Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net (loss) income attributable to Amtech Systems, Inc.	$ (2,372)	$ 6,452
Denominator:
Weighted-average shares used to compute basic EPS (in shares)	14,220	14,781
Common stock equivalents (in shares)	0	517
Weighted-average shares used to compute diluted EPS (in shares)	14,220	15,298
Basic (loss) income per share attributable to Amtech shareholders (USD per share)	$ (0.17)	$ 0.44
Diluted (loss) income per share attributable to Amtech shareholders (USD per share)	$ (0.17)	$ 0.42
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	1,198	120

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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