code: function showBackdrop(scope, element, options) {
  
          // If we are not within a dialog...
          if (options.disableParentScroll && !$mdUtil.getClosest(options.target, 'MD-DIALOG')) {
            // !! DO this before creating the backdrop; since disableScrollAround()
            //    configures the scroll offset; which is used by mdBackDrop postLink()
            options.restoreScroll = $mdUtil.disableScrollAround(options.element, options.parent);
          } else {
            options.disableParentScroll = false;
          }
  
          if (options.hasBackdrop) {
            // Override duration to immediately show invisible backdrop
            options.backdrop = $mdUtil.createBackdrop(scope, "md-select-backdrop md-click-catcher");
            $animate.enter(options.backdrop, $document[0].body, null, {duration: 0});
          }
  
          /**
           * Hide modal backdrop element...
           */
          return function hideBackdrop() {
            if (options.backdrop) options.backdrop.remove();
            if (options.disableParentScroll) options.restoreScroll();
  
            delete options.restoreScroll;
          };
        }
docs: Show modal backdrop element...