access the ZIP_FILE variable properly?

Jenkinsfile

@@ -55,17 +55,17 @@ pipeline {
             cd /tmp/url-shortener
             rm -rf features spec coverage db/*.db .git* Jenkinsfile *.html 
             cd /tmp
-            tar -czvf $ZIP_FILE url-shortener/
-            mv /tmp/$ZIP_FILE $CUR_DIR/
+            tar -czvf ${ZIP_FILE} url-shortener/
+            mv /tmp/${ZIP_FILE} $CUR_DIR/
             '''
         }
-        archiveArtifacts artifacts: '*.tgz'
+        archiveArtifacts artifacts: '${ZIP_FILE}.tgz'
       }
     }
     stage('Clean up deliverable') {
       steps {
         sh 'rm -rf /tmp/url-shortener'
-        sh 'rm -rf $ZIP_FILE'
+        sh 'rm -rf ${ZIP_FILE}'
       }
     }
   }