on said projected etching curve.

11. A laser interferometer endpoint detection system comprising:

beam forming means adapted to produce a narrowly focussed beam spot, said beam forming means including a laser source adapted to produce a laser beam, beam expanding means positioned in the path of said laser beam and adapted to produce an expanded laser beam, and beam focussing means positioned in the path of said expanded laser beam and adapted to produce a focussed laser beam;

scanning means coupled to said beam forming means and adapted to scan said beam spot across a surface of a semiconductor wafer in discrete steps;

detection means responsive to a reflected portion of said beam spot which is reflected from said semiconductor wafer, said detection means including a photodetector and detection optics separate from said beam focussing means for focussing said reflected beam on said photodetector;

environmental isolation means for isolating said beam forming means and said detection means from said semiconductor wafer; and

control means having an input coupled to said detection means and having an output coupled to scanning means, said control means being operative to cause said scanning means to scan across a preferred flat area which is wider than said beam spot plus one discrete step, said control means being further operative to develop an endpoint detection signal when the output of said detection means indicates the cessation of a characteristic etching curve.

12. A laser interferometer endpoint detection system as claimed in claim 11 wherein said beam forming means further includes optical isolation means disposed between said laser source and said beam expanding means along said path of said laser beam, whereby said optical isolation means minimizes reflection of said laser beam back to said laser source.

13. A laser interferometer endpoint detection system as claimed in claim 12 wherein said optical isolation means includes polarizing means.

14. A laser interferometer endpoint detection system comprising:

beam forming means adapted to produce a narrowly focussed beam spot;

scanning means coupled to said beam forming means and adapted to scan said beam spot across a surface of a semiconductor wafer;

detection means responsive to a reflected portion of said beam spot which is reflected from said semiconductor wafer;

environmental isolation means for isolating said beam forming means and said detection means from said semiconductor wafer, said environmental isolation means including window means disposed between said beam forming means and said semiconductor wafer and adapted to thermally isolate the beam forming means side of said window from the semiconductor wafer side of the window, said window means including a plurality of spaced-apart panes; and

control means having an input coupled to said detection means and having an output coupled to said scanning means, said control means being operative to cause said scanning means to scan across a preferred flat area, said control means being further operative to develop an endpoint detection signal when the output of said detection means indicates the cessation of a characteristic etching curve.

15. A laser interferometer endpoint detection system as claimed in claim 14 further comprising heating means for heating at least the pane closest to said semiconductor wafer.

16. A laser interferometer endpoint detection system in claim 14 wherein said environmental isolation means further comprises enclosure means at least partially enclosing said beam forming means.