How make attribute unique in xsd?

XML:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<p:configuration xsi:schemaLocation="http://ift.tt/1HL3jyc ATMScreenConfiguration.xsd " xmlns:p="http://ift.tt/1HL3jyc" xmlns:xsi="http://ift.tt/ra1lAU">
  <p:screen screenNumber="001" screenName="Do you need more time" />
  <p:screen screenNumber="002" screenName="Do you need more time" />
  <p:screen screenNumber="004" screenName="Currency prefix for amount entry" />
  <p:screen screenNumber="004" screenName="Thousands separator for amount entry" />
</p:configuration>

XSD:

...
<xsd:element name="configuration" type="configuration.configuration.type">
  <xsd:unique name="uConfiguration">
    <xsd:selector xpath="screen" /> 
    <xsd:field xpath="@screenNumber" /> 
  </xsd:unique>
</xsd:element>

I whant attribute 'screenNumber' of 'screen' tag be unique. Where I've made a mistake?